BOTANY VOLUME 3 - EVOLUTION AND SYSTEMATICS - 2007

11. SYSTEMATICS AND PHYLOGENY

11.2. Bacteria, Fungi, Plants

Third subdivision: pteridophytes (Pteridophytina)

Pteridophytes, like bryophytes, are primarily plants adapted to a terrestrial lifestyle, but their Tissues and Organs are significantly more functionally differentiated.

In The life cycle of modern pteridophytes, the sporophyte dominates (Fig. 11.128, H). It is an independent green plant, and in clubmosses, horsetails, and true ferns, it is differentiated into stem, leaves, and roots. In contrast, the extinct early ferns were usually composed of entirely uniform, leafless, dichotomously branched axes (telomes; see Fig. 11.130, A, B). Like modern Psilotaceae, they lacked true roots.

The haploid gametophyte of pteridophytes is called a prothallus (Fig. 11.128, A). It usually lives for only a few weeks, reaches no more than a few centimeters in diameter, and often resembles a simple thalloid liverwort in appearance. Typically—though deviations from this rule are highly diverse—it is a simple green thallus attached to the soil by unicellular, filamentous rhizoids. Numerous antheridia and archegonia develop on it. As in bryophytes, Fertilization is possible only in Water, i.e., when the prothalli are wet.

After fertilization, the diploid generation develops from the zygote—the sporophyte (Fig. 11.128, H: 3, 4), which in ferns, however, looks completely different and is much more highly developed than in bryophytes. Perhaps only the Initial Stages of its development are similar to those of bryophytes. In most species, the prothallus soon dies off (if fertilization is prevented, it can live for several years); however, the sporophyte seedling grows into an independent perennial plant with roots, stem, and leaves: the pteridophyte plant proper (see Fig. 11.128, H: 4B; 11.148, A; 11.150; 11.152, A). The fern sporophyte is thus a true leafy plant.

Class="center">Fig. 11.128. Pteridophytina, Pteridopsida:

A, B — Dryopteris filix-mas: A — prothallus (from below) with archegonia (ar), antheridia (an), and rhizoids (r), B — prothallus with a young sporophyte (5x); C — G — development of a fern sporangium (C — E — Asplenium, 300x; F, G — Polypodium, 200x): C — first Divisions of the primordium arising from a single epidermal Cell, D — division into an outer layer — the sporangium wall (wa) and a central cell (as) — the archesporium, which has already cut off a tapetal cell (t), E — the archesporium has differentiated into tapetal Cells and sporogenous tissue (sg), F — Cells of the annulus of the sporangium wall (wa) have thickened walls; tapetal cells have lysed, spore mother cells (sm) have formed spore tetrads (s), G — mature sporangium with spores; H — diagram of fern development: light lines — haploid phase, dark lines — diploid phase, 1 — spore, 2 — prothallus with ♀ and ♂ gametangia, 3 — prothallus with a young sporophyte, 4 — sporophyte (greatly reduced) with sori of sporangia, 5 — immature single sporangium (greatly enlarged) from a sorus, 6 — mature sporangium with spore tetrads; 7 — spores (s); b — first leaf; w — ROOT; 1 — 5 in C, D — successively formed cell walls; G — gametophyte; S — sporophyte; R! — reduction division

The Telome Theory. Organs typical of leafy plants obviously arose from leafless, dichotomous telomes through the following main processes: overtopping, planation, fusion, reduction, and incurvation (bending).

Box 11.9. Vascular Plants

Pteridophytes and seed plants have true vascular bundles (and not just conducting strands, see below) in their stems, leaves, and roots, which serve to transport substances (water with minerals, assimilates). Therefore, they are called vascular plants (Tracheophyta), in contrast to bryophytes, which lack such vascular bundles¹. True, in bryophytes with an internal water conduction system, conducting strands are already found that resemble the ancestral types of vascular bundles. However, these strands are less sharply demarcated from the surrounding tissue, and no Lignin deposition occurs in The Cell walls of the conducting elements. Accordingly, the water-conducting elements of bryophytes lack wall thickenings of the tracheid and vessel types found in vascular plants. The collective system of vascular bundles in the stem of vascular plants is called the stele. With the increasing adaptation of vascular plants to terrestrial life, steles became more highly developed in terms of substance conduction and providing mechanical support to the stem (proto-, actino-, plecto-, poly-, siphono-, eustele, atactostele; see Stele Theory, Box 4.3).

¹ Vascular plants received this name because their water-conducting elements are tracheary—tracheids and vessels, whereas in bryophytes they are hydroids (or are absent entirely). — Editor's note.

The differentiation and division of labor of the originally uniform telomes, which still made up the body of early ferns, into supporting main axes and lateral axes must have occurred As a result of overtopping (Fig. 11.129, A, B). In this process, the dominant main axis grows more vigorously than the sister axes it outgrows, which become its lateral appendages (B) and increasingly take over the function of assimilation. During planation, the lateral axes began to branch in a single plane (C, D). Through congenital fusion, these telomes, already arranged in one plane, were transformed into flat, leaf-like lateral organs (D, G). This is apparently how large, complexly divided leaves with originally dichotomous venation (macro- or megaphylls) arose. Thalli arranged in three dimensions can fuse with one another, resulting in a thicker parenchymatous axis that now contains not just a single vascular bundle (protostele) (Fig. 11.130, C), but two or more (Fig. 11.129, H–K). This significantly increases the strength of the axis. Through reduction (F, G), it is believed that small, more or less awl-shaped leaves with a single vein (microphylls) could have arisen. However, it is not entirely clear whether we are dealing here with outgrowths of the axes, i.e., organs 'sui generis' that cannot be derived from telomes. The process of incurvation can be traced, for example, in the sporangia-bearing axes of horsetails (L, M) (see Fig. 11.141, F).

Fig. 11.129. Diagram of the five elementary processes which, According to the telome theory, led to the appearance of the SHOOT: A, B — overtopping; C, D — planation; E — G — reduction; H — K — fusion (also D, N); L — N — incurvation

Embryo development. In modern pteridophytes, soon after the first cell divisions of the fertilized egg, there typically arise, In addition to the FOOT (haustorium), the root, stem¹, and leaf apices, which develop in the growing embryo still attached to the prothallus (see Fig. 11.128, B; 11.155) into the primary root, primary stem², and first leaf (cotyledon), respectively. The presence of a root is characteristic of most pteridophytes. The end of the seedling axis opposite to the 'shoot pole' could be called the 'root pole'; however, only in seed plants does the main root develop from it (see Fig. 3.1), whereas in pteridophytes the first root arises laterally on the axis as an endogenous adventitious Structure³ (see Fig. 11.155, B: w). The pteridophyte seedling is thus not bipolar, as in seed plants, but unipolar⁴. The embryonic root (see Fig. 11.128, B: w) quickly dies off, but subsequently, numerous adventitious roots arise on the shoots (primary homorhizy, see 4.4.1).

¹ Correctly: shoot apex. — Editor's note.

² Primary shoot. — Editor's note.

³ The primary root of seed plants is formed below the suspensor and, strictly speaking, is endogenous. — Editor's note.

⁴ The embryo of the vast majority of pteridophytes is just as bipolar as that of seed plants, only curved according to its lateral position relative to the prothallus that nourishes it. — Editor's note.

Fig. 11.130. Psilophytopsida:

A — E — Rhyniales: A — Rhynia gwynne-vaughanii, reconstruction (1/4x), B — Aglaophyton (Rhynia) major, reconstruction (1/5x), C — E — Rhynia: C — cross-section of a telome showing the protostele (50x), D — sporangium, longitudinal section (2x), E — spore tetrad (100x); F — H — Zosterophyllales: Zosterophyllum rhenanum: F — gametophyte (= Sciadophyton), G, H — sporophyte: G — young, attached to the gametophyte, H — mature with sporangia; a — outer cortex; e — epidermis; i — inner cortex; p — phloem; x — xylem

The three primary organs in most pteridophytes grow through The activity of apical initials (see 3.1.1.1; Fig. 3.2, A; 3.6, A). The heavily leafy shoot branches dichotomously or laterally (but never from the leaf axils!). Roots have a root cap (see Fig. 3.6, A); lateral roots arise not from the pericycle, but from the innermost layer of the primary cortex (see 4.4.2.2). Leaves (at least in the most highly developed ferns) are anatomically largely similar to the leaves of seed plants. The epidermis of the aerial parts is typically covered by a cuticle (an important prerequisite for a terrestrial lifestyle at some distance above the ground!) and has Stomata (see 3.2.2.1), although epidermal cells most often still contain METABOLISM/14.html">Chloroplasts. Well-developed vascular bundles consisting of phloem and xylem run through the stems, roots, and leaves, appearing in their typical evolutionary form for the first time in pteridophytes. Their water-conducting elements are lignified tracheids; very rarely (e.g., in Pteridium), vessels are also found (see Fig. 11.149: t). There are no specialized mechanical elements in the vascular bundles yet; nevertheless, the water-conducting elements are often reinforced with annular or other thickenings of their cell walls (see Fig. 11.149). Concentric vascular bundles (specifically, with xylem surrounded by phloem) predominate, numbering one or several, although other bundle types also occur. In pteridophytes, a general evolutionary sequence of vascular bundles can be traced (see Box 4.2, Fig. A). Lignification of tracheids increases their hydraulic conductivity and simultaneously strengthens the shoot. Thus, in contrast to bryophytes, pteridophytes were able to develop into richly branched, sometimes even arborescent (tree-like) plants. The cell walls of mechanical tissue located outside the vascular bundles typically also contain lignin. The presence of roots ensures intensive water absorption and allows The formation of larger leaves that produce assimilates. Nutrient transport is carried out by longitudinally elongated sieve cells (see 3.2.4.1). Secondary thickening due to cambial activity in modern pteridophytes is characteristic of only a few representatives and is weakly expressed, although it was characteristic of some extinct groups of pteridophytes.

Sporangia with meiospores (see Fig. 11.128, G, H: 6) are formed on leaves, and only in the most primitive classes on undifferentiated telomes. The structure of sporangia varies. Leaves bearing sporangia are called sporophylls. They are often simpler in structure than assimilating leaves (trophophylls), and are arranged in groups within a strobilus: such aggregations of sporophylls can be called "flowers"¹. For better spore dispersal, sporophylls are usually raised relatively high above the substrate.

¹ This has no longer been done since the turn of the 19th and 20th centuries. — Editor's note.

Sporangia contain an archesporium with sporogenous tissue (see Fig. 11.128, H: 5, E: sg); its cells round up, separate from one another, and become spore mother cells (usually 16), each of which yields 4 haploid meiospores after Meiosis, often arranged in a tetrahedron.

Surrounding the sporogenous tissue are cells, often in several layers, adjacent to the sporangium wall. They serve to nourish the spores and collectively represent the tapetum (E: t). Cells of the secretory tapetum release their contents through their walls. The cell walls of the plasmodial tapetum lyse, and the released protoplasts merge into a periplasmodium. The latter then penetrates between the young spores of the disintegrating tetrads, nourishes them, participates in the Formation of the spore wall (perispore), and is consumed in the process (F, G).

Spores. The spore wall consists of an inner endospore and a resistant outer exospore, on which the perispore is deposited as ornamentation of various appearances. Spores, which range in color from brownish to yellow, are almost always devoid of chlorophyll.

In most (more primitive) pteridophytes, all spores are of the same type. Upon germination, they develop into prothalli that usually bear both antheridia and archegonia. However, in more evolutionarily advanced forms, the prothalli are dioecious². In some groups of pteridophytes, this Separation of sexes led to the appearance of two forms of meiospores: nutrient-rich megaspores (= macrospores), which arise in megasporangia (= macrosporangia) and germinate into relatively large female prothalli, and microspores, which are formed in microsporangia and give rise to smaller male prothalli (see Fig. 11.137). Based on this character, a distinction should be made between homosporous (isosporous) and heterosporous (heterosporous) groups — a distinction that has arisen independently multiple times in different evolutionary lineages of pteridophytes.

² More precisely, unisexual. — Editor's note.

1. Class: Psilophytes (Psilophytopsida)

The now-extinct Psilophytopsida are the ancestral group of pteridophytes. Their vegetative body was constructed of telomes, which were naked in representatives of primitive families, and covered with outgrowths in advanced ones. Sporangia were located at the tips of dichotomously branched axes or laterally on the main or lateral branches. All genera were homosporous. True roots were not yet present; instead, there were rhizomoids covered with rhizoids. Only simple proto- or actinosteles were developed in the axes.

Psilophytes were the oldest terrestrial plants to possess vascular bundles and stomata. They appeared at the Silurian-Devonian boundary (i.e., approximately 400 million years ago), rapidly achieved significant diversity of forms, and became extinct by the beginning of the Late Devonian¹.

¹ Individual representatives still existed at the end of the Late Devonian. — Editor's note.

1. Order: Rhyniales. The Rhyniales (morphologically the most primitive representatives) had a vegetative body in the form of naked, dichotomously branched telomes, often still with terminal sporangia.

The genus Cooksonia, occurring from the Late Silurian to the Early Devonian, includes the oldest terrestrial plants discovered to date. Their dichotomously branched telomes, about 10 cm in height and containing an internal protostele, bore sporangia whose width exceeded their length.

Rhynia (including Aglaophyton¹, formerly assigned to Rhynia) — the type genus of the family Rhyniaceae, represented in the Early Devonian of Scotland by two species (see Fig. 11.130, A, B) — reached 1–2 m in height². The sporophyte, at least in one species (B), grew from creeping axes³ ("rhizomes") — horizontal to arching, locally covered with non-septate rhizoids. The vertical, circular in cross-section, dichotomously branched telomes were leafless.

¹ Aglaophyton is so distinctive that it cannot be included not only in Rhynia, but even in the order Rhyniales. It is possible that it should not be included in Tracheophyta at all. — Editor's note.

² This is a highly overestimated value. — Editor's note.

³ These axes were part of the sporophyte. — Editor's note.

The axes had a cuticle and stomata of a still very simple structure (see 3.2.2.1) and were evidently assimilating organs. Thus, Rhynia species were terrestrial plants and formed reed-like thickets. The vascular bundle consisted of hydroids in Aglaophyton (Rhynia) major and Horneophyton, or of tracheids with very simple annular and spiral wall thickenings in Rhynia gwynne-vaughanii. The protostele (see Box 4.3, Fig. A; Fig. 11.130, C) in some species already contained metaxylem; typical sieve cells with sieve areas in the outer tissue of the bundle — the phloem — were not yet present¹. There was also no secondary thickening. Relatively large sporangia, ranging in shape from cylindrical to clavate, were located at the ends or on the sides of the axes². They had a wall composed of several cell layers and dehisced by a longitudinal slit. Numerous tetrads of homospores were formed within them (Fig. 11.130, D, E).

¹ Phloem is not preserved in the fossil state; therefore, it is unjustified to deny its presence in Rhyniaceae. — Editor's note.

² In Rhyniaceae, sporangia were always terminal. — Editor's note.

In Rhynia gwynne-vaughanii, lateral branches occurred as a result of overtopping; the sporangia were also partially subjected to overtopping (see Fig. 11.130, A).

In the genus Horneophyton¹, which is outwardly similar to the genus Rhynia, the elongated sporangia, arranged together in dense groups of 2–4, resemble the sporogonia of Sphagnum in their structure: the sporogenous tissue, like a bell, enclosed a columella consisting of longitudinally elongated cells. The sporangia dehisced by an apical pore.

¹ Horneophyton significantly differs from both Rhynia and Sphagnum. — Editor's note.

Due to the absence of tracheids, Aglaophyton (Rhynia) major and Horneophyton are no longer considered vascular plants.

2. Order: Zosterophyllales (-aceae)¹. Widespread globally in the Lower Devonian, the species of this order also had a vegetative body in the form of naked, dichotomously branched axes, but their lateral sporangia, which dehisced along a preformed transverse slit, were most often grouped into spike-like aggregations (see Fig. 11.130, H). They are considered an ancestral group of lycophytes, with which they are often grouped.

¹ Zosterophyllales, together with lycophytes, apparently represent an evolutionary Lineage parallel to all other vascular plants, and there is no reason to include them in Psilophytopsida. — Ed. note.

It has been shown that the gametophyte of some genera (e.g., Zosterophyllum) was a small, star-shaped branched plant (see Fig. 11.130, F), which formed umbrella-like gametangiophores on upward-curving stalks, with archegonia in the center and antheridia at the periphery. The young sporophyte resulting from fertilization detached from the gametangiophore and subsequently developed into an independent plant (see Fig. 11.130, G, H). Accordingly, it is incorrect to view the "rhizomes" of Rhyniaceae as gametophytes that remain associated with the sporophyte throughout their life, like the gametophytes of mosses.

Thus, these ancient pteridophytes had a heteromorphic Morphology/12.html">ALTERNATION OF GENERATIONS, but these generations were still more or less equally developed. From here, evolution could have proceeded both toward mosses (gametophyte dominance) and toward more advanced pteridophytes (sporophyte dominance)².

² This is a clear anachronism. According to modern data, bryophytes are closer to Algae than vascular plants are. Therefore, they should be considered either a sister group to all vascular plants, or even the ancestors of vascular plants. — Ed. note.

As with some algae (e.g., Halycystis-Derbesia in Chlorophyta), different generations of Zosterophyllales were also initially described under different generic names, until only recently was it possible to establish that they belong to the same life cycle. Thus, the name Taeniocrada corresponds to the sterile, and Sciadophyton to the fertile gametophyte, while the sporophyte generation is named Zosterophyllum (see Fig. 11.130, F). Lyonophyton, with creeping, terete axes and a conducting bundle containing hydroids, as well as vertical, saucer-shaped gametangiophores expanded at the top, presumably represented the gametophyte generation of Aglaophyton major (see Fig. 11.130, B). It was even possible to identify the structure of its antheridia with spermatogenous cells.

Fig. 11.131. Psilophytopsida: A — Psilophyton, fertile branch (3/4x); B — Lycopodiopsida, Asteroxylon mackiei, cross-section through an actinostele: xylem shown in dark, phloem in light (10x); C — the same, reconstruction (1/3x)

3. Order: Trimerophytales (-aceae). The species of this order, more highly organized compared to members of the previous order, occurred from the Lower to Middle Devonian. On prostrate (sympodially branching) main axes, dichotomously or trichotomously branching lateral axes were formed, with sporangia located at their tips, already clustered in groups. This order is considered an ancestral group of the "Progymnospermae" (see Fig. 11.288).

Dawsonites, in contrast to Trimerophyton, had recurved sporangia on curved axes. Psilophyton ornatum bore small emergences on its axes (Fig. 11.131, A) and thus represents a transition to Lycopodiopsida¹.

¹ Both the structural features and the geological range of Psilophyton exclude it from the ancestors of Lycopodiopsida. The authors have presented long-outdated views on THE ORIGIN OF lycophytes. — Ed. note.

Psilophytopsida, being the first land plants, represent the ancestral group for other, phylogenetically more advanced pteridophytes¹, and possibly even certain gymnosperms.

¹ See the previous note. — Ed. note.

2. Class: Lycophytes (Lycopodiopsida)

The sporophyte of lycophytes, which is often dichotomously branched, has simple, unsegmented, small leaves (= microphylls), usually spirally arranged. Except in a few fossil forms, the sporangia are located singly adaxially or at the Base of the leaves (sporophylls); they are most commonly arranged in terminal strobili. According to the telome theory, the arrangement of sporangia on leaves may have arisen according to the scheme shown in Fig. 11.133. Along with homospory, heterospory is widespread. Spermatozoids in a few representatives are multiflagellate (Isoetes), but most commonly biflagellate—this distinguishes lycophytes from all other pteridophytes. It is assumed that Lycopodiopsida originate from simply structured psilophytes that had lateral emergences on their axes and terminal strobili of sporophylls (Zosterophyllales)².

² Zosterophyllales did not have sporophylls. — Ed. note.

1. Order: Asteroxylales (-aceae). The branches were covered with needle- or spine-like emergences, arranged loosely or more or less densely, giving the plant a resemblance to modern clubmosses.

In the stems of Asteroxylon mackiei, which occurs together with Rhynia in the Lower Devonian of Scotland, there was a stele that was star-shaped in cross-section (actinostele — see Box 4.3, Fig. A; Fig. 11.131, B). The rays of the star represented branching lateral strands running to the attachment sites of the lateral emergences, which, however, lacked a vascular bundle. The xylem of the stele consisted of annular and helical tracheids. Sporangia were borne directly on the stem or in the axils of its emergences.

2. Order: Protolepidodendrales. Now extinct, the representatives of this order already developed sporangia on or in the immediate vicinity of the leaves. Despite the loose arrangement of the leaves, they resemble modern clubmosses (Lycopodiales) in appearance. Representatives of both major families—Drepanophycaceae¹ and Protolepidodendraceae—are found in Lower and Middle Devonian deposits. The relationships of Drepanophycus (Fig. 11.132, A) are unclear (possible link to Zosterophyllaceae); the sporangia were located not on the leaves, but between them on short stalks supplied with a vascular bundle. The leaves of Protolepidodendron (B) were still dichotomously divided at the tip; the sporangia were located on the upper side of the leaf-like structures (sporophylls; partly also on dichotomously branched lateral axes).

¹ Drepanophycaceae apparently occupy an intermediate position between Asteroxylales and Lycopodiales, and there is no reason to include them in Protolepidodendrales. — Ed. note.

Fig. 11.132. Lycopodiopsida, Protolepidodendrales: A — Drepanophycus spinaeformis, Lower Devonian (1/4x); B — Protolepidodendron scharyanum, Middle Devonian (1/4x)

Fig. 11.133. Transition (A — D) from the terminal arrangement of sporangia in Psilophytopsida (A) to their epiphyllous arrangement in Lycopodiopsida (D)

3. Order: Lycopodiales. Representatives of this predominantly modern order are usually grouped into a single family (Lycopodiaceae). These are herbaceous evergreen plants (400 species; 9 of which are found in Germany), densely covered with more or less needle-like leaves. Secondary thickening of the stems, as in the orders mentioned above, is absent.

In Lycopodium (see Fig. 11.134), the dichotomously branching shoot appears monopodial² due to the overtopping of one of the branches (see 4.2.5). The shoots are long and creeping². Dichotomously branched roots, which have several initial cells in the apex, arise from the underside of the shoots. The leaves, which are small, subulate, and essentially spirally arranged², have an unbranched midrib, but are otherwise similar to the leaves of Asteroxylales.

² Only in some species. — Ed. note.

The mesophyll of L. clavatum is homogeneous; only in a few species is it possible to distinguish between palisade and spongy parenchyma. The epidermal cells of the leaf lack chloroplasts. As is always the case with dichotomy, branching is not associated with the leaves.

The Vascular System of the stem is a highly dissected plectostele derived from an actinostele (see Box 4.3, Fig. A) with sieve cells in the phloem that bear sieve areas on their longitudinal walls but do not yet possess sieve plates. The plectostele is surrounded externally by a sheath of non-lignified cells, the outer layer of which contains starch; this is followed by a one- or two-layered endodermis with lignin in its thin cell walls. As in all pteridophytes, the endodermis in clubmosses is the innermost layer of the cortex. The outer cortex consists of highly lignified sclerenchyma cells (Fig. 11.134, L).

Fig. 11.134. Lycopodiopsida, Lycopodiales, Lycopodium clavatum:

A — unopened antheridium, longitudinal section (75x); B — spermatozoids (400x); C — young, unopened archegonium; D — dehisced archegonium ready for fertilization (75x); E — mature prothallus (2x); F — prothallus with young sporophyte (3/4x); G — sporophyll in cone-like strobili; H — sporophyll with dehisced sporangium (8x); J, K — spores in two views (400x); L — transverse section of the stem (100x); b — leaf base with cavity h; e — epidermis; en — endodermis; h — cavity; p — phloem; r — cortex; s — starch sheath; x — xylem

Some of the branches exhibit negative geotropism. Their sporophylls are often arranged in dense, spike-like strobili above a more sparsely leaved section of the shoot (Fig. 11.134, G); their formation consumes the apical meristem, so that the strobilus terminates the shoot. The sporophylls (H) have the appearance of broad scales and bear a single large, flattened, reniform sporangium at the base on the upper side, from which numerous meiospores of uniform size (isospores) (J, K) are shed. From the margins of the sporophylls, scarious lobes hang down, each covering the next lower sporangium like an 'indusium'.

The sporangial wall consists of several cell layers, bordered internally by a secretory tapetum. The sporangium dehisces by two Valves, the slit running across its apex along a line distinguishable by its specialized Cell Structure. Until mature, the spores remain united in tetrads; their multilayered exospore is covered by a network of prominent ridges¹ (see Fig. 11.134, J, K). In nature, the spores germinate only after 6 to 7 years, and, utilizing their stored nutrients, initially form a 5-celled pre-prothallus (Fig. 11.135, A). After a period of dormancy, this develops further only when fungal hyphae penetrate its lower cells, establishing a mycorrhiza-like association (B: p).

¹ This is not a separate covering, but the ornamentation of the exospore itself. — Ed. note.

The prothalli (see Figs. 11.134, E, F; 11.135) live underground and are heterotrophic, whitish, tuberous bodies up to 2 cm in size, with tuberculate lobes and unicellular rhizoids that absorb water. Mycorrhizal Fungi living in the cells of their peripheral layers undoubtedly play a major role in their Nutrition (Fig. 11.135, B, C). Under natural conditions, sexual maturity is reached only after 12 to 15 years, and the total lifespan of the prothalli can be about 20 years. In sterile, axenic culture, however, the entire development takes only a few months. In some species, the upper part of the prothalli emerges above the ground, turning green in the light. The prothalli are monoecious and bear numerous sex organs, mainly in the upper part (see Figs. 11.134, A–D; 11.135, C: a, ag). Antheridia (a) are somewhat sunken into the tissue. They are multicellular, and each cell, except for the wall cells, gives rise to a single oval spermatozoid bearing only two flagella at its apex (see Fig. 11.134, B). Archegonia (see Figs. 11.134, C, D; 11.135, C: ag), which are also sunken, often have numerous neck canal cells (up to 20, though reduction to a single one can occur); the uppermost wall cells are shed upon opening. After a series of cell divisions, the fertilized egg cell develops into an embryo, whose suspensor (Fig. 11.135, D, E: e) pushes it into the tissue of the prothallus. The Development of the haustorium, which absorbs nutrients from the prothallus, and of the first leaves (ba), which remain scale-like, is shown in Fig. 11.135, E. The first root arises adventitiously on the shoot.

In Lycopodium, the sporophylls are aggregated into strobili terminating short lateral branches; in Huperzia, trophophylls and sporophylls are produced successively throughout the year on vertical, dichotomously branched shoots. Diphasium has sporophyll strobili like Lycopodium, but its shoots are flat and dorsoventral, with scale-like leaves.

Fig. 11.135. Lycopodiales:

A, B — Lycopodium, L. annotinum, Development of the prothallus: A — 5-celled colorless pre-prothallus with spore coat (sh), rhizoidal cell (r), basal cell (b), apical cell (s) (580x); B — young prothallus with an endophytic fungus (p) inhabiting its lower cells; the apical cell has divided into three cells of the apical meristem (s1, s₂, s₃) (470x); C — E — Diphasium, D. complanatum: C — mature prothallus with antheridia (a), archegonia (ag), and cells containing fungal hyphae (shown in solid black) (24x); D, E — embryo development: D — embryo after the first cell divisions; the basal wall 1 separates the suspensor initial (e) from the embryo body initial; walls 2 and 3 perpendicular to it (the latter in the plane of section), as well as wall 4 parallel to it, yield groups of 4 cells arranged in two tiers, of which the cells between walls 1 and 4 form the haustorium, and the lowermost forms the shoot; E — intermediate stage of development (112x); ba — apex of the first leaf; ss — shoot apex

Lycopodites species from the Upper Devonian were already very similar to modern representatives of this family. Thus, the appearance of the clubmoss has remained unchanged for more than 300 million years.

While we have so far dealt with isospory, the subsequent orders evolved toward heterospory. In the leaf axils of their representatives, There is a small outgrowth — the ligule (ligula) (Fig. 11.136, C).

4. Order: Selaginellales. In appearance, the herbaceous Selaginella, or spike-mosses, somewhat resemble mosses¹. However, in the structure of their reproductive organs and Anatomical Features, they are clearly pteridophytes — and thus true vascular plants. In temperate latitudes, they are represented by only a few species, whereas in the tropics, by contrast, there are more than 700 species.

¹ Hence their German name Moosfarne (literally 'moss-ferns'). — Translator's Note.

Habit. Selaginella species have shoots that are partly prostrate, partly erect, and profusely dichotomously branched; some form mats, while others have climbing shoots that ascend several meters up shrubs.

The stem bears small, scale-like leaves. They are arranged spirally or, most commonly, in 4 rows on dorsoventral shoots: 2 rows of small, so-called dorsal leaves and 2 rows of larger ventral leaves (11.136, A; see Fig. 4.67, B; anisophylly). The leaves have only a single unbranched midrib and only rarely possess palisade parenchyma in addition to spongy parenchyma; in some species, the mesophyll cells contain only a single large, cup-shaped chloroplast. On their upper side at the base, the leaves of Selaginella bear a small, scarious, chlorophyll-free scale of epidermal origin — the ligule (see Fig. 11.136, C), which serves as a water-absorbing organ: it very rapidly absorbs rainwater flowing down the stem, and in some species is connected to the vascular bundle by tracheids.

Fig. 11.136. Lycopodiopsida, Selaginellales, Selaginella: A — S. helvetica, plant with a strobilus (s) of sporophylls (2x); B — S. kraussiana, megaspore with a germinating sporophyte (10x); C — S. lyallii, longitudinal section through the leaf base (b) (250x); D — S. selaginoides, longitudinal section through a strobilus with megasporangia (below) and microsporangia (above); in the sporophylls cut in the median plane, the ligule (l) is visible above the insertion of the sporangium (6x); e — stem epidermis; t — tracheids; w — rhizophore

At the branching points of the shoots in many species, cylindrical, elongated, downward-growing, dichotomously branched, colorless, and leafless axes — rhizophores (Fig. 11.136, A: w) — arise exogenously, and tufts of roots arise endogenously at their free ends. Under certain conditions, rhizophores can produce leaves, like typical shoots. The STRUCTURE OF THE stele varies from a central protostele and distele to a siphonostele. There is no secondary thickening. Vessels with scalariform wall thickenings are extremely rare. The endodermis of the stem (for example, in S. kraussiana) consists of tube-like, separated cells with Casparian strips (trabeculae).

5. Order: Lepidodendrales (lepidophytes). These “arborescent clubmosses” (Fig. 11.138), reaching heights of up to 40 m and trunk diameters of up to 5 m, reached their peak in the Carboniferous (see Fig. 11.163) and constitute a significant component of coal. Their linear, spirally arranged leaves (microphyll-type, which, however, reached up to 1 m in length) had stomata in two longitudinal grooves on the lower side. After they shed, characteristic scars and leaf cushions remained on the trunk surface (Fig. 11.138, B, D). The trees were anchored to the soil by repeatedly dichotomously branched rhizophores spreading over the surface (wet soil!), which, like the trunk, exhibited secondary thickening (A, C); numerous relatively weak roots of a peculiar structure (so-called appendages) grew exogenously from them, later shedding and leaving numerous scars on the rhizophore, which is why the latter were called stigmariae.

Fig. 11.138. Lycopodiopsida, Lepidodendrales: A, B — Sigillaria: A — reconstruction (1/80x), B — leaf cushions (2.5x); C — E — Lepidodendron: C — reconstruction (1/200x), D — leaf cushions (natural size), E — strobilus of sporophylls (natural size)

Leaves had a simple, occasionally dichotomously branched vascular bundle, but they lacked palisade tissue. The scars, visible in pairs (Fig. 11.138, B) or two pairs (D) on the leaf scar next to the vascular bundle scar, represent the exit points of strands of loose tissue that served for aeration and ran parallel to the leaf traces in the primary cortex. Trunks had a siphonostele (Fig. 11.139, A); the thin-walled phloem was still poorly differentiated. A ring of low-activity cambium produced new tissues during secondary thickening. In this process, the scalariform tracheids (with somewhat atypical thickenings) of the secondary xylem had a uniform diameter. The secondary xylem, with uniseriate rays already present in some forms, resembled the wood of modern conifers (but the tracheid pits were scalariform rather than pitted, and furthermore, as in almost all plants from the Carboniferous of the Northern Hemisphere, there were no annual rings). However, all secondary xylem apparently played no significant role either in providing mechanical support to the trees or in water conduction. The trunks also contained a meristem corresponding to the phellogen; it deposited derivatives inward particularly intensively, resulting in an extremely thick cortex compared to the wood (in Lepidodendron, up to 99% of the cross-sectional area (!), which is why species of this genus were called “bark trees”; see Fig. 11.139, A). This bark consisted mainly of mechanical tissue; furthermore, it apparently participated in water absorption via ligules that persisted for a long time after the leaves were shed.

The trunks of Sigillariaceae (Fig. 11.138, A) were covered with longitudinal rows of more or less hexagonal leaf cushions (B) (during Secondary Growth in thickness, they increased due to dilatation). Their simple leaves, reaching up to 10 m in length and up to 10 cm in width, were arranged in a cluster at the end of unbranched or only weakly dichotomously branched trunks. In the lower part of the crown, large strobili of sporophylls hung from very short lateral branches.

In Lepidodendraceae (Fig. 11.138, C), the spirally arranged leaves, reaching several decimeters in length, sat on rhombic leaf cushions (D). Their trunks were profusely dichotomously branched and bore terminal strobili of sporophylls on branches up to 34 m long, superficially resembling conifer cones (C, E). Very numerous, scale-like, imbricately arranged sporophylls covered and protected the sporangia. Lepidodendrons were almost exclusively heterosporous; some of them had only a single megaspore up to 6 mm or more in the megasporangium; in some representatives (Lepidostrobus major), the megaspore was partially fused with the sporangium wall, so that the development of the gametophyte had to occur inside the sporangium. The gametophytes were similar to those of Selaginellaceae (Fig. 11.139, B).

Fig. 11.139. Lycopodiopsida, Lepidodendrales: A — Lepidodendron, Cytology/practical/72.html">Cross section of the trunk (diagram), B — Bothmstrobus mundus, Longitudinal section of a megaspore with a gametophyte (35x); C — Lepidocarpon lomaxi, longitudinal section of a megasporangium (8x); ap — outer periderm; ar — outer primary cortex; b — leaf cushion; h — integument (megasporophyll); ip — inner periderm; ir — inner primary cortex; m — pith; p — phloem; pt — gametophyte; s — spore wall; sp — sporangium wall; x — xylem

Of great interest are certain Carboniferous forms (the herbaceous Miadesmia, Selaginellales, and the arborescent Lepidocarpon, Lepidodendrales) with seed-like structures; despite the lack of close relationship, they are grouped together under the name “Lepidospermae”.

In these “seed” clubmosses, the megasporophyll was positioned as an integument around the sporangium (Fig. 11.139, C: h); this integument was open at the top and could admit airborne microspores, which ensured the fertilization (though it is not yet known how) of the gametophyte (pt) located inside the single megaspore. This entire complex remained on the parent plant and developed there into a seed¹, in the formation of whose coat both the megasporangium wall and the integument (megasporophyll) participated. The megasporophylls were grouped into a cone-like strobilus, similar to that of modern gymnosperms².

¹ This structure is not homologous to the seeds of seed plants. — Editor's note.

² The cone of gymnosperms only superficially resembles the strobili of sporophylls; its Organization is fundamentally different. — Editor's note.

6. Order: Isoetales. The order is currently represented by the family Isoetaceae with two genera. Approximately 60 species of the genus quillwort (Isoetes) (Fig. 11.140) are aquatic or wet-soil perennial herbs with a corm-like, shortened, occasionally dichotomously branched axis that can reach a very great age.

From 2–3 longitudinal furrows at the base of the axis arise rows of dichotomously branched roots, while the upper part of the axis bears long, awl-shaped leaves arranged in a rosette (reaching up to 1 m in length in some species!).

The leaves, traversed by 4 air canals, have a longitudinal depression (pit, "fovea") on the upper side of their expanded base. Most of the leaves are sporophylls, each bearing a single sporangium within the fovea; only the innermost leaves of the rosette are sterile, though they do not differ in shape from the others. Behind the fovea, there is a ligule in the form of a leathery, triangular structure with a sunken base (Fig. 11.140, B, C).

Fig. 11.140. Lycopodiopsida, Isoetales:

A–C — Isoëtes lacustris: A — habit of the plant (1/2x), B — basal part of the leaf with ligule (li) and fovea (f) (2x), C — its longitudinal section (4x); D–M — I. setacea; development of the male gametophyte and formation of spermatozoids (500x); N — I. malinverniana, spermatozoid (1,100x); O–Q — female gametophyte: O — Stylites andicola, gametophyte in the ruptured spore wall with archegonia (ar), the right one with a ventral canal cell (b) and an egg cell (o) (60x); P, Q — Isoetes echinospora, development of an archegonium from a single superficial cell (250x); e — exine, with intine inside; f — fovea; h — neck wall cells; hk — neck canal cell; m — microspores; p — prothallial cell; s — spermatogenous cells; t — trabeculae; w — wall cells; z — central cell; gives rise to the ventral canal cell (b)¹

¹ Upon division, it forms the ventral canal cell and the egg cell. — Editor's note.

The outer leaves of the rosette bear megasporangia with numerous megaspores, while the younger, inner leaves bear microsporangia containing very numerous microspores. The sporangial walls are lined internally by a secretory tapetum. The gametophytes are extremely reduced and develop within the microspores (♂) or megaspores (♀), respectively. In their early developmental stages, male gametophytes are very similar to those of Lycopodium, which are still enclosed within the spore wall (see Fig. 11.135, A). Otherwise, they resemble the male gametophytes of Selaginella, but produce only 4 spirally coiled spermatozoids bearing a tuft of flagella at their anterior end. The female gametophyte (Fig. 11.140, O; shown for the closely related genus Stylites) is similar in structure to that of Selaginella and fills the entire megaspore. It develops a small number of archegonia where the spore wall ruptures. The embryo developing within the female gametophyte inside the megaspore lacks a suspensor.

Evolution. In Stylites (2 species from Peru) — the second genus of the order, which is probably evolutionarily more ancient — the stems, covered with leaf scars, are larger (15 cm); there is only one longitudinal furrow with roots; and there is a more pronounced tendency toward dichotomous branching. Individuals of extinct representatives of Isoetales from the families Pleuromeiaceae and Nathorstianaceae were significantly larger than those of extant species. This applies to a lesser extent to the Lower Cretaceous Nathorstiana, and more clearly to Pleuromeia from the Buntsandstein, whose unbranched trunks, about as thick as a human arm (with short leaves and apical heterosporous strobili), reached a height of 2 m. Modern quillworts stand at the end of an evolutionary lineage that originates from the sigillarians (Lepidodendrales), which had relatively long leaves and weakly branched or completely unbranched trunks, though significantly larger, and progresses through Pleuromeia, Nathorstiana, and the modern species of Stylites, showing a progressive shortening of the stem.

It follows from the above that modern Lycopodiopsida can often be linked to extinct forms. The latter were represented in the Carboniferous by numerous arborescent genera; they were much more highly developed than today (see Fig. 11.163), and in some evolutionary lineages (Lepidospermae) they reached the level of organization of seed plants. As the climate became increasingly arid at the end of the Paleozoic, the arborescent lycophytes, with their inefficient water absorption and transport structures, became extinct or were displaced by emerging forms with more advanced vascular systems (e.g., Cordaitidae) (see Fig. 11.163). Herbaceous lycopods and selaginellas, on the other hand, proved to be so well adapted that they have persisted unchanged for about 300 million years to the present day ('persistent types'). Admittedly, they no longer play any significant role in modern landscapes, whereas arborescent lycophytes, along with calamites and some tree ferns, dominated the Carboniferous forests, shaping their appearance (see 11.3.2.2; Fig. 11.295).

3. Class: Horsetails (Equisetopsida) (= Sphenopsida)

This class differs from the preceding one (II) in several characters: the sporophylls are clearly distinct from the trophophylls; in modern species, several sporangia are borne on umbrella-like sporangiophores, but never in the leaf axils; the tapetum is plasmodial (in Lycopodiopsida, it is secretory); the shoot is clearly differentiated into nodes with whorled leaves and internodes.

General characteristics of horsetails: small leaves relative to the stem — so-called microphylls, which, unlike in other pteridophytes, are arranged in whorls. The stem, usually whorled-branched, is clearly differentiated into nodes and long internodes (see Fig. 11.143, A, B). The sporophylls¹, which differ from the assimilating leaves, are usually peltate with a central stalk, from the underside of which a large number of sporangia hang, and are grouped into cone-like apical strobili.

¹ The foliar nature of horsetail sporangiophores has not been proven. — Editor's note.

The morphological uniformity of modern horsetails (Fig. 11.141, E, K) stands in stark contrast to the rich diversity of forms among fossil Equisetopsida.

Fig. 11.141. Equisetopsida, Equisetaceae Equisetum:

A — ♀ gametophyte with archegonia on the lower side (17x); B — spermatozoid (1,250x); C — embryo; 1, 2 — quadrant walls; from the upper half, located above the basal wall 1, the stem¹ (s) and the first whorl of leaves (b) develop, from the lower half — the root (w) and the haustorium (h) (165x); D — LATERAL VIEW OF ♀ gametophyte with a young sporophyte (shown darker) having leaf whorls and a root; E–L — Equisetum arvense: E — fertile shoots (f) arising from buds of the underground rhizome, and a starting to grow sterile (vegetative) shoot (v) (1/2x); F and G — sporophylls with sporangia, dehisced in G (6x); H — spore with two spiral bands (elaters) of the perispore (360x); J — spores with elaters uncoiled in the dry state, at lower magnification than in H (100x); K — sterile, vegetative shoot (1/2x); L — transverse section of the stem, with black xylem and carinal canals (ck) in the bundles, with sclerenchyma strands (ss) in the ridges and valleys (16x); a — archegonium; b1, b₂ — first leaf whorls; c — cytoplasm; cg — chlorophyllous tissue; e — endodermis; g — flagella; k — Cell Nucleus; m — lysigenous pith cavity; so — row of stomata; vk — vallecular canal

¹ Actually the shoot apex. — Editor's note.

1. Order: Sphenophyllales.

Representatives of this order are completely extinct. Their fossil remains from the Paleozoic (from the Upper Devonian to the Permian) are characterized by (mostly 6-merous) whorls of leaves, which are forked or wedge-shaped at the apex, with dichotomously branched Veins (see Fig. 11.142, A). They were herbaceous plants, reaching a length of about 1 m, possibly climbing. In appearance, they resembled modern Galium species. In the thin, jointed, sparsely branched

stems, a single triarch vascular bundle ran; secondary thickening was present (reticulate tracheids and tracheids with bordered pits) (B). Well-defined strobili were homosporous in some species and presumably heterosporous¹ in others.

¹ It was not the strobili, but the plants themselves that were homosporous or heterosporous. — Editor's note.

2. Order: Equisetales. The representatives of this order constitute the main group of the class, widespread from the late Devonian to the present day, and are characterized by the presence of a pith cavity in the stem, which is surrounded by a ring of collateral vascular bundles², to which secondary wood³ is adjacent externally in arborescent Paleozoic representatives.

² The strands of Vascular Tissues in the stems of Equisetales are not identical to collateral vascular bundles. — Editor's note.

³ Secondary wood was located outside the primary xylem but inside the phloem; therefore, it is incorrect to state that it was adjacent to the ring of bundles externally. — Editor's note.

Horsetails (Equisetaceae) are currently represented by only one genus — Equisetum, all species of which (32) are similar to each other in the Main Features of their structure and development.

Habit. From an underground creeping rhizome, which is often located deep in the soil, vertical aerial shoots grow, each with a single apical cell (see 3.1.1.1; Fig. 3.2, A, B), usually living for only one year. They either remain simple or branch, forming whorls of second-, third-, etc. order axes (see Fig. 11.141).

The ribbed axes consist of elongated internodes. At the nodes, separated from each other by these internodes, there are whorls (see 4.2.2) of microphylls in the form of pointed Teeth, each with a single vascular bundle, which fuse at their bases into a sheath surrounding the stem (see Fig. 11.141, E). The bases of the internodes, where they grow intercalarily, are enclosed by these sheaths. At the nodes, there is a single ring of inner xylem with phloem external to it (siphonostele). In the internodes, the vascular system is represented by separate strands located in the parenchyma (eustele¹; see Box 4.3, Fig. A).

¹ This is not a eustele, but an arthrostele, since its individual strands are not leaf trace bundles. — Ed. note.

In the nodal region, there are leaf trace strands consisting of protoxylem, arranged one after another below and above the primordia of lateral branches. The strands located above the primordia enter the leaves only in the region of the node above. The vascular bundles, together with their leaf trace strands, alternate in adjacent internodes (Fig. 11.142, E). Lateral branches break through the sheaths between the leaves.

Fig. 11.142. A, B — Sphenophytales, Sphenophyllum: A — S. cuneifolium, portion of a shoot with forked and undivided leaves (1/3x); B — S. plurifoliatum, transverse thin section of a stem; inside, a triangular primary xylem with three protoxylem groups, surrounded by secondary xylem (7x); C — E — Equisetales, Calamitaceae: C — Calamites carinatus, reconstruction (1/200x); D, E — Arthropitys communis: D — transverse thin section through a portion of wood (10x), E — tangential thin section through a young shoot (10x); b — leaf trace; c — carinal canal; i — infranodal canal; m — pith; ms — medullary ray; sx — secondary xylem; x — xylem

Due to The small size of the leaf blades, which soon lose chlorophyll, the function of assimilation is taken over by the green stems. The collateral (see 3.2.4.3) vascular bundles are very poor in xylem. The oldest PARTS OF THE xylem soon disappear, and large cavities are formed in their place, which in cross-section appear as a ring of so-called carinal canals (see Fig. 11.141, L). A large air-bearing intercellular space (pith cavity) arises in the expanded pith, and a ring of so-called vallecular (furrow) canals (under the furrows On the surface of the shoot) arises in the primary cortex. On the inner side, the primary cortex is usually delimited by a single- or double-layered endodermis with Casparian strips on the anticlinal cell walls.

The outer walls of the stem epidermal cells in horsetails are more or less heavily impregnated with silica (which is why they were formerly used for polishing metalware). Stomata are located in the furrows between the Ribs, always in pairs in longitudinal rows. They possess the following unique features: the guard cells are completely covered by the subsidiary cells; with an increase in turgor, the guard cells become rounded, and the movement is transmitted to the subsidiary cells through the thickenings of adjacent walls, causing the stomata to open.

Sporangia are formed on peculiar sporangiophores. The latter are arranged in several alternating whorls, which, due to highly shortened internodes, are aggregated at the tips of the shoots into cone-like strobili (see Fig. 11.141, E). The sporangiophores themselves ('sporophylls') are peltate (table-like on a single stalk), bearing 5 — 10 sac-like sporangia on their underside (F, G); they are supplied by concentric vascular bundles.

The sporogenous tissue in a younger sporangium is surrounded by a multilayered wall. Its innermost layer (plasmodial tapetum), as a result of the dissolution of cell walls, forms a periplasmodium, which penetrates between the rounding spores and is consumed in the formation of their walls¹. Thus, by the time the spores mature, only the two outer cell layers of the sporangium wall persist; the epidermal cell walls have annular and helical thickenings. The sporangia dehisce by a longitudinal slit on the inner side due to cohesive forces between the cell walls of the sporangium wall and water, the volume of which in these cells decreases due to evaporation.

¹ The plasmodial tapetum secretes substances used in the formation of spore walls, but it is incorrect to state that it is 'consumed' in the formation of these walls. — Ed. note.

The dehisced sporangium of extant *Equisetum* species releases numerous green spores with a uniquely structured wall. On the spore wall proper, which consists of an endospore and an exospore, the periplasmodium deposits a multilayered perispore. Its outermost layer consists of two narrow, parallel bands (elaters; see Fig. 11.141, H, J) that are spatulate at their tips. Under moist conditions, they are spirally coiled around the spore. As the spores dry, the elaters uncoil, yet remain attached at their centers to each other and to the endospore (I). In doing so, they extend, but coil up again when moistened (see 8.4); through these hygroscopic movements, they facilitate the dispersal of spores in tangled groups. Consequently, gametophytes often grow in dense clusters. The spores remain viable for only a few days.

All spores are identical. They germinate into deeply lobed, green prothalli (see Fig. 11.141, A).

The prothalli are deeply lobed, dorsiventral, curled plates that can be monoecious or dioecious¹. Sex determination in potentially bisexual prothalli is phenotypic, influenced by environmental factors. Under starvation conditions, predominantly ♂ gametophytes develop. Development to sexual maturity takes only 3 to 5 weeks—apparently this rapid pace is necessary to complete the water-sensitive and poorly competitive (compared to mosses) gametophyte phase as quickly as possible. In contrast to female gametophytes, male gametophytes are heavily pigmented with carotenoids—a phenomenon also known in mosses (antheridial walls) and fungi (♂ gametes of Allomyces), which may serve as protection against mutagenic radiation.

¹ Correctly: bisexual or unisexual. — Editor's note.

Antheridia are embedded in the tissue of the prothallus, while archegonia project above its surface¹. Coiled spermatozoids are produced in numbers of about 250 to 1,000 per antheridium and possess numerous flagella (see Fig. 11.141, B).

¹ Only the necks of the archegonia project above The surface of the prothallus. — Editor's note.

The first division (basal wall, 1–1 in Fig. 11.141, C) divides the zygote into two halves, both of which, in contrast to Lycopodium (see Fig. 11.135, D), participate in the formation of the embryo; no suspensor develops. The first leaves appear at the shoot pole, immediately arranged in a whorl, and form a ring around the shoot apex, which continues to grow by means of a tetrahedral apical cell (see Fig. 3.2, A). The primordium of the first root lies lateral to the longitudinal axis of the embryo (see Fig. 11.141, C: w); it grows downward, penetrating the tissue of the prothallus (D).

Most species of the genus Equisetum, which is distributed from the tropics to cold zones, prefer damp habitats. The South American E. giganteum and some other tropical species, being climbing plants, reach up to 12 m in length, whereas the height of species native to Germany (such as E. telmateia) is at most 2 m.

In E. arvense—the field horsetail (see Fig. 11.141)—as in other species with annual aboveground parts, short lateral Branches of the underground rhizomes develop into rounded overwintering tubers containing storage Materials; however, evergreen species also exist (e.g., E. hyemale).

In some horsetail species, some aboveground shoots are sterile and profusely branched, while others bear strobili of sporangiophores at their tips, branching later and only weakly, or not producing lateral branches at all (see Fig. 11.141, E, K).

Both of the following families are completely extinct. The species of Archaeocalamitaceae, which occurred only in the Lower Carboniferous, had forked leaves (Fig. 11.143, A) that were positioned directly above one another at adjacent nodes, corresponding to the course of the vascular bundles through the nodes.

Calamitaceae differ from Equisetaceae in the following characteristics. On the reproductive axes, whorls of peltate sporangiophores alternated with whorls of lanceolate leaves ("bracts") (Fig. 11.143, C). Along with homosporous species, there were also heterosporous ones (D). The spores lacked elaters. Species of the genus Calamites (see Fig. 11.142, C), which was widespread in the Upper Carboniferous and Permian, were an important component of the coal-swamp forests and, along with lepidodendrids and sigillarians, made a significant contribution to the formation of coal. Specimens of some species reached heights of 30 m, and due to the extensive development of secondary wood, their trunk diameters reached up to 1 m (C, D); however, like Equisetum, they had a large pith cavity ("pipe trees"). In most species, the trunks bore whorls of branches, but in some, they were unbranched. The vascular bundles (just as in Equisetum) branched into three small bundles at the upper end of the internode. Two lateral branches fused, each with a lateral branch of the adjacent bundle, to form the bundle of the next internode, while the third branch extended outward as a leaf

trace¹ (E). Radially running "infranodal canals", which arose from the lysis of thin-walled cells, probably served for aeration. The leaves (Fig. 11.143, B) were simple, lanceolate, and single-veined; at the tip of the leaf—as on the leaf teeth of extant, water-guttating horsetails—there was a hydathode. Corresponding to the alternation of primary bundles in successive internodes, the leaves were arranged in alternating whorls.

¹ On p. 250, this part of the conducting system was correctly described as a tubular section of the arthrostele ("siphonostele"). — Editor's note.

Fig. 11.143. Equisetales: A — Archaeocalamitaceae, Archaeocalamites radiatus (1 /3x); B — D — Calamitaceae; B — Annularia stellata (1/2x), C — Calamostachys binneyana, longitudinal thin section of a strobilus with sterile leaves (4x), D — Calamostachys casheana, tangential thin section through a sporangiophore bearing three megasporangia and one microsporangium (22x)

The class Equisetopsida reached its peak diversity in the Paleozoic and is now entirely extinct, except for the genus Equisetum (see Fig. 11.163). This genus unites the remnants of more highly developed forms, since during the Mesozoic, some Representatives of the genus Equisetites were arborescent forms with secondary growth. Thus, our modern horsetails are merely relicts, which we cannot, however, consider descendants of the heterosporous Paleozoic representatives, as heterospory must always be derived from homospory, and not vice versa. Therefore, modern horsetails must have originated from earlier, still homosporous forms. Some of the extinct forms (Calamites, Sphenophyllum) were heterosporous; the genus Calamocarpon reached the level of seed formation, as in Lepidospermae.

4. Class: Whisk ferns (Psilotopsida)

The extant species of Psilotum show a certain superficial resemblance to extinct psilophytes (dichotomous branching of shoots, absence of roots). However, the lateral sporangia fused into synangia, as well as true leaves (microphylls), indicate that whisk ferns are far removed from Psilophytopsida and are evolutionarily more advanced.

On the other hand, besides the absence of roots, the presence of a gametophyte that still possesses conducting tissues can be regarded as a primitive character. However, the absence of roots is also interpreted as a secondary simplification (cf. the rootless water fern Salvinia, and the loss of roots in epiphytes)¹.

¹ The absence of roots in whisk ferns is associated with their mycotrophy. — Editor's note.

The single order Psilotales includes small, perennial, dichotomously branched herbs (Fig. 11.144, A) with forked leaves (C). Aerial stems have an actinostele or an incipient siphonostele with a lignified pith (B). Leafless, protostelic rhizomes lack roots (even the embryo lacks a root primordium) but bear filamentous rhizoids and contain mycorrhizal fungi. The leaves are scales¹ (microphylls, very small and veinless in Psilotum) arranged in a loose spiral. The sporangia have a multilayered wall, are fused in groups of three² into synangia (C, D), and do not yet possess a true tapetum (homospores receive nourishment from sterile cells of the archesporium, which not only surround the sporocytes but are also interspersed among them). The synangium sits on a very short stalk in the axil³ of a pair of scales with a forked tip.

¹ The entire aerial structure, including the scale-like formations and the axes bearing them, can be compared to a fern frond. — Editor's note.

² In Tmesipteris — two. — Editor's note.

³ The synangium is located on the adaxial lobe of the pinnule. — Editor's note.

Fig. 11.144. Psilotopsida, Psilotaceae: A — E — Psilotum, P. triquetrum: A — habit (12x), B — cross-section of a stem with an actinostele (more precisely, a siphonostele) (40x), C — portion of a shoot with a synangium in the axil of a forked leaf (2.5x), D — cross-section of a synangium (8x), E — spores (250x); F, G — Tmesipteris, T. tannensis: F — habit (12x), G — sporophyll (2.5x); H — K — Psilotum, P. triquetrum: H — prothallus (15x), J — cross-section of a prothallus (40x), K — spermatozoid (990x); an — antheridia; ar — archegonia; c — outer green layer of the cortex; ep — epidermis; m — inner cortex; my — mycorrhizal cells; p — phloem; x — xylem

The gametophytes, or prothalli, reach several centimeters in length, are radially symmetrical, and branched (Fig. 11.144, H). They are colorless and live underground, sustained by mycorrhizal fungi (J: my). On their surface, they bear multicellular antheridia, in which numerous multi-flagellated spermatozoids develop. Small archegonia (with only one, rarely two, neck canal cells) are slightly sunken into the gametophyte tissue. Particularly robust prothalli have a vascular bundle with lignified annular tracheids and an endodermis.

Only two genera are assigned to Psilotopsida: Psilotum and Tmesipteris (each with two species, occurring in the tropics primarily as epiphytes). In Tmesipteris, the forked leaves are somewhat larger. They are decurrent on the stem, and their blades are parallel to the stem. However, they cannot yet be equated without reservation to the leaves of higher plants⁴. Sometimes the forked aerial parts are interpreted as the axes of fronds (leaves) bearing pinnate leaflets, which are more strongly reduced in Psilotum than in Tmesipteris. Forked branchings only rarely correspond to true dichotomy (longitudinal division of a tetrahedral apical cell); most often they occur because a cell adjacent to the old apical cell becomes a second apical cell. The rhizoids of the gametophyte and sporophyte develop gemmae, which serve for Vegetative Reproduction.

⁴ They correspond to pinnules on the rachis of a fern frond. — Editor's note.

Psilotales have not been found as fossils. However, they are still regarded as ancient relicts that nevertheless show clear links to the Pteridopsida discussed below (via Gleicheniaceae with the New Caledonian genus Stromatopteris, and via Schizaeaceae).

5. Class: Ferns (Pteridopsida) (= Filicopsida)

Representatives of this class of pteridophytes, the true ferns, are characterized by large, often complexly dissected megaphylls, which are also called "fronds." The sporangia, originally located terminally, are situated along the leaf margin or on its lower side in more advanced forms. Shoots, roots, and leaves grow — as in the previously discussed class Equisetopsida — mostly through the activity of a single apical cell, rather than a group of apical cells as in representatives of Lycopodiopsida.

The fronds — usually petiolate and multi-veined — are typically coiled at the tip when young (with the exception of Ophioglossales). This coiling arises due to the faster growth of the abaxial (lower) side of the young primordia (of leaves or telomes, respectively), and only later does the organ unroll. The leaves, which develop in this acroplastic manner (see 4 3.1.2), bear numerous sporangia on their lower side, often clustered into groups (sori). The stem is usually unbranched or sparsely branched. The origin of leaf blades from telome systems and the shift of sporangia to the lower side of the leaf due to the stronger growth of its upper side can be envisioned according to the diagram shown in Fig. 11.145, A — H. A fossil transitional form is shown in Fig. 11.145, J. The origin of large pinnate fronds can be conceptualized in a similar way. Although representatives of the extinct Primofilices and the extant fern groups Eusporangiatae, Leptosporangiatae, and Hydropterides show certain phylogenetic relationships with one another, they are not natural taxa but evolutionary grades, i.e., they reflect the evolutionary pathway of various characters.

Fig. 11.145. Pteridopsida: A — D — transition from a fertile telome to a sporophyll; E — H — shift of sporangia to the lower side of the leaf; J — sporophyll of Acrangiophyllum (a fern-like plant of uncertain systematic position) from the Upper Carboniferous (7x); K — M — venation of pinnules of fern-like plants; K — flabellate venation (Archaeopteris, Upper Devonian), L — pinnate venation (Alethopteris, a representative of seed ferns from the Upper Carboniferous), M — reticulate venation (Linopteris, Upper Carboniferous) (12x)

Evolutionary grade: Primofilices¹ (= Protopteridiidae)

¹ A significant portion of this group, as circumscribed here, belongs to the Progymnospermae and is unrelated to ferns. — Editor's note.

The Psilophytopsida are considered to be the ancestors of the Pteridopsida. The Primofilices represent The Link Between them; on the one hand, they still resemble psilophytes, but on the other hand, they exhibit features of higher development. Common to all Primofilices was the presence of terminal sporangia (Fig. 11.146, B), as well as the fact that the elements of the pinnate leaves did not lie in a single plane ('three-dimensional fronds'). The transition from psilophytes to Primofilices was so gradual that for some forms (Protopteridium and Pseudosporochnus, Fig. 11.146, A–E) it is doubtful whether they should still be classified as psilophytes or already as ferns¹. The stratigraphic range of the Primofilices corresponds to their phylogenetic position: they appeared in the Middle Devonian and became extinct in the Lower Permian.

¹ Protopteridium can be classified as Progymnospermae, but by no means as ferns. — Editor's note.

Within the Primofilices, there is a transition from almost tufted, forked branches with terminal sporangia (e.g., Pseudosporochnus; Fig. 11.146, A, B) to wedge-shaped, flattened, irregularly dichotomously branched leaves and similar sporophylls with marginal sporangia (e.g., Cladoxylon; Fig. 11.146, M)². In some species, sporophylls were interspersed with sterile leaves, while in others they occurred in groups, although they were not yet organized into 'flowers'³. The progressive lateral fusion of telomes led to the appearance of broad, flat leaves with dichotomous venation, which were already present in the Upper Devonian (see Fig. 11.145, J; cf. also Adiantum of the modern Leptosporangiatae — see Fig. 4.60, A).

² Describing the sporangia of Cladoxylon as marginal on the sporophylls would be premature. — Editor's note.

³ Flowers are also absent in all other ferns; they are unique to angiosperms. — Editor's note.

Later, the dichotomous venation of ferns and seed ferns gradually evolved into reticulate venation. In the Upper Devonian, flabellate venation with dichotomously branching veins occurred; in the Lower Carboniferous, pinnate venation appeared for the first time, and in the Upper Carboniferous, reticulate venation, which provided the leaf with water and nutrients in the most efficient manner (see Fig. 11.145, K–M).

A prerequisite for The Emergence of such flat leaf-like structures was the arrangement of telomes in a single plane. In primitive forms, however, they were still partly arranged perpendicular to one another (as is still the case today, for example, in Ophioglossales); even flattening could still be absent, so that the 'leaves' were circular in cross-section. Both features occurred in Stauropteris (Fig. 11.146, G) from the Upper Carboniferous; palisade parenchyma could already be present in such cylindrical leaf-telomes.

The Primofilices were predominantly isosporous; a few species were already heterosporous (Archaeopteridales)¹. They had thick-walled sporangia, i.e., they were eusporangiate. In some representatives, specialized mechanisms of sporangial dehiscence were already present. The Diversity of steles also increased, ranging from protostele to eustele.

¹ Archaeopteridales are not ferns, but progymnosperms, Progymnospermae. — Editor's note.

The Origin of the characteristics typical of modern Pteridopsida can thus be traced back to the Primofilices group, so that the megaphyllous Pteridopsida, together with the Equisetopsida, can be derived from the Psilophytopsida as an evolutionary lineage parallel to the microphyllous Lycopodiopsida.

Systematics. The Primofilices as a whole constitute a highly heterogeneous group that can be subdivided into 5 orders.

1. Order: Pseudosporochnales². Representatives of this order occur in the (Lower to) Middle Devonian; for example, Pseudosporochnus (Fig. 11.146) barely reached a height of 1 m, having an unbranched trunk and many equally developed, weakly dichotomously branched lateral axes that bore numerous thin, dichotomizing twigs. In some cases, the tips of the branches were slightly flattened — the beginning of planation and fusion (in terms of the telome theory). The lateral branches with their expanded assimilating laminae can be regarded as precursors to large, pinnately decompound leaves (megaphylls), or fronds.

² Most likely, this order should be merged with Cladoxylales. — Editor's note.

2. Order: Protopteridales¹. In the structure of their 'fronds', representatives of this order also more or less resemble psilophytes. Some genera had a stout trunk with scalariform tracheids² in the secondary wood. The genera Hyenia and Calamophyton, previously regarded as primitive Equisetopsida, also belong to this group according to recent studies, as do Protopteridium (Fig. 11.146, C), Aneurophyton, Tetraxylopteris, Rhacophyton, and Pertica (Fig. 11.146, F)³.

¹ These are not ferns, but progymnosperms. — Editor's note.

² True Protopteridales had pitted tracheids in the secondary xylem. — Editor's note.

³ Protopteridium, Aneurophyton, and Tetraxylopteris are progymnosperms; Hyenia and Calamophyton are representatives of Cladoxylales (= Pseudosporochnales); Rhacophyton belongs to the primitive Zygopteridales; and Pertica belongs to the Trimerophytales. It is incorrect to assign them to a single order. — Editor's note.

3. Order: Cladoxylales. Species of this order (Cladoxylon, Fig. 11.146, J) lived from the Middle Devonian to the Lower Carboniferous. The structure of their stele, which consisted of numerous individual bundles with a V-shaped cross-section (Fig. 11.146, L), is different from that of all other vascular plants¹.

¹ The Cladoxylales include plants with a cladoxylalean-type stele. Plants in which the stele structure cannot be reliably determined are placed in the order Pseudosporochnales. The retention of both orders is justified solely by the paleobotanical practice of establishing separate classifications for different organs and for organs of different states of preservation — in contrast to the neontological practice of classifying whole organisms. — Editor's note.

Fig. 11.146. Primofilices:

A–B — Pseudosporochnales: A — Pseudosporochnus, reconstruction, B — same, tip of branch (natural size); C–F — Protopteridales: C–E — Protopteridium hostimense, Devonian, C — frond (1/4x), D — sterile, E — fertile pinnules (3x), F — Pertica quadrifaria, Devonian, branch with sporangia (s); G, H — Coenopteridales, Stauropteris oldhamia, Carboniferous: G — section of sterile frond, reconstruction (natural size), H — same, sporangium with the wall region along which it dehisced (35x); J–M — Cladoxylales, Cladoxylon scoparium, Middle Devonian: J — section of branch (2/3x), K — leaflets (2x), L — transverse section of plectostele (4x), M — group of sporangia (2x)

4. Order: Coenopteridales. Species of this order (from the Upper Devonian to the Lower Permian, peaking in the Lower Carboniferous) are fully characterized by fronds that are still branched in three-dimensional space (Stauropteris, Fig. 11.146, G; Botryopteris, the climbing fern Ankyropteris, and many others¹). In the sporangial wall of some species, a group of thick-walled cells can already be recognized, which ensured its dehiscence along a predetermined line of rupture, as in the sporangium of Osmunda (see Fig. 11.156, A).

¹ Ankyropteris and Botryopteris had true pinnate fronds, quite comparable to the fronds of most modern ferns. — Editor's note.

5. Order: Archaeopteridales². This (heterosporous) order includes the Upper Devonian cosmopolitan genus Archaeopteris, represented by many species (see Fig. 11.288, C). These were already slender trees with bipinnately compound, three-dimensional fronds. The spatulate pinnules had pinnate venation (see Fig. 11.145, J); two stipules were located on either side of the frond base, as in Marattiales. The lower pinnules of fertile fronds bore sporangia located along the margin facing the apex of the frond. Microsporangia contained numerous microspores 0.03 mm in diameter, while megasporangia contained 8–16 megaspores 0.3 mm in diameter (see Fig. 11.288, C: 8). In trunks reaching heights of up to 9 m and thicknesses of up to 1.5 m, there was a well-developed secondary xylem of tracheids with araucarioid wall pitting (see 11.2, Araucariaceae). Archaeopteris thus combines features of ferns³ and gymnosperms.

² These are progymnosperms, not ferns. — Ed. note.

³ Its resemblance to ferns is convergent. — Ed. note.

Some authors group Protopteridales—an intermediate link between Psilophytopsida and Filicopsida—and Archaeopteridales, which is evolutionarily intermediate between Psilophytopsida and Gymnospermae, into the group "Progymnospermae". In terms of secondary growth in thickness and several other characteristics, they represent a vast transitional group between pteridophytes and gymnosperms¹.

¹ Progymnospermae represents a transitional group between Trimerophytales and gymnosperms. — Ed. note.

Evolutionary stage: eusporangiate ferns (Eusporangiatae) (= Ophioglossidae)

Sporangia, which have a multilayered wall (Fig. 11.147, F), each develop from several cells.

1. Order: Ophioglossales. Members of the order (the sole family Ophioglossaceae, with about 80 homosporous species) have fronds consisting of an assimilating (lacking palisade tissue) green part and a fertile yellowish part arranged perpendicularly to it (Fig. 11.147, A, D). The leaves are three-dimensional fronds and in this respect correspond to the ancestral type of fronds (cf. Primofilices). In the spore-bearing part, growth in width is suppressed.

Longitudinal growth is driven not by a single large apical cell, but by several initial cells forming the shoot apex. On the short underground stem, usually only a single frond develops annually, featuring a long stipe and a small leathery sheath. When young, it is not coiled. In the lower parts of the stem, there is a protostele, which higher up divides into a tube of vascular bundles. The highly reduced, underground prothalli, just a few millimeters long, lack chlorophyll and are multilayered nodules, often perennial, which exist due to Symbiosis with mycorrhizal fungi. Antheridia and archegonia are embedded in the tissue of the prothallus (Fig. 11.147, C). The embryo, arising from the fertilized egg, lives underground for several years in some species.

In Botrychium, both the assimilating part of the frond with dichotomously forked veins and its spore-bearing part are pinnate. Rounded sporangia are located along the mar-

gins of the pinnae and do not fuse with each other (Fig. 11.147, D, E). In the underground shoot, slight secondary thickening occurs (the only such case among all modern ferns).

In Ophioglossum, the green part of the leaf is Tongue-shaped with reticulate venation, while the yellow spore-bearing part is a simple cylinder. The sporangia are embedded in the tissue, arranged in two rows, and laterally fused (Fig. 11.147, A). Plant nutrition is evidently aided by mycorrhizal fungi, which are always present in the roots. In Ophioglossum simplex, the leaf usually lacks assimilating tissue entirely and only bears sporangia. The prothallus is cylindrical (Fig. 11.147, C). The chromosome number is extraordinarily high (in O. vulgatum n = 256, in O. reticulatum n = 630).

Fig. 11.147. Eusporangiatae, Ophioglossales:

A–C—Ophioglossum, O. vulgatum: A—sporophyte (1/2x), B—longitudinal section of the apex of the fertile part of the leaf (2x), C—prothallus with antheridia (a), archegonia (ag), young sporophyte (s) with the first root, fungal hyphae (h) in the prothallus tissue (10x); D–G—Botrychium, B. lunaria: D—sporophyte (1/2x), E—sporangia, bottom view, F—longitudinal section of an immature sporangium with a multilayered wall; inside are spore mother cells surrounded by tapetal cells (10x), G—section of the prothallus (35x); e—embryo

2. Order: Marattiales. This order, restricted to the tropics, also includes primitive and ancient ferns. Their modern species bear a cluster of fronds on a short tuberous stem; these fronds are usually several meters long, multiply pinnate, coiled when young, and equipped with paired stipules at the base. Venation is open (cf. Ophioglossum with reticulate venation). In some genera, the homosporous sporangia fuse laterally into capsule-like, multilocular synangia that later dehisce sharply (Fig. 11.148, B, C); in other genera, they are free and grouped into sori.

Fig. 11.148. Eusporangiatae, Marattiales: A—tree fern Megaphyton (family Marattiaceae), reconstruction (Upper Carboniferous). Scars from fallen fronds are arranged in two rows on the trunk. The base of the trunk is reinforced by a mantle of downward-growing roots; B—Ptychocarpus unitus, Upper Carboniferous, underside of pinnules with synangia (8x); C—the same, transverse section of a synangium (60x)

Long-lived prothalli contain endophytic mycorrhizal fungi, yet they live on the soil surface as autotrophic, multilayered thalli. Externally, they resemble liverworts. Sunken antheridia and archegonia develop on their underside.

Modern Marattiales comprise approximately 200 species from several genera. They inhabit tropical rainforests: for example, Angiopteris is found in Asia (with fronds up to 5 m long!), Danaea in South America, and Marattia throughout the tropical belt.

Evolution. The first Marattiales appeared in the Carboniferous. Apparently, they originated from homosporous Primofilices, almost all of which had multilayered sporangial walls. Until the Rotliegend, their species diversity was greater, and they were more widely distributed than they are today. These were trees with trunks covered by roots, reaching heights of up to 10 m (the most robust and widespread plant was Asterotheca arborescens). At that time, they completely dominated the Leptosporangiatae. Megaphyton was particularly striking, with fronds arranged in two rows rather than spirally (Fig. 11.148, A).

Evolutionary stage: leptosporangiate ferns (Leptosporangiatae) (= Pterididae)

The sporangium (see Fig. 11.128) develops from a single epidermal cell and has a thin wall, which, after early lysis of the tapetum, consists of a single layer of cells. The ferns classified here are predominantly shade-loving plants. Their numerous species (90% of all Pteridopsida, about 9,000 species) are distributed worldwide; they are most widely represented in the tropics, where they are highly diverse: from dwarfs just a few millimeters in size (for example, Didymoglossum species from the family Hymenophyllaceae) to palm-like trees up to 20 m tall (see Fig. 11.150). The trunk of tree ferns, usually as thick as a human arm (family Cyatheaceae, genera Cyathea, Dicksonia, Cibotium), does not branch and bears a rosette of multiply pinnate fronds up to 3 m long at the apex. In Germany, by contrast, ferns occur predominantly as herbaceous plants with an underground perennial, vertical or ascending, sparsely branched rhizome, which in Pteridium can reach up to 40 m in length and live up to 70 years¹.

¹ The rhizomes of several European species, including Pteridium aquilinum, are horizontal and branch intensively. — Ed. note.

The trunks—or rhizomes in herbaceous forms—usually have a central protostele when young, which in older parts transitions into siphonostelic and polystelic structures of various shapes (see Box 4.2, Fig. A), most commonly with central xylem and peripheral phloem (Fig. 11.151, A; 11.149; see also 3.2.4)². Occasionally, vessels even develop (as in Pteridium aquilinum, Fig. 11.149). The vascular bundle is surrounded by an endodermis (Fig. 11.149). There is no secondary thickening, and trunk rigidity is achieved differently than in Lycopodiopsida and Equisetopsida: numerous leaf trace bundles usually run for a long distance through the cortex and, together with sclerenchyma plates (Fig. 11.151, A), reinforce the stem (see also 3.2.3). In some tree ferns, trunk strength is also enhanced by a mantle of rigid adventitious roots. Sometimes this covering can be extremely thick (up to several decimeters!).

² Ferns possess a dictyostele, sometimes polycyclic, rather than a polystele. Siphonosteles and dictyosteles are found in younger, rather than older, parts of the stem (rhizome). — Ed. note.

Fig. 11.149. Leptosporangiatae, Pteridales, Pteridium aquilinum. Vascular bundle, transverse and longitudinal sections (100x); e — endodermis; g — ground parenchyma; s — sieve cells; t — scalariform vessels

Megaphylls, which arose from many telomes, exhibit various venation patterns. Ancient dichotomous leaves (see Fig. 11.157) or dichotomous veins (see Fig. 4.60, A; on leaves with fused pinnules) are rare; as a rule, they occur only in the leaves of seedlings and young plants. Fully developed leaves are highly diverse. Often, the fronds are pinnately compound (for example, bi- to quadripinnate in Pteridium aquilinum, bracken; bipinnate in Dryopteris filix-mas, male fern; once-pinnate in Polypodium vulgare, common polypody). However, simple leaves with a dominant central vein and less prominent lateral veins also occur (Phyllitis scolopendrium, hart's-tongue fern, Fig. 11.151, C). Their very long, sometimes unlimited apical growth, in contrast to the leaf growth of seed plants, is due to the activity of a two-sided apical cell, which, however, is often later replaced by a group of initial cells.

The leaf often develops over several years. In bracken (Pteridium aquilinum), for example, only one leaf is initiated annually on each short branch of the rhizome, requiring 3 years to develop fully. After dying off, the leaves leave behind large, conspicuous scars (see Fig. 11.150) — especially in tree ferns, where they persist for several years after fully expanding. In their histological structure (the presence of palisade and spongy parenchyma), these leaves closely resemble those of higher land plants; however, the epidermal cells in ferns usually contain chloroplasts.

Fig. 11.150. Pteridopsida, Cyatheales, Cyathea crinita. Tree fern from Sri Lanka (1/100x)

Sporangia are formed in large numbers along the margin or, most commonly, on the underside of the leaves (see Box 4.1, Fig. A; Fig. 11.152, B–D). In their external appearance, sporophylls usually differ only slightly from sterile leaves (trophophylls); however, in representatives of some genera, they have a distinctly different appearance — primarily due to the reduction of the pinnule laminae (cf. Matteuccia, Blechnum, Osmunda).

Fig. 11.151. Pteridopsida: A — Pteridium aquilinum, transverse section of the rhizome (7x); B — Asplenium nidus, growth diagram; C — Phyllitis scolopendrium (1/4x); al — outer vascular bundle; e — epidermis; il — inner vascular bundle; p — parenchyma; s — sclerenchyma plates; sr — sclerenchyma ring

Typically (for example, in Aspidiales, to which the vast majority of European ferns belong), sporangia are grouped in large numbers into sori. These sporangia arise on a projection of leaf tissue — the Placenta (Fig. 11.152, B; the placenta is also called the receptacle) and, in many species, are covered and protected until maturity by a membranous outgrowth of the leaf surface — the so-called indusium (B–D). An individual sporangium, when mature, is a small stalked capsule containing a large number of meiospores of almost always equal size (isospory!)¹. A highly characteristic feature is the annulus, which varies in structure; in Polypodiaceae, it runs as a prominent row of cells with heavily thickened radial and inner walls over the dorsal side and apex of the sporangium to the middle of its ventral side (see Fig. 11.156, D). By means of a cohesive mechanism between the annulus cell walls and the water contained within them (with the participation of the stomium cells, see 8.4), the annulus effects the dehiscence of the sporangium and the ejection of the spores (see Fig. 8.37).

¹ In leptosporangiate ferns, most commonly 64 spores develop in a sporangium. — Ed. note.

Fig. 11.152. Pteridopsida, Aspidiales, Dryopteris: A — habit (1/4x); B — section through a sorus: showing the placenta with sporangia and a peltate indusium (30x); C — pinnule with young sori still covered by the indusium; D — the same, at a later stage, with shriveled indusia (3x)

A short-lived haploid prothallus (see Figs. 11.128, A, B; 11.153) develops from the germinating spore, which does not exceed a few centimeters in length and, as a rule, forms both antheridia and archegonia. Sex determination is normally environmental. The prothalli are potentially bisexual. Only in representatives of the Australian genus Platyzoma (family Gleicheniaceae) are Two Types of spores formed, which develop respectively into unisexual, i.e., ♂ and ♀ prothalli (Fig. 11.153, D, E).

Fig. 11.153. Pteridopsida: A, B — Aspidiales, development of the prothallus of Matteuccia struthiopteris from a spore (70x): A — 11-day-old, B — 21-day-old, with the apical cell (s) and the segments (I–X) formed by it; C — Hymenophyllales, Trichomanes rigidum, filamentous prothallus with archegoniophores (a), one of which bears a young sporophyte; D, E — Gleicheniales, Platyzoma microphyllum (20x), ♂ prothallus; E — the same, ♀ prothallus

First, a filamentous, rhizoid-bearing 'protonema' grows, which, however, is strongly developed in only a few forms; in these cases, for example, in Trichomanes (Hymenophyllaceae) and Schizaea (Schizaeaceae), it bears antheridia on its branches and archegonia on special multicellular lateral branches (Fig. 11.153, C). Usually, the filamentous stage is very short-lived and, after forming only a few cells, produces a wedge-shaped, two-sided apical cell at its tip; the segments cut off by this cell divide further (A, B). In this way, a usually Heart-shaped, prostrate, thin, thalloid prothallus is formed (see Fig. 11.128, A).

Finally, the apical cell is replaced by several initials. Antheridia and archegonia arise on the side protected from direct light, i.e., usually on the side facing the soil and moisture; by the end of their development, they are not embedded or only slightly embedded in the tissue of the prothallus¹. Archegonia are usually formed later than antheridia. Under very poor nutritional conditions, the formation of archegonia is completely suppressed.

¹ The archegonia have their venter embedded in the tissue of the prothallus, as in other Pteridophyta (with the exception of Hymenophyllaceae and Schizaeaceae, which have filamentous prothalli). — Ed. note.

An antheridium is a rounded, protruding structure that sits sessilely directly on a single epidermal cell; it originated from it as a papilla-like outgrowth separated by a transverse wall (Fig. 11.154). The spermatozoids formed within it are coiled like a corkscrew and have numerous flagella (F). As in all archegoniates, the main volume of the spermatozoid is occupied by The Nucleus. Initially, it bears a vesicle-like cytoplasmic remnant at its posterior end containing small Plastids and starch grains as reserve material, which, however, is discarded upon entering the archegonium.

Fig. 11.154. Pteridopsida, Aspidiales: A — E — development of the antheridium in Dryopteris filix-mas (250x), explanation in the text; F — spermatozoid of Thelypteris palustris (3000x); G — N — development of the archegonium in Dryopteris filix-mas (20x), explanation in the text

Archegonia arise in the multilayered central portion of older prothalli, each by the division of a single superficial cell. Within the archegonial primordium, a neck canal cell and a central cell are formed; the latter divides again into the egg cell and the ventral canal cell (Fig. 11.154, M). In some species, there may be several neck canal cells (for comparative evolutionary aspects, see Overview). The archegonium becomes ready for fertilization after The breakdown of the ventral and neck canal cells, the dissolution of the mucilage contained within them, and the opening of the neck at the apex. Spermatozoids are attracted to the neck of the archegonium and the egg cell (see 8.2.1.1) by chemotaxis.

After the first divisions of the zygote (Fig. 11.155, A), the shoot apex (s) of the embryo differentiates endoscopically next to the future foot (f), while the primordia of the first leaf (b) and root (w) face the neck of the archegonium. The root arises on the suspensorless embryo not opposite the shoot, but laterally to the longitudinal axis of the embryo, as in all pteridophytes. Since the archegonium is located on the underside of the prothallus, the shoot and the first leaf of the embryo must bend upward (B) upon emerging from the archegonium, exhibiting negative geotropism. The sporophyte remains connected to the prothallus via the foot (f) for some time until the latter dies off. Numerous adventitious roots are later added to the primary root. THE POSITION OF the embryo axis cannot be altered by gravity or light; consequ

ently, even the prothallus of Leptosporangiatae must exhibit polarity, which is then transmitted to the cytoplasm of the egg cell.

Fig. 11.155. Pteridopsida, Pteridales Pteridium aquilinum. Embryo development: A — after the first divisions of the zygote in the archegonium; B — at a later stage, the haustorium is embedded in the enlarged venter of the archegonium (a); b — first leaf; f — haustorium; p — prothallus; s — shoot apex; w — root

Adventitious (brood) buds are often found on the leaves; they shed and participate in vegetative reproduction. The transformation of stems and even leaves into creeping stolons also serves vegetative reproduction. In some species, deviations from the normal alternation of generations occur due to apogamy and apospory (see 10.1.3.3); this is most common in polyploid forms, many of which in Pteridopsida have high chromosome numbers.

Systematics. The Classification of the numerous species of leptosporangiate ferns is based on differences in sporangium structure and their position on the sporophyll. Several orders are distinguished based on these characters.

In representatives of the first three orders (1–3), the annulus on the sporangia is either absent or transverse.

Fig. 11.156. Leptosporangiatae. Sporangia: A — Osmunda regalis (Osmundales, stomium open, 40x); B — Anemia caudata (Schizaeales); C — Hymenophyllum dilatatum (Hymenophyllales); D — Dryopteris filix-mas (Aspidiales, stomium in surface and lateral views) (B — D — 70x)

1. Order: Osmundales. Sori are absent, and sporangia lack an annulus; the sporangial wall ruptures at its apex due to a group of thick-walled cells (Fig. 11.156, A). Indusia and scales are absent. Prothalli are long-lived, often even perennial. Species of this order, grouped into a single family (Osmundaceae), have sporangia either on specialized sporophylls (Osmunda cinnamomea) or on specific parts of trophophylls: in the royal fern (O. regalis), the upper pinnae of the otherwise normal-looking fronds are modified, while in O. claytoniana, the middle ones are. The family is known since the Late Carboniferous and is currently represented by only a few genera.

2. Order: Gleicheniales. Sessile sporangia have an annulus running transversely above the middle. They are clustered in small groups into sori, which, however, are not protected by an indusium. In the fossil record, representatives of this order are known from the Late Carboniferous; today, they are widely distributed in the tropics. Fronds are (pseudo-)dichotomously branched, with "dormant" buds in the ramifications (Fig. 11.157). The placement of the genus Platyzoma (with pronounced heterospory, see Fig. 11.153, D, E) within this order is controversial.

Fig. 11.157 Pteridopsida, Gleicheniales, Gleichenia circinata, Australia (1/5x)

3. Order: Schizaeales. Sessile sporangia located along the leaf margin dehisce by a longitudinal slit via a transverse annulus located at the very apex (see Fig. 11.156, B). This order is also known in the fossil record from the Late Carboniferous; currently, its distribution is restricted mainly to the tropics. Leaves in Schizaea (-ceae) resemble grass leaves but are dichotomous; in Anemia (-ceae) they are pinnate with a fertile lower pair of pinnae; in Lygodium (Lygodiaceae) they are climbing.

The next three orders (4–6) have an obliquely running annulus, as, for example, in Hymenophyllum (see Fig. 11.156, C) of the 4th order.

4. Order: Hymenophyllales. Sporangia are almost sessile. Sori are located along the leaf margin, each on a receptacle (an extension of the leaf vein), which is sometimes greatly elongated, and are protected by a cup-shaped or bivalve indusium. The leaves are usually thin, with a single-layered blade lacking stomata. Reliable fossil remains are known only from the Tertiary period. About 650 extant species are known. They inhabit mainly the humid forests of the tropics and subtropics, such as species of the genera Hymenophyllum (although H. tunbrigense is very rarely found in Europe) and Didymoglossum.

5. Order: Matoniales. Sessile sporangia are clustered in small groups into sori, which are covered by a peltate indusium. This order, widely distributed in the Mesozoic, is currently represented by only three species inhabiting the Malay Archipelago.

6. Order: Cyatheales (including Dicksoniales). Stalked sporangia are clustered in sori on the surface or along the margins of the leaves. These plants appeared in the Jurassic. Currently, these are predominantly tree ferns (palm-like trees up to 20 m high) in the montane forests of the tropics and subtropics. Genera represented by many species include Cyathea (including Alsophila, see Fig. 11.150), Dicksonia, and Cibotium.

In the following orders, the sporangia, which usually have a well-developed stalk, dehisce by means of a longitudinal annulus (Fig. 11.156, D), or in rare cases, a vertical annulus.

7. Order: Polypodiales. Sori are formed on the underside of the leaf; the indusium is absent. Fronds of perennial plants are pinnately lobed or once-pinnate. Along with the genus Polypodium, this order also includes the genera Drynaria (see Fig. 11.162, B), Platycerium (see Fig. 11.162, A), Microsorum, and Pyrrosia.

8. Order: Pteridales. Sori are located along the margins of the pinnae. A common representative of this order is the bracken fern, Pteridium aquilinum (Dennstaedtiaceae), with twice- to four-times pinnately compound fronds up to 2 m long and long creeping rhizomes. The sori are covered on one side by the reflexed margin of the leaf, and on the other by an indusium. The genus Acrostichum belongs to the same family. In Adiantum (Adiantaceae), primitive fan-like leaf venation is observed (see Fig. 4.60, A), with the reflexed margin protecting the sori; they lack an indusium. Ceratopteris (Parkeriaceae) is an aquatic plant with sterile floating leaves and fertile aquatic leaves that bear globose, almost sessile sporangia with a vertical annulus, not grouped into sori. This order also includes the families Gymnogrammaceae with the genus Anogramma, Sinopteridaceae with the genera Notholaena and Cheilanthes, and Davalliaceae with the genus Davallia.

9. Order: Aspidiales. Sori, protected by an indusium, are located on the underside of the leaf. The leaf blade is occasionally simple or lobed, but most often once- to four-times pinnate. This order, which includes several families, comprises most of the fern species known in Germany. In species of the family Thelypteridaceae (Thelypteris), the sori are situated along the midrib of the pinnule; in Aspleniaceae (Asplenium, Ceterach), they are on the sides of the terminal VEINS OF THE pinnules. The latter family also includes the hart's-tongue fern, Phyllitis, with entire, tongue-shaped fronds (see Fig. 11.151, C). Representatives of Aspidiaceae often form peltate or reniform indusia (for example, Dryopteris, see Fig. 11.12, C; Bolbitis). Very closely related to them are the Athyriaceae, with often elongated sori (as in the lady fern, Athyrium filix-femina). In the ostrich fern, Matteuccia struthiopteris, which also belongs to the latter family, the sporophylls differ from the green trophophylls, which overlap in a funnel-like manner, just as in representatives of the next order.

10. Order: Blechnales. In the only representative of this order found in Germany, the hard-fern (Blechnum spicant), several dark brown sporophylls are arranged within a rosette of green trophophylls, with sori elongated along the entire length of the narrow fertile pinnae. In addition to the genus Blechnum, the genus Salpichlaena is also included here.

Evolutionary grade: water ferns (Hydropterides) (= Salviniidae)

The water ferns include only a few genera of herbaceous plants living in water or marshes. They are all heterosporous. Their mega- and microsporangia are thin-walled, lack an annulus, and are contained within special receptacles located at the base of the leaf (e.g., Fig. 11.158, C). The meiospores are covered by a unique perispore derived from the plasmodial tapetum.

Fig. 11.158. Hydropterides, Salviniales. Salvinia, S. natans:

A — lateral view of a shoot portion showing rounded sporangium receptacles (3/4x); B — the same, top view (3/4x); C — receptacles of megasporangia (ma) and microsporangia (mi) in longitudinal section (8x); D — microsporangium (55x); E — microspores embedded in a frothy mass (250x); F — megasporangium with a megaspore surrounded by a perispore (p) in longitudinal section (55x); G — J — ♂ prothallus: G — division of a microspore into three cells I — III (860x), H — mature prothallus in lateral view, J — ventral view. Cell I has divided into prothallial cells pz₁ and pz₂ (pz₁ — non-functional rhizoidal cell); cell II — into sterile cells sz₁, sz₂ and two spermatogenous cells sp1, each of which forms 2 spermatozoids; cell III — into sterile cells sz₃, sz4 and two spermatogenous cells sp2. Cells sp1 sp1 and sp₂ sp₂ represent two antheridia (an antheridium includes not only spermatogenous cells but also wall cells. — Ed. note); cells sz₁—sz₄ — their walls; numbers 1 — 1 and 2 — 2 mark the position of the first cell walls (640x); K — embryo (e) in longitudinal section, prothallus (pr) with chloroplasts, b₁ — b₃ — first leaves (100x); ar — remnant of the archegonium; es — exospore; f — haustorium; s — spore cell; ss — shoot apex; sw — sporangium wall; wb — water leaf

Water ferns include two orders with about 100 species.

1. Order: Salviniales (the sole family Salviniaceae). The species belonging to this order are free-floating aquatic plants. In the German flora, the genus Salvinia is represented by the now rare floating fern S. natans, which bears three leaves at each node of its sparsely branched shoot.

The two upper green leaves floating on the water surface are oval (Fig. 11.158, A), with numerous large intercellular spaces. In contrast, the submerged leaves (wb) are divided into numerous colorless, Hair-covered, filiform lobes hanging down in the water. They function as the absent roots (heterophylly¹, see 4.3.2). At the bases of the submerged leaves sit several spherical sporangial receptacles (A); they enclose the sporangia arising on a Column-like placenta (C).

¹ Correctly: anisophylly. — Ed. note.

In its development, the placenta corresponds to a modified lobe of a submerged leaf, whereas the wall of the receptacle can be regarded as a double-layered indusium. It arises as a ring-like ridge which, in the form of a cup and finally a sphere, grows over the placenta with its sorus of sporangia, and then closes tightly at the apex.

Each such receptacle contains a single sorus—consisting either of more numerous microsporangia or of a smaller number of megasporangia (Fig. 11.158, C:mi, ma); sex Determination of the gametophyte is thus diplomodificatory. Both types of sporangia are stalked and, when mature, have a single-layered wall (D, F); within them, meiospores arise after reduction division.

The microsporangia contain 64 microspores formed in tetrads. They are embedded in a foamy, hardening substance (perispore, Fig. 11.158, E).

Each microspore develops into a short, tubular ♂ gametophyte consisting of only a few cells. It forms only 2 antheridia (Fig. 11.158, H). Each antheridium produces (from 2 spermatogenous cells giving rise to 4 spermatids) 4 spermatozoids; they escape through a rupture in the cell walls. Thus, the gametophyte is highly reduced. Its development occurs inside the sporangium, which does not dehisce. Instead, the gametophytes elongate like a pollen tube, locally piercing its wall to release the spermatozoids.

Megasporangia are larger than microsporangia and also have a single-layered wall (Fig. 11.158, F), but contain only one megaspore, as only one of the 32 arising spores (from 8 spore mother cells) completes its development at the expense of the others. The megaspore is densely packed with protein bodies, oil droplets, and starch grains; at its apex is a denser cytoplasm containing the nucleus. The brown wall (exospore) is covered by a thick, cellular coat—the perispore. The megaspore remains inside the sporangium, detaches with it from the mother plant, and floats on the water surface. Upon germination, a small-celled ♀ gametophyte (Fig. 11.158, K) and a large, nutrient-rich cell (s) lying behind it arise at the apex of the megaspore: this cell serves to nourish the gametophyte and does not divide further, although its nucleus undergoes mitoses to produce numerous parietally arranged daughter nuclei¹.

¹ Both the small-celled tissue and the large multinucleate cell are parts of the same gametophyte. — Ed. note.

The spore wall opens by three valves, the sporangium wall also ruptures, and the gametophyte is exposed as a small, dorsiventral structure. Although it contains chloroplasts, it utilizes the reserve substances in the large cell (s). It develops several archegonia, but only one egg cell, after fertilization, yields an embryo, which embeds itself via its haustorium into the expanding and finally rupturing venter of the archegonium (Fig. 11.158, K). If none of the archegonia are fertilized, new archegonia are formed.

The second genus, Azolla, is predominantly tropical; these delicate, profusely branched floating plants bear closely overlapping leaves arranged in two rows, and long roots on the underside of the shoot (Fig. 11.159, A).

Fig. 11.159. Hydropterides, Salviniales. Azolla:

A—plant, top view (4x); B—shoot apex, from above (12x); C—the same, in cross-section (12x); D—longitudinal section of the upper leaf lobe; Anabaena azollae in the cavity (70x); E—♂ (above) and ♀ (below) sori (20x); F—microsporangium (65x); G—megasporangium surrounded by an indusium, containing a megaspore with floats (65x); H—part of a massula with glochidia (160x); J—megaspore with the upper half of the indusium removed to show the floats, with three massulae attached to the epispore by means of glochidia (65x)

Each leaf is divided into two lobes, of which the upper floats and assimilates, while the lower is submerged in water and participates in water absorption (A, B); furthermore, on certain lateral branches, the lower leaf lobes are modified into sporangial receptacles and covered by an outgrowth of the lobe. The cyanobacterium Anabaena azollae, which fixes atmospheric nitrogen, lives as a symbiont in the cavities of the upper lobe; therefore, Azolla is used in rice paddies as a green manure (cf. Cyanobacteriota). Azolla is of interest for its adaptations to ensure reliable fertilization. After release from the microsporangium, the 64 microspores are found grouped in several numbers within 5 to 8 rounded, floatable clumps formed by the periplasmodium—the so-called massulae. Each massula bears on its surface stalked, anchor-like appendages—glochidia (H, J)—which also arise from the substance of the periplasmodial tapetum. These hooks serve for attachment to the megaspore, which moves in water with the aid of a special air-containing float (0,1). It is formed from the highly vacuolated periplasmodium at the apex of the megasporangium. The gametophyte develops as in Salvinia.

2. Order: Marsileales. This order includes genera whose species inhabit waterlogged soil. The genus Marsilea (Marsileaceae), which until recently was represented in Germany by the now extinct species M. quadrifolia (Fig. 11.160, A), is characterized by a creeping, branched rhizome with solitary leaves on long petioles, bearing closely set pairs of leaflets. The nyctinastic movements characteristic of these leaves are not found in other ferns. Above the base of the petiole arise stalked, oval sporangial receptacles, in pairs or, in other species, in larger numbers. In contrast to the Salviniales, the wall of each receptacle in the Marsileales corresponds in origin to an assimilating leaflet, in which the sorus primordia are shifted inward due to the enhanced growth of the lower side (B). Therefore, these receptacles are called sporocarps.

Fig. 11.160. Hydropterides, Marsileales:

A—Marsilea quadrifolia, habit (2/3x); B—section of a young sporocarp; dots indicate the sorus primordium (200x); C—mature sporocarp (8x); D—dehisced sporocarp of M. salvatrix (natural size); E—germinated microspore with two antheridia (150x); F—spermatozoid (700x); G—archegonium (150x); H—Pilularia globulifera, habit (2/3x); g—ring of cartilaginous tissue; I—vascular bundle; me—megasporangium; mi—microsporangium; s—sporocarp; ss—chambers with sori

The sori, consisting of a megasporangium and numerous microsporangia, are arranged in rows enclosed in chambers (C). Upon spore maturation, a ring of cartilaginous tissue (g) girdling the sporocarp swells and pulls the chambers with sori outward (D). The male gametophyte, which remains enclosed within the microspore wall (E), forms two antheridia in which corkscrew-like, spirally coiled spermatozoids (F) arise. The female gametophyte develops a single archegonium (G). The genus Pilularia, with the species P. globulifera occurring in Germany, differs from Marsilea in having simple, linear leaves, at the bases of which spherical sporocarps arise singly, also corresponding in origin to an assimilating leaflet (Fig. 11.160, H). There are four chambers with sori in the sporocarps of Pilularia. The leaves of species of both genera, as in most other ferns, are circinate when young (A, H; acroplastic growth, see 4.3.1.2).

As fossils, Azolla is known from the Lower Cretaceous, Salvinia from the Upper Cretaceous, and Pilularia from the Miocene. In North America, Salvinia became extinct during the Miocene.

Distribution and Ecology of Pteridophytes

Pteridophytes are distributed across all climatic zones, but they reach their greatest size (tree ferns!) and highest species diversity—primarily Pteridopsida and Lycopodiopsida—in the tropics. Like bryophytes, pteridophytes prefer damp habitats, although individual species also extend into arid regions. They avoid saline habitats; only the fern Acrostichum aureum inhabits mangrove swamps throughout all tropical regions.

In adapting to various terrestrial environments, pteridophytes acquired highly adaptive morphological and physiological features and are represented by the same life forms (see Fig. 4.19) as seed plants. In terms of their water relations, they occupy an intermediate position between bryophytes and seed plants. The prothallia of most pteridophytes are just as sensitive to desiccation as the protonemata of mosses. However, the mature green plants of most fern species regulate their internal water content themselves (homoiohydric pteridophytes)—in contrast to bryophytes, which are completely dependent on external moisture and are highly desiccation-tolerant. Admittedly, in some poikilohydric species of Selaginella (Lycopodiopsida) and ferns (Pteridopsida), such as Ceterach, Notholaena, and Cheilanthes, the evergreen leaves can dry out to an air-dry state and resume vital activity upon rehydration even after many months.

The internal water conduction system plays a crucial role in maintaining water relations; some pteridophytes additionally utilize adaptations for capillary absorption and water storage (ligules in Selaginellaceae and Lepidodendrales).

Very few xerophytes, for example among Pteridopsida, are protected from desiccation by a waxy bloom, a covering of dry scales and hairs, or even possess succulent shoots (Davallia) or leaves (for example, some species of Polypodium). In inhabitants of wet habitats (hygrophytes), we observe guttation, either through hydathodes on the teeth of Equisetum leaf sheaths or through peculiar hydropotes in some ferns (Fig. 11.161).

Fig. 11.161. Leptosporangiatae, Polypodiaceae. Polypodium vulgare, hydropote (80x)

Along with evergreen species of the genera Lycopodium, Selaginella (Lycopodiopsida), Equisetum (E. hyemale, Equisetopsida), and Polypodium (Pteridopsida), a significant portion of pteridophytes in temperate and cold zones are summer-green (cf. bryophytes in this respect). Pteridophytes in various evolutionary lineages, some as early as the Devonian and Carboniferous, developed arborescent forms (phanerophytes; life forms, see Fig. 4.19); for example, among Lycopodiopsida—Sigillaria, Lepidodendron, Pleuromeia; among Equisetopsida—Calamites; among Pteridopsida—various Primofilices, eusporangiate (Megaphyton, Fig. 11.148, A) and leptosporangiate ferns (Cyathea, see Fig. 11.150). The prerequisites for this were the strengthening of the axes (partly due to secondary thickening, but more often through other adaptations and an efficient design of conducting tissues). Phanerophytes also include climbing (lianas) and epiphytic pteridophytes, which are common especially in the tropics; for example, the tropical Gleicheniaceae (climbing plants); Lygodium and Salpichlaena with twining rachises up to 15 m long; certain species of the genus Polypodium that climb tree trunks in the tropics using roots; Platycerium and Drynaria as epiphytes with sterile leaves forming a nest for humus accumulation (Fig. 11.162). There is reason to assume that all extant pteridophytes descended from evergreen arborescent ancestors with originally autotrophic prothallia. Subsequently, other life forms arose: chamaephytes, some of which have mycotrophic prothallia (Lycopodium), while others have prothallia that exist at the expense of spore reserves (Selaginella); hemicryptophytes with autotrophic, short-lived prothallia (e.g., Dryopteris); geophytes with mycotrophic (e.g., Ophioglossum) or autotrophic prothallia (e.g., Pteridium, Equisetum); and therophytes (e.g., Anogramma). Except for the prothallia of some species and the largely mycotrophic Ophioglossum simplex, pteridophytes, like mosses (but cf. Cryptothallus, Bryophytina), did not transition to a heterotrophic lifestyle (parasitism). A few representatives have secondarily adapted to aquatic life (as hydrophytes): for example, among Pteridopsida—Salvinia and Azolla (Salvi-

niales) as floating plants, as well as Ceratopteris (Pteridales). Some Ceratopteris species live as floating or more or less submerged plants, while others inhabit wet soil. Bolbitis heudelotii and Microsorum pteropus cultivated in aquariums produce only sterile fronds underwater, whereas sori develop exclusively on leaves emerging from the water. Species of the genus Isoëtes (Lycopodiopsida) inhabit partly periodically flooded soil, partly the bottom of lakes, often at a depth of 1–3 m.

Fig. 11.162. Epiphytic ferns with dimorphic leaves (heterophylly: sterile leaves forming a nest for humus accumulation, and sporotrophophylls):

A—Platycerium alcicorne; B—Drynaria quercifolia (1/6x)

Pteridophytes compete particularly with individuals of similar life forms: tree ferns, for example, compete with gymnosperms, palms, and woody dicotyledons; fern prothallia, as well as the sporophytes of Hymenophyllaceae, compete with mosses and Lichens; Equisetum with Juncaceae and Cyperaceae; Salvinia with Lemnaceae, etc. At the same time, some pteridophytes prove to be highly competitive and, under favorable conditions, occur in such Abundance that they form pure stands, like bracken (Pteridium aquilinum) along forest edges or water horsetail (Equisetum fluviatile) along muddy lake shores. Some species are globally distributed, such as bracken (sometimes up to 4 m high as a climbing plant) or stag's-horn clubmoss (Lycopodium clavatum); Examples of restricted, disjunct, and endemic ranges can also be given.

Overview of Pteridophytes. Pteridophytes can be viewed as an evolutionary branch that arose in parallel with bryophytes from a common ancestral group, possibly already terrestrial ("Propsilophytopsida"), which in turn descended from algal ancestors. Among algae, only streptophytes are suitable candidates as ancestors of these early land dwellers. Conversely, the previously discussed evolution from bryophytes to pteridophytes—for example, from forms like Anthoceros, through an increase in size, differentiation, and increasing independence of the sporophyte (Sporogonites)—seems unlikely. While mosses have not undergone significant progressive development since the Carboniferous, i.e., they were already "fully developed" about 250 million years ago, pteridophytes reached their peak only after that time.

While mosses conquered the land by means of their gametophytes and thus remained restricted to specific adaptive zones, pteridophytes and (above all) seed plants achieved a dominant role in shaping terrestrial vegetation through the Progressive development of their sporophytes. The evolutionary advantage of sporophytic plants is probably related to their genetic stability (buffering effect) and an increased frequency of recombination (after each syngamy event, the resulting diploid nuclei reproduce mitotically; consequently, numerous diploid nuclei enter meiosis), as well as the development of protective adaptations for the gametophyte, which is sensitive to terrestrial conditions. The diploid sporophyte of pteridophytes is highly developed and diverse, which, in contrast to mosses, became possible due to the progressive evolutionary development of lignified (and thus highly efficient) vascular bundles (conducting both water and organic substances¹). The development of true roots has a similar effect. Since, in addition, the epidermis is cutinized, the shoot can grow upward into the illuminated airspace, develop leaves, and assimilate carbon dioxide. Its supply of organic substances is thus independent of the gametophyte, which eliminates The Need for further growth of the latter.

¹ The author of this section apparently has a peculiar view of The Role of different tissues in conducting various substances, as well as The Significance of lignification. The idea expressed by the author should probably be treated with great caution. — Editor's note.

Apical growth is most often carried out by means of an apical cell (Fig. 3.2, A). In contrast, eusporangiate ferns and some representatives of Lycopodiopsida (Lycopodium, Selaginella—in the latter genus, still partly by means of an apical cell) grow due to multiple initials. Among Psilophytopsida, the genus Rhynia² already exhibited this progressive feature.

² The structure of the telome apex in Rhynia is not precisely known. — Editor's note.

The position of the embryo (see Fig. 11.155) is most often endoscopic (the shoot apex points away from the neck of the archegonium—Leptosporangiatae, as well as Lycopodium, Selaginella), rarely exoscopic (Eusporangiatae, with the exception of Marattiales, Psilotum, Equisetum, Isoëtes).

In ancestral forms, all spores are identical (isospory); in more highly organized forms, differentiation into micro- and megaspores already appears. Heterospory arose repeatedly and independently in different classes of pteridophytes (Lycopodiopsida; Equisetopsida—both in calamites and sphenophylls—and Pteridopsida); this is associated with the division of labor between small ♂ prothallia and larger ♀ prothallia. In the form of "seed clubmosses," pteridophytes, independently of seed plants, reached a level of development equivalent to seed formation with an extreme degree of heterospory.

The haploid gametophyte of pteridophytes remains (unless too strongly reduced) at the level of thallus organization (prothallium) and occasionally develops tracheids (Psilotum). It completes its development early, forming antheridia and archegonia, which are often simpler in structure than those of mosses; large multicellular gametangia are considered primitive compared to small ones consisting of few cells. While mosses (Bryopsida) have numerous neck canal cells in the archegonium (10–30 or more), liverworts (Marchantiopsida) have 4–8, hornworts (Anthocerotopsida) have 6, and pteridophytes often have only one or a few. In Anthocerotopsida, the epidermal cell that becomes the archegonial initial no longer forms stalk cells, i.e., unlike Marchantiopsida and Bryopsida, it immediately divides into an axial cell and three archegonial wall cells. In pteridophytes, There are also no divisions yielding archegonial wall cells¹. The antheridia and archegonia of Bryopsida and Marchantiopsida are initiated exogenously and are free, i.e., only later become surrounded by gametophyte tissue. In Anthocerotopsida and pteridophytes, they are already enclosed in gametophyte tissue at early Selection/3.html">Stages of development (endogenous formation)².

¹ This statement is highly controversial in reality. — Editor's note.

² In all embryophytes (except for the antheridia of Anthocerotophyta), gametangia develop from a surface cell, i.e., they are always exogenous. — Editor's note.

Fig. 11.163. Development of the major plant groups in Earth's history

¹ Obsolete names.

Evolutionary History. Psilophytopsida, Lycopodiopsida, and Equisetopsida reached their greatest peak in both diversity of forms and abundance of individuals during the Paleozoic. The representation of Pteridopsida increased significantly as early as the Mesozoic, and they are better preserved to this day than the other two surviving classes (Fig. 11.163). Their forms, which dominated from the Carboniferous to the Triassic, survive today as only a few species, whereas the families that dominate at present did not appear until the Mesozoic.