BOTANY VOLUME 3 - EVOLUTION AND SYSTEMATICS - 2007

11. SYSTEMATICS AND PHYLOGENY

11.2. Bacteria, Fungi, Plants

First Phylum: Green Algae I (Chlorophyta)

Almost all Levels of Organization are found in green Algae. With the exception of amoeboid forms (which, however, are sometimes represented by reproductive Cells), all types have evolved here, even parenchymatous and pseudoparenchymatous (represented, for example, by Ulva and Codium, respectively). These can be microscopic unicellular organisms, filamentous algae—either unbranched or branched, often forming dense tufts (see Fig. 11.90)—as well as complex foliose thalli that partially resemble higher plants.

The formation of transverse Cell walls occurs primarily in phycoplasts: here, during telophase, microtubules assemble in the equatorial plane between the dividing daughter nuclei. This gives rise to a cell plate with pores for plasmodesmata (see 2.2.1).

Flagellated cells are typically pear-shaped and radially symmetrical. Two or four (rarely many) flagella of equal length are located at the anterior end of The Cell (terminally); these motile cells are isokont and possess "smooth" flagella. They often contain contractile vacuoles (usually two) and, in the posterior part (relative to the direction of movement), a curved or cup-shaped parietal chloroplast¹ with or without an eyespot (stigma; see Fig. 11.94, A). The red eyespot consists of globules containing carotenes; unlike in Euglenophyta, Eustigmatophyta, and Heterokontophyta, it is not associated with a flagellar Swelling.

¹ In a few representatives, the chloroplast may also occupy a central position. — Translator's Note.

The term "smooth flagella" means they do not bear tubular hairs, although they may sometimes have very fine hairs or scales of various types.

The ULTRASTRUCTURE OF THE flagellar apparatus in the region of flagellar insertion is of great importance for chlorophyte systematics (Fig. 11.88). It consists of basal bodies (i.e., the intracellular bases of the flagella), microtubular roots along with their associated structures, and a rhizoplast (or rhizoplasts). The latter represent connections between the basal bodies of the flagella and the Cell Nucleus. The flagella are most commonly inserted in a cruciate pattern: four microtubular roots arranged in a cross anchor the basal bodies of the flagella within the cell. In the "1 o'clock–7 o'clock" arrangement (abbreviated as the 1–7 arrangement), the two basal bodies are positioned, when viewing the cell from above, like the numbers 1 and 7 on a clock face. The "12 o'clock–6 o'clock" arrangement and the common "11 o'clock–5 o'clock" variant are to be understood in the same way.

Class="center">Fig. 11.88. Chlorobionta, Chlorophyta. Microtubular ROOT System of the flagellar apparatus. View of the basal bodies with roots of two and four microtubules in a cruciate arrangement. Upper row (A, C, E)—biflagellate; lower row (B, D, F)—quadriflagellate cells. A, B—"12 o'clock–6 o'clock" type: hypothetical ancestral flagellar apparatus in which the basal bodies (each pair of opposing flagella) are aligned; C, D—"11 o'clock–5 o'clock" type: basal bodies (compared to A, B) are slightly offset counterclockwise relative to each other; E, F—"1 o'clock–7 o'clock" type: basal bodies (compared to A, B) are slightly offset clockwise relative to each other

Box 11.8. Uses of Algae

Iodine can be extracted from the ash of various brown algae (Phaeophyceae: Laminariales); this was the primary method of its production until the 1930s. Suitable brown algae can accumulate iodine in their cells (up to 0.3% of dry weight) from seawater, where its concentration is only 0.000005%. In addition, brown algae yield alginates, whose colloidal properties find diverse Applications in the textile, food, photographic, and cosmetic industries. Global production is around 14,000 tons per year. They are used, for example, in the preparation of ice cream, puddings, ointments, toothpaste, dietary weight-loss products, drug capsules, adhesives, paints, etc. Soda and mannitol are also obtained from brown algae. In China and Japan, brown algae are used as food (kombu).

Polysaccharides for medicinal and industrial purposes are obtained from the cell walls of several species of red algae (Rhodophyta). These include carrageenan from Chondrus crispus and Gigartina mamillosa from the North Sea coast (known in dried form as "Irish moss") and Agar from various Pacific florideophytes (such as Gelidium and Gracilaria species), and more recently also from European species. Japan, producing 2,000 tons of agar annually, is its most important producer (used for microorganism cultures, and in the food and pharmaceutical industries). Porphyra (cultivated along the marine coasts of East Asia on plantations consisting of nets suspended near the Water surface) is widely used as food (nori), especially in East Asian countries.

Green algae (Chlorophyta) are of less economic importance than red and brown algae. In Western Siberia, filamentous green algae are harvested on a massive scale (about 1,000,000 tons annually from several thousand square kilometers) to be used for paper manufacturing, as well as insulation and construction Materials (algilite). Coccoid green algae (Chlorella, Scenedesmus) are suitable for biotechnology due to their high photosynthetic activity and potential for mass cultivation. Research in this area is aimed at producing Proteins and Vitamins for Human and Animal Nutrition (maximum yields in outdoor cultures in the tropics reach 5 tons per hectare monthly). Using "algal bioreactors", biological gas exchange (CO₂ for O₂ during Photosynthesis) can be achieved. Corresponding devices have been tested to use these algae as sources of oxygen and nutrients (for example, for spacecraft).

Other algae. Rocks formed from diatoms (Bacillariophyceae) (diatomaceous earth, polishing slates, kieselguhr) were formerly used as materials for construction, cleaning, and polishing. Even today, they are still used as a filtering medium (for example, for water purification) or as an absorbent and filler. Rock composed of coccoliths (Haptophyta) was formerly used as writing chalk.

During sexual reproduction, the vast majority produce flagellated Gametes. In this process, two gametes copulate (cf. Fig. 11.89, F); these are often very similar to zoospores (asexual spores) and arise in unicellular gametangia. Male gametes are typically flagellated, while female gametes (egg cells) can also be non-motile (see, for example, Fig. 11.100, E). The simple haplontic life cycle is often complicated by the presence of a diplophase (i.e., becoming haplodiplontic). Some representatives have become diplontic due to the reduction of the gametophyte (such as Caulerpaceae). Thus, the originally zygotic Morphology/12.html">ALTERNATION OF GENERATIONS becomes intermediate or gametic. The product of copulation—the zygote—in freshwater forms is most commonly a thick-walled, rounded resting cell (cystozygote), often colored red by carotenoids.

Fig. 11.89. Ulvophyceae: A–K—Ulothrix, U. zonata: A—young filaments with a rhizoidal (r) cell (300x); B—fragment of a filament with releasing zoospores, which arise two per cell; C—individual quadriflagellate mitozoospores; D—formation and release of smaller biflagellate gametes from a filament fragment; E—gametes; F—their copulation; G, H—zygote; J—zygote germinating after a period of dormancy; K—formation of meiozoospores in the zygote (B–K—480x); L—Ulva, U. lactuca (sea lettuce) on a rock, cells along the margin are colorless due to the release of zoospores (1/2x); M—Enteromorpha, E. intestinalis, copulation of anisogametes and zygote (1,800x); 1–3—stages prior to zygote formation

The green algae comprise approximately 450 genera and 7,000 species, most of which (about 90%) inhabit the plankton or benthos (see Box 11.7) of freshwaters. Some species with larger thalli are also found in seas, usually near the shores; in marine plankton, by contrast, green algae constitute only a minor fraction. Some green algae live out of water: in damp soil or on its surface, as epiphytes on trees, etc. Certain species can even withstand significant desiccation and are distinct terrestrial plants. Some live as symbionts in Lichens or as intracellular endosymbionts in lower animals ("zoochlorellae", e.g., in Hydra). Some representatives have lost their assimilatory pigments and live heterotrophically. Their relationship to autotrophic forms of green algae can be established by other characters. The systematic position of certain green algal genera remains unclear for now.

1. Class: Prasinophyceae

Representatives of this class possess unique scales on the cell surface, as well as two or four identical flagella (rarely only one flagellum). The monadoid (e.g., Pyramimonas, Pedinomonas, Platymonas), and partly also palmelloid and coccoid organisms belonging here inhabit mainly marine plankton, with only a few species occurring in freshwaters. Platymonas convolutae is an endosymbiont of a marine flatworm.

In the subsequent classes (2–7), the flagella are inserted in a cruciate pattern, and the flagellar apparatus shows an 11–5 arrangement of its ultramicroscop

ic structures (in the 8th class, Chlorophyceae, by contrast, the arrangement is 1–7 or 12–6).

2. Class: Ulvophyceae

Unicellular (coccoid) and multicellular—filamentous, parenchymatous, or siphonocladous—representatives of this class, with the exception of reproductive cells, lack flagella. The flagellar apparatus (11 — 5 arrangement) shows a clear Displacement of the basal bodies relative to each other. Cell wall formation occurs by furrowing without the involvement of a phycoplast. Plasmodesmata are absent in the cross-walls. The walls contain polysaccharides. Most species occur in marine or brackish waters.

Thalli are represented predominantly by unbranched filaments, which elongate due to ("diffuse") transverse division of many or all cells (filamentous level of organization). In the genus Monostroma, older filaments become flattened due to longitudinal cell divisions in a single plane. A large, leaf-like, green, two-layered parenchymatous thallus is formed in the alga Ulva lactuca (Fig. 11.89, L), which inhabits the coastal marine zone. Enteromorpha, also an alga of coastal waters but also found in saline inland waters, has a tubular thallus, circular or flattened in cross-section. Polarity is only weakly expressed in some species. Thus, for example, in Ulothrix, it is determined by the presence of a single colorless rhizoidal cell incapable of division (Fig. 11.89, A). The cells have a nucleus and a single parietal, band-like chloroplast, which is either closed-cylindrical, cylindrical with longitudinal openings, or shaped like a curved plate with one or more pyrenoids. After nuclear division, The cell wall immediately invaginates to partition the daughter cells (cf. Chlorococcales).

Asexual reproduction occurs by zoospores, sexual reproduction by the copulation of flagellated gametes. The life cycle is either purely haplontic with

zygotic alternation of nuclear phases (Ulothrix)¹ or haplodiplontic as a heterophasic alternation of generations (Ulva).

¹ In Ulothrix, immediately before Meiosis, the zygote germinates into a small pear-shaped plantlet; the diploid nucleus migrates into it, where it undergoes meiosis. This plantlet is often interpreted as the diploid stage of this alga (Codiolum stage), in which case one can no longer speak of a zygotic alternation of nuclear phases. — Translator's note.

1. Order: Codiolales (Ulotrichales). A species frequently found in fresh waters, Ulothrix zonata (Fig. 11.89, A), forms unbranched, intercalarily growing filaments. Their short cells contain a single band-like chloroplast, which lies against the cell wall in the form of a ring open on one side. The filaments are attached to stones and other substrates by a rhizoidal cell, which is usually colorless. Except for the rhizoidal cell, any other cell can serve for reproduction; in doing so, nothing remains in the cells. In the life cycle (see Fig. 11.101, A; 11.89, A — K), 4-flagellated haploid mitozoospores provide asexual reproduction. They have an eyespot and a chloroplast and are formed simultaneously by the Cleavage of the protoplast, which previously becomes quadrinucleate, into uninucleate motile cells that escape (B) through a lateral pore in the wall of the mother cell (= sporocyst). After a phase of active movement, they attach themselves by secreting mucus, the flagella and eyespot disappear, and the cells grow into a new haploid, polar filament. Under unfavorable conditions, isogametes are formed in a similar manner but in much larger numbers (D, E); they are morphologically similar to zoospores but smaller and possess only 2 flagella. Gametes of different mating types (+, -) fuse (F) in pairs to form a zygote (G). The zygote, initially swimming by means of 4 flagella, retracts its flagella, rounds up (H), becomes enclosed in a thick wall (cystozygote), and is colored red by carotenoids. It represents a resting stage that ends with meiosis and subsequent germination into 4 — 16 haploid meiozoospores (K)². During this process, segregation into the two mating types occurs. Meiozoospores attach to the substrate laterally with the formation of a rhizoid, so that their longitudinal axis becomes the transverse axis when division begins. Now haploid filaments develop with genotypic determination of mating type (A), which, In addition to gametes, can also produce mitozoospores. Thus, these plants are haplontic with zygotic alternation of nuclear phases. The zygote (J), which forms a rod-like appendage, can in part be regarded as an extremely underdeveloped sporophyte.

² The Structure into which the zygote germinates is sometimes interpreted as a diploid generation (see the previous footnote). — Translator's note.

Codiolales inhabit both fresh water and seas, and partly also soil. The thalli of aquatic representatives are often attached to the substrate.

In Monostroma grevillei, the leaf-like parenchymatous (male or female) thallus produces gametes. The zygote develops into an independent vesicle (Codiolum) that penetrates the calcareous shells of marine animals. In some species, the size of the zygote increases 20 (or more) times from its initial diameter.

2. Order: Ulvales with the genera Ulva (sea lettuce) and Enteromorpha (gutweed). Here, a heterophasic isomorphic alternation of generations occurs: a diploid sporophytic phase is intercalated.

This situation can be briefly examined using the example of Ulva—sea lettuce. The gametophyte and sporophyte have morphologically identical, leaf-like parenchymatous thalli (Fig. 11.89, L). The alternation of generations is analogous to that in species of the genus Cladophora (see Fig. 11.101. B). The gametophytes are genotypically (+ and -) differentiated. Isogamous copulation occurs between the biflagellate gametes of opposite mating types. The zygotes formed in this way germinate immediately into a diploid sporophyte, as in many marine algae (consistently under favorable growth conditions, in contrast to freshwater forms). It produces quadriflagellate zoospores, the formation of which is associated with both meiosis and genotypic Sex Determination. From the zoospores, haploid gametophytes of different sexes arise again. Both the gametophyte and the sporophyte are preceded by a filamentous protonema, which, As a result of longitudinal cell divisions, becomes a leaf-like parenchymatous thallus.

The genus Enteromorpha differs from the genus Ulva in having anisogamy. Sex Determination of the gametophyte is genotypic; male thalli produce smaller ♂ gametes, while female thalli produce larger ♀ gametes with a green chloroplast (cf. Fig. 11.89, M). In Hormidium, chloroplast movement was studied (see 8.2.2).

The order Acrosiphonales is also related to the orders discussed. The solid fraction of the cell wall consists of felt-like interwoven Cellulose fibrils (the type of cellulose differs from the crystalline cellulose of the Cladophorophyceae discussed below and most leafy plants). Thalli of Spongomorpha (gametophyte) can be interwoven like felt. The alternation of generations is heteromorphic, with a Codiolum-type sporophyte. Urospora also belongs here (see Fig. 11.102).

3. Class: Trebouxiophyceae

This class includes algae living outside of water¹, including lichen symbionts, ranging from coccoid to branched-filamentous organization. The flagellar apparatus (in reproductive cells) with basal bodies displaced relative to each other (11 — 5 arrangement) shows features found only in this class. Daughter cells resulting from schizogeny (rapidly successive cell divisions within the mother cell) are surrounded by a newly formed cell wall. In this respect, Trebouxiophyceae are similar to Chlorococcales of Chlorophyceae, but their mitosis proceeds differently². Species of the genera Trebouxia and Chlorella, characterized by a coccoid thallus type (some with aplanospores, see Fig. 11.97, J), are represented as symbionts in lichens³, and are also found in the Cytoplasm of lower animals (Chlorella vulgaris in Ciliates, Chlorohydra, etc., cf. 9.2).

¹ Species of the genus Chlorella are very widely distributed not only outside of water, but also in fresh waters. — Translator's note.

² It should be clarified what exactly the difference is: in Trebouxiophyceae during mitosis, the centrioles are located at the sides of the spindle (i.e., the spindle is metacentric), whereas in other green algae and many other "protists" they are at the spindle poles. This is the main feature of Trebouxiophyceae. — Translator's note.

³ Species of the genus Chlorella do not occur in lichens. — Translator's note.

The Prasiolaceae are also closely related here. Prasiola stipitata has a peculiar life cycle. In the purely vegetative upper PARTS OF THE leaf-like sporophyte, meiosis occurs, followed by mitosis. The gametophyte developing in this way remains fused with the sporophyte throughout its life. Certain cell fields of the gametophyte produce egg cells, while others produce small, biflagellate ♂ gametes (genotypic sex determination, oogamy). The released reproductive cells then fuse into a zygote.

The subsequent classes are highly isolated; their phylogenetic relationships with other Chlorophyta have not yet been clarified.

4. Class: Cladophorophyceae

Species of this class belong exclusively to the siphonocladous type of organization; branched, sometimes unbranched filamentous thalli are formed by multinucleate cells. The cell walls consist of cellulose in the form of fibrils and with the same structure as in land green plants. Motile reproductive cells have 2 or 4 flagella; the flagellar apparatus (11 — 5 arrangement) is characterized by the overlapping of basal bodies. They inhabit mainly seas, less frequently fresh waters.

The often abundantly branched thalli of species of the single order Cladophorales are multicellular, and each cell is multinucleate. Multinucleate cells are also found in species of other orders (for example, Hydrodictyon of Chlorococcales). Species of the genus Cladophora most often have a heterophasic isomorphic alternation of generations (see Fig. 11.101, B). In this case, each generation can reproduce asexually. Isogametes are biflagellate, whereas meiozoospores are equipped with 4 flagella (in freshwater species, they have 2). Cladophora glomerata, which often forms tufts up to 30 cm or more in length in fresh waters, reproduces only asexually¹.

¹ There are reports that C. glomerata also has a sexual process. — Translator's note.

Filament bundles of *Cladophora* species, frequently found on hard substrates in freshwaters (including running waters) and in seas (Fig. 11.90), are attached at the base by a rhizoid-like cell and exhibit predominantly apical growth. Branching occurs by the protrusion of a "stem" cell (each time beneath a centripetally formed septum); they continue to grow as a septum forms at an acute angle to the longitudinal axis. The parietal chloroplast is reticulate². It contains pyrenoids with starch grains. Cellulose microfibrils of the cell wall are arranged in layers at different angles, giving it great strength. As in Ulvophyceae, motile reproductive cells (zoospores and isogametes) arise in cells that are virtually indistinguishable externally, but usually at the tips of lateral branches (*Siphonocladus* is a marine alga).

² In fact, *Cladophora* does not have a single chloroplast, but rather many elongated, irregularly shaped METABOLISM/14.html">Chloroplasts that join at their ends to form a parietal network, so that they appear as a single reticulate plate. — Translator's note.

Fig. 11.90. Cladophorophyceae, *Cladophora*: A — general view (1/3x); B — branching; C — gametangium with gametes (B, C — 250x)

The order Valoniales (Fig. 11.91, E; formerly assigned to Bryopsidophyceae), which is closely related to Cladophorales, is distinguished by a unique mechanism of Cell Division. The protoplast of the initially non-cellular, sac-like thallus divides into several fragments of various sizes, which often round up while still inside the parent thallus and become surrounded by new cell walls. In this way, a multicellular pseudoparenchymatous thallus can arise. *Valonia* thalli, containing a large vacuole, many nuclei, and numerous parietal chloroplasts, represent a convenient subject for studying cell wall permeability (see Fig. 2.66, B).

Fig. 11.91. Bryopsidophyceae: A — D — Halimedales: A — *Caulerpa prolifera*, thallus (12x), B — *Halimeda tuna*, thallus (1/2x), C, D — *Codium tomentosum*: C — thallus (1/2x), D — cross-section of the thallus (15x); E — Valoniales (now assigned to Cladophorophyceae); *Valonia utricularis*, thallus (1 1/2x); F — K — Bryopsidales; F, G — *Derbesia marina* ("Halicystis ovalis"); F — gametophyte (3x), G — ♂ and ♀ gametes (500x); H, J, K — *Derbesia marina*: H — fragment of the sporophyte thallus (30x), J — sporangium (120x), K — zoospore (400x); g — gametangium; u — utricle (swollen cell of the cortical layer)

5. Class: Bryopsidophyceae (= siphonous algae)

Siphonous algae, which are particularly widespread in warm seas, exhibit an extremely diverse appearance. Their thallus lacks transverse septa; at most, a system of trabeculae (braces) may be present. The cell wall (which contains mannan and Xylan along with cellulose) thus surrounds a single, polyenergid protoplast containing numerous small, discoid chloroplasts. Only the structures housing reproductive cells are partitioned off by a septum (siphonal level of organization). The thalli of some species consist of complex networks of filamentous elements. In addition to the pigment Complement characteristic of green algae, siphonous algae possess siphonaxanthin and siphonein—accessory pigments distinctive to this class.

Sexual reproduction is anisogamous, less frequently isogamous. Reproductive cells have 2, 4, or many flagella. The flagellar apparatus (11 — 5 orientation) shows overlapping basal bodies. According to recent studies, the life cycle is predominantly haplontic, but a heteromorphic alternation of generations with dikaryotic-diploid sporophytes is also possible. This can be illustrated by the example of *Derbesia-Halicystis*. The gametophyte consists of a vesicle-like gametangium 0.5 — 3 cm in size, from which a perennial rhizoid arises. Due to ignorance of its connection to the sporophyte, this plant was formerly placed in a separate genus ("Halicystis" = gametophyte of *Derbesia*; Fig. 11.91, F). Dioecious "Halicystis" plants produce anisogametes with two flagella of equal length (G). The zygote develops into a sporophyte with a branched-filamentous thallus — *Derbesia* (H). In the ovoid sporocysts of this plant, which is initially dikaryotic and later locally diploid, stephanokont meiospores (meiozoospores with a crown of flagella) arise after meiosis. The alternation of generations is heteromorphic with a slight predominance of the dikaryotic-diploid sporophyte, which reaches up to 10 cm in height (see Fig. 11.101, C). The cell wall composition in these generations, as in *Bryopsis* (see below), is different.

1. Order: Bryopsidales. Species of this order lack amyloplasts used for starch formation. Species of the type genus *Bryopsis* form creeping "rhizomes" prostrate on the substrate, and erect branches that are pinnately branched in their upper part. The life cycle is haplontic. The cell wall of the plants producing haploid gametes consists mainly of xylan, while the zygote wall consists mainly of mannan. *Derbesia* (*Halicystis*) occurs along the Atlantic coast.

Codiaceae are characterized by thalli up to several meters long (for example, some species of the genus *Codium*; Fig. 11.91, C, D), which consist of an interwoven mass of branched, non-septate filaments, stiffened by rings on the cell wall. The cell wall consists mainly of mannan. Fossil Codiaceae are known as early as the Lower Paleozoic.

2. Order: Halimedales. Cells of representatives of this order possess both chloroplasts and amyloplasts. The cell walls do not contain mannan.

The family Caulerpaceae, with the genus *Caulerpa* distributed in warmer seas and represented by A wide variety of forms, is characterized by thalli consisting of colorless creeping main axes up to 1 m long, with rhizoids extending downward into the substrate and upward-growing blades of various shapes that can reach several decimeters in size (A). Large plants consist of a single multinucleate giant cell, the outer wall of which is supported only by repeatedly branching trabeculae. The cell wall contains mainly xylan. Whether meiosis occurs during gamete formation (they are released as green clouds, after which the emptied plant dies) remains unclear.

In Representatives of the genus *Halimeda* (B), which is widespread in warmer seas, the thallus consists of discoid segments whose walls are calcified.

6. Class: Dasycladophyceae

Dasycladophyceae (Dasycladales) differ from typical siphonous algae (the previous class) in the radial Symmetry of the thallus and Hair-like appendages (see Fig. 4.1), which can be partially shed, leaving scars. The central axis bears whorled lateral branches. Species of this class are found exclusively in marine environments.

The cell wall consists mainly of mannan. The thallus is formed by a 'stem cell' attached to the substrate by rhizoids and lateral branches arising from it in whorls (Fig. 11.92, B). These branches can be simple or branched and often terminate in a gametangium.

Fig. 11.92. Dasycladophyceae, Dasycladales. *Cymopolia barbata*: A — upper part of the plant (4x); B — longitudinal section through a portion of the thallus, the calcareous coating is shown by fine dots (40x); C — G — *Acetabularia mediterranea*: C — mature thalli (natural size), D — longitudinal section through the 'cap', with a corona of sterile hairs above, and scars of fallen whorls of sterile hairs below (6x), E — dehisced cyst releasing gametes (100x), F — gametes (300x), G — copulation (30x)

As a subject of morphogenetic research, *Acetabularia* is particularly famous (Fig. 11.92, C — G). This alga bears an umbrella-like 'cap' on an undivided stalk, which consists of radially arranged, closely appressed chambers. A corona of short cells is formed above and below the cap. From the upper corona, an additional whorl of slender, apically branched hairs arises, which quickly die off as the cap matures. The thallus initially has only a single primary nucleus, which remains unchanged in the rhizoid for a long time. After The Development of the cap, it divides into numerous haploid secondary nuclei, which migrate into the chambers, whereupon the formation of thick-walled cysts begins. Cysts are released after the decay of the cap, open by a lid, and release gametes (E). The zygote (G), resulting from the copulation of two isogametes, attaches to the bottom and grows into a new diploid thallus. According to recent studies, *Acetabularia* is not a diplont, as even the primary nucleus appears to be haploid. However, according to another view, meiosis occurs during the Formation of secondary nuclei; in that case, the life cycle would be diplontic (with gametic meiosis).

The outer layers of the cell wall in Dasycladaceae are very heavily calcified (B), so that after the death of the thallus, a perforated calcareous tube remains; this underlies the significant rock-forming role of fossil Dasycladaceae, for example, in the Alpine Triassic. Since the Cambrian, a total of 120 genera of Dasycladophyceae are known across all formations, whereas today there are only 10 genera. Based on fossil finds, we can trace the evolution from simple forms, in which branches arise irregularly from the stem cell, to highly differentiated ones, such as species of the genus *Acetabularia*.

7. Class: Trentepohliophyceae

The filamentous thallus is often heterotrichous, i.e., differentiated into prostrate and erect filaments, which may be branched (Fig. 11.93, C). Cells are uninucleate. In some forms, the prostrate filaments fuse into a flat disc (*Cephaleuros*), as in some representatives of Klebsormidiophyceae (*Coleochaete*, see Fig. 11.106, A). In their heterotrichous thallus structure, Trentepohliophyceae resemble the not yet discussed Chaetophorales of Chlorophyceae (*Stigeoclonium*; see Fig. 11.99, A, B). Exclusively characteristic of Trentepohliophyceae are accessory structures in the form of columns in the flagellar apparatus (11 — 5 orientation) with overlapping basal bodies, as well as bilateral keel-like depressions in the flagella.

Fig. 11.93. Trentepohliophyceae: A, B — *Pleurococcus*, *P. naegelii* (600x); C — E — *Trentepohlia*: C — *T. aurea*, portion of a prostrate filament with erect branches (at the apex of the right erect filament, one cell bears a zoosporangium, from the second, the zoosporangium has detached; 500x), D — *T. umbrina*, zoosporangium releasing zoospores (300x), E — *T. umbrina*, abscission of the emptied sporangium (300x)

The cell wall, composed of polysaccharides, may additionally form a layer of sporopollenin. During cell division, new cell walls are initiated in the phragmoplast. Most species are terrestrial algae (for example, epiphytes on tree bark or stone).

Trentepohlia (Fig. 11.93, C) is frequently found as a symbiont in lichens or as a terrestrial alga on rocks (T. aurea on limestone, and T. iolithus, which has a violet-like odor, on silicate rocks) and tree trunks, and in the tropics also on leathery leaves. ADAPTATION TO A terrestrial lifestyle is also manifested in the fact that sporocysts containing zoospores are often shed as a whole. Biflagellate reproductive cells copulate with each other as gametes or serve for asexual reproduction (facultative determination of function). Very common green coatings on tree bark and rocks are caused by algae of the "Pleurococcus" type (Apatococcus and Desmococcus)¹; some of these terrestrial algae no longer form motile cells, i.e., they have undergone a corresponding reduction.

¹ In most modern systems, the genus Desmococcus belongs to the class Trebouxiophyceae (see above). — Translator's note.

8. Class: Chlorophyceae

Evolutionarily, the Chlorophyceae, in contrast to the Prasinophyceae (1), Ulvophyceae (2), and Trebouxiophyceae (3), despite their often primitive organization, should perhaps be considered a later-diverging group. This class includes unicellular or colonial forms with or without flagella, as well as filamentous (trichal and siphonal) species. The cell wall in monadoid species consists of Glycoproteins, and in non-motile representatives, of polysaccharides, including cellulose. During cell division, new transverse walls arise in phycoplasts (see Introduction to Chlorophyta); such walls are often perforated by plasmodesmata. Flagellar arrangement belongs to the cruciate type; the flagellar apparatus shows a 1–7 or 12–6 arrangement (of basal bodies, etc.; see Fig. 11.88). Species of this class inhabit predominantly fresh waters, with a small portion in brackish and marine waters, or even in terrestrial environments.

1. Order: Volvocales. This order includes flagellated unicellular forms that can aggregate into colonies (see 5.1). In this order, the

transition from unicellular forms to colonies with varying levels of differentiation and increasing polarity is clearly traceable. Radially symmetrical cells have 2, 4, or 8 apical, "smooth" flagella of equal length (cf. Fig. 11.97, D). They arise on both sides of the apical papilla.

Reproduction of unicellular species occurs asexually via zoospores, which are formed by repeated successive longitudinal Divisions of the mother cell contents into 2–16 protoplasts (Fig. 11.94, B) and are released upon the rupture of the mother cell wall, which has thus become a sporocyst. During sexual reproduction (in 10% of Chlamydomonas species), biflagellate gametes or egg cells and spermatozoa fuse.

In isogamy (Fig. 11.94, C), the copulating gametes are completely identical in size, appearance, and movement, and generally do not differ from vegetative cells. Under appropriate circumstances, they can either copulate arbitrarily with each other or develop vegetatively (facultative determination of function). Thus, here we are dealing with the rudiments of sexuality. The gametes may belong to the same mating type (homothallism) or, despite apparent similarity, differ genetically (heterothallism with "+" or "-" gametes, for example, Chlamydomonas reinhardtii). The determination of the function of reproductive cells partly depends on environmental conditions. A nitrogen-rich medium (NН₄! ions) determines exclusively the formation of vegetative cells. Ca ions promote the determination of gamete function.

Fig. 11.94. Chlorophyceae, Volvocales. Chlamydomonadaceae:

A — Chlamydomonas angulosa (1 100x); B — the same, four daughter cells inside the mother cell (1 100x); C, D — Chlamydomonas botryoides, copulation of two isogametes (250x); E — Chlamydomonas paradoxa, zygote (500x); F — Chlamydomonas monoica, resting cystozygote (500x); G — Stephanosphaera pluvialis, germinating hypnozygote (300x); H, J — Chlamydomonas braunii, copulation of anisogametes (400x); K — Haematococcus pluvialis (cell surrounded by a thick mucilaginous layer, 330x): c — chloroplast; g — flagellum; k — nucleus; p — pyrenoid; s — eyespot; v — contractile vacuole

In species with anisogamy (Fig. 11.94, H, J), smaller ♂ gametes copulate with larger ♀ gametes. In Chlamydomonas suboogama, the flagella of the ♀ gametes are non-functional, which can be seen as a transition to the next group of species.

Oogamy. In Chlorogonium oogamum, ♀ gametes are completely devoid of flagella. They emerge from the mother cell in an amoeboid fashion (Fig. 11.95, D) and become egg cells. The egg cell is fertilized by spermatozoa, which are produced in numbers of 64 or 128 as light-green, biflagellate, needle-like structures in ♂ individuals through successive divisions (Fig. 11.95, B). In Chlamydomonas coccifera, reproduction occurs as gameto-gametangiogamy, since the entire ♀ cell loses its flagella, becomes an oogonium, and is fertilized by a spermatozoon.

Fig. 11.95. Chlorophyceae, Volvocales: A — E — Chlorogonium oogamum (240x): A — vegetative cell, B — ♂ cell with spermatozoa, C — ♀ cell with egg cell, D — release of the egg cell, E — egg cell surrounded by spermatozoa; F — Stephanosphaera pluvialis (250x); G — Pandorina morum (160x); H — the same, formation of daughter colonies (walls of the mother cells are already partially destroyed, 150x)

Thus, even in these unicellular forms, a progressive development can be traced from isogamy through anisogamy and oogamy to the fusion of ♂ gametes with the oogonium.

Flagellated reproductive cells usually arise in large numbers (2–64) within the mother cell through repeated longitudinal divisions. In iso- and anisogamy, they fuse in pairs to form zygotes (Fig. 11.94, C–E), whereby the tips of the flagella of the partners usually contact first and twist around each other (C). During copulation, glycoproteins act as gamones (see 8.2.1.1), which attract gametes of different mating types to each other and ensure temporary adhesion of the flagella. The zygote has 4 flagella and is initially still motile (planozygote). Later, the flagella are retracted, and the zygote, which becomes thick-walled, can enter a resting state (cystozygote; F). Gametes are always formed naked, but they can also be surrounded by a wall, in which case during copulation the contents must escape through this wall. Upon germination of the zygote (G), meiosis occurs, with the resulting motile cells distributed in a 1:1 ratio between the two mating types (+ and -). Thus, the motile cells are meiozoospores, the alternation of nuclear phases is zygotic, and the life cycle is haplontic. In certain cases, entire individuals are involved in gamete formation.

Occurrence. Volvocales are planktonic organisms widely distributed in fresh waters. They can occur in such large numbers that the water appears completely green; Volvocales are absent in seas.

In many species, development is promoted by the uptake of organic substances (mixotrophy; see 9.1), so they are found partly in organically polluted water bodies. A few species (for example, Polytoma uvella) live purely saprotrophically. Although they lack chlorophyll, the chloroplast present in ancestral forms can still be distinguished here as a colorless plastid. Instead of thylakoids, it contains a system of randomly arranged tubules. Similar Plastids are observed in yellow, non-photosynthetic mutants of Chlamydomonas obtained by ultraviolet irradiation.

Classification of the Volvocales. Exclusively naked representatives are found in the small, possibly ancestral family Polyblepharidaceae. While Polyblepharides, as far as is known, reproduces only by longitudinal division into two cells, more highly developed forms also undergo sexual reproduction with phenotypic or genotypic determination of the mating type (+ and -). Dunaliella salina belongs to the latter group, lives in highly saline waters, and is colored red by carotenoids.

The family Chlamydomonadaceae differs from the Polyblepharidaceae in having a cell wall. Initially, the chloroplast occupies a central position; in most Chlamydomonas species, it is parietal, and in more highly organized forms, it is reticulate-perforated or even fragmented into separate small discs. Sexual reproduction undergoes progressive evolution up to oogamy.

The swimming speed of flagellated cells, for example in Chlamydomonas, is approximately 10 times the cell length per second during phototaxis. Near the Base of the flagella, There are two contractile vacuoles that contract alternately to expel water. They maintain the constancy of the cell's osmotic pressure. Each cell contains a single cup-shaped chloroplast, in the "base" of which there is usually a pyrenoid, around which starch is deposited (see 2.2.9.1 and Fig. 11.94, A, cf. 11.96, B), and at the anterior end, an eyespot (stigma, see Fig. 11.94 A). Starch formation in chloroplasts is not exclusively associated with the pyrenoid. The pigment granules (carotenoid globules) making up the eyespot together form 3–8 rows. Glycoproteins (including hydroxyproline and arabinose linked to galactose) and polysaccharides (but not cellulose!) are involved in the Formation of the cell wall (if present, as for example in Chlamydomonas).

Fig. 11.96. Chlorophyceae, Volvocales, Volvox: A — individual with 6 daughter individuals (50x); B — single cell with plasmodesmata extending laterally to neighboring cells (1000x); C — cell connection, general view (500x); D — J — development and inversion of the daughter sphere (D — 250x, E — F — 350x, G — J — 250x); K — part of a monoecious individual with 5 egg cells and 2 sperm packets (200x); L — egg cell surrounded by spermatozoa (265x); M — spermatozoon (1000x); A — J, M — V. aureus; K, L — V. globator

Some species (Haematococcus pluvialis, see Fig. 11.94, K) color rain puddles red due to the presence of carotenoids. Chlamydomonas nivalis causes "red snow" in high altitudes and the Arctic. Some Chlamydomonadaceae (and other flagellates) colonize bare ice and snow even in winter (see Box 11.7, p. 199). Species of the genus Carteria have 4 flagella.

The family Volvocaceae represents the result of further evolutionary development towards colony formation. Individual cells, often resembling Chlamydomonas, are linked to one another by mucilage or also by plasmodesmata. In Oltmansiella, 4 cells, and in Gonium, 4 to 16 cells are united into flat plates, with all flagella pointing in the same direction. Colonies of Stephanosphaera species, which live in rain puddles (see Fig. 11.95, F), form a crown of 4, 8, or 16 cells with stiff appendages; the chloroplasts usually contain 2 pyrenoids each. In Pandorina, 16 Chlamydomonas-like cells, and in Eudorina and Pleodorina, 32 or 128 such cells respectively, are united into a single hollow sphere. In all colonies, the flagella beat synchronously, which is mediated by plasmodesmata (see 2.2.7.3). From Pandorina through Eudorina to Pleodorina, a polar differentiation of characters occurs relative to the direction of movement in water (eyespot size, cell size, reproductive capacity, etc.). At the end of their individual development, single cells do not die but divide or are used to form reproductive cells. The highest level of organization, assessed by cell number, differentiation, and polarity, is found in Volvox (Fig. 11.96). Volvox colonies are hollow spheres visible to the naked eye, up to several millimeters in size, filled with mucilage. The sphere is formed by numerous (up to several thousand) cells; for example, V. globator has up to 16 000 cells. Each cell has 2 flagella, an eyespot, and a chloroplast. The cells are connected to each other by wide plasmodesmata (B; C). Only certain cells, scattered in the posterior part of the sphere, are capable of reproduction. Most cells serve for photosynthesis and locomotion; however, they also differ in stigma size, which gradually decreases as cell size increases, from the anterior to the posterior pole (polarity!). The anterior pole of the sphere is additionally defined by the direction of its movement in water. The Volvox sphere can be regarded no longer as a colony, but as a multicellular individual. Individual cells are no longer totipotent. Since only a fraction of the cells is capable of reproduction, most Cells of the sphere die after the formation of daughter spheres or gametes (a "corpse" as a consequence of multicellularity).

During asexual reproduction in Volvox (Fig. 11.96, D — J), individual, relatively large cells (D) at the posterior pole of the colony divide longitudinally several times to form a hollow sphere (F), which eventually becomes an open hollow sphere (G). The daughter sphere formed in this way turns inside out (H) and sinks, with its flagella now oriented outwards, into the interior of the mother sphere, which is still filled with mucilage. In this way, several daughter spheres (A) are formed, which are released only after the disintegration of the mother individual.

Sexual reproduction in Eudorina and Volvox is oogamous. Within individual larger (generative) cells, either green egg cells (one per cell, 6 — 8 in total) or numerous small spermatozoa, arranged as a plate upon release, arise (Fig. 11.96, K, M). Volvox globator is a monoecious species, while V. aureus and V. carteri are dioecious. The development of asexually produced spheres into ♂ or ♀ individuals in dioecious species is induced by a sex hormone (glycoprotein). It is produced by ♂ individuals (or their spermatozoa) and is required for young spheres determined as ♂ or ♀ to develop further into individuals of the respective sex. If the sex hormone is absent, only asexual Volvox spheres are formed. After Fertilization, the egg cell develops into a thick-walled resting zygote, which undergoes meiosis upon germination. In all Volvocaceae, therefore, the cells of a colony always originate from a single ancestral cell.

Adjacent to the forms described above are non-motile unicellular or colony-forming green algae that still partially exhibit characteristics of monadoid algae, such as contractile vacuoles, eyespots, and flagellated stages. They are sometimes treated as a separate order, Tetrasporales.

2. Order: Chlorococcales (= Protococcales). Cells with only a single nucleus and a single chloroplast lack flagella in the vegetative state and are therefore non-motile. Only during reproduction can biflagellate motile cells (zoospores, Fig. 11.97, D; or gametes) arise. The latter are usually naked and only become enclosed in a wall (encyst) after swimming for some time. In some representatives, only flagella-less "aplanospores" (K) are formed. Sexual reproduction, known in very few representatives, is isogamous, involving flagellated gametes (e.g., Pediastrum and Hydrodictyon); oogamy is extremely rare. Zygotes germinate with a reduction division, so that the life cycle proceeds exclusively in the haplophase. In some species, characteristic colonies are formed from unicellular units (see 5.1; e.g., Pediastrum, Fig. 11.98; Scenedesmus, see Fig. 11.97, D). The Chemistry of the polysaccharide cell wall is largely unknown; in Pediastrum, silicic acid occurs in it, and in some species, sporopollenin is deposited.

Fig. 11.97. Chlorophyceae, Chlorococcales. A — G — Chlorococcum (1000x): A — vegetative cell with a cup-shaped chloroplast (c) having a small indentation only at the anterior end, with a pyrenoid (p), k — slightly translucent nucleus, B — division into 8 daughter cells, C — release of zoospores in a vesicle, which later dissolves, from the inner layer of the mother cell membrane (m), D — free-swimming zoospore with apical flagella of equal length, E — the same, having entered a resting state; eyespot and flagella are still present, F, G — development to stage A with the loss of the eyespot (a) and vacuoles (v); H — K — Chlorella, C. vulgaris (500x): H — vegetative cell, J, K — division into 8 aplanospores; L, M — Scenedesmus, S. acutus (1000x): L — 4-celled colony, M — division; N — R — Hydrodictyon, H. utriculatum. N — young net within a cell of the mother net (15x), O — mesh of a young net (80x), P — part of an older cell with zoospores, Q, R — fusion of zoospores into a new net in the parietal protoplast (P — R — 10x)

During cell division (e.g., in Chlorococcum), A number of naked daughter cells are often formed first, which subsequently become enclosed in cell walls simultaneously. Kirchneriella, studied in more detail using Electron Microscopy, deviates from this pattern: septa containing cell wall material arise immediately after divisions (successively) but soon disappear again. The resulting 4 daughter cells move apart and each becomes surrounded by its own new cell wall before they individually leave the mother cell.

As in the Volvocales, we find a progressive series here, but it is directed from unicellular species to colonies (see 5.1) that are arranged as plates or hollow spheres.

However, the ontogeny of Kirchneriella (see above) suggests that the unicellular forms of this genus arose from multicellular (colonial) structures.

Unicellular forms, ranging from spherical to ellipsoidal, are represented by species of genera such as Chlorococcum (with zoospores; Fig. 11.97, D) and Oocystis. Groupings of cells arranged in a single row in simple forms, usually numbering 4 (or 8), are found in species of the genus Scenedesmus (Fig. 11.97, L, M), which are widespread in fresh waters. More complex colonies are found in the genus Pediastrum, which is also common in fresh waters: they look like elegant flat plates (Fig. 11.98, A) floating freely in the water, somewhat resembling Gonium but without flagella. The mature colony of Coelastrum is three-dimensional, with cells forming a hollow sphere (E). In the water net (Hydrodictyon reticulatum), which floats freely in fresh water, cylindrical cells join at their ends in groups of 3 to 4 to form a sac-like colony in the shape of an elongated, closed, hollow network of many meshes (see Fig. 11.97, N).

Fig. 11.98. Chlorophyceae, Chlorococcales: A–D — Pediastrum, P. granulatum. A — discoid colony of cells, three of which are undergoing division; a vesicle containing 16 motile cells is emerging from a fourth cell, B — motile zoospores in the released vesicle, C — after 4.5 h, the grouping of 16 daughter individuals into a colony has begun, D — the same, lateral view (300x); E — Coelastrum, C. proboscideum (550x)

Sexual reproduction is by isogametes that are smaller than zoospores. Upon germination of the zygote, four meiozoospores are initially formed, which, after swimming for a short time, transform into thick-walled "polyhedra". Only the latter then germinate into new Hydrodictyon colonies, which are initially much smaller than mature ones. During asexual reproduction, species of all these genera produce motile zoospores or non-motile aplanospores, which, however, are not released individually. Very early on, they bond together due to the adhesion of their walls, forming a colony characteristic of the species in terms of cell number and appearance (Figs. 11.97; 11.98). This association can occur shortly after release from the mother cell within a mucilaginous vesicle (Fig. 11.98, A) or even inside the mother cell itself, so that upon its breakdown, a colony of the appropriate cell number is already present, albeit initially small. No further cell divisions occur within the colonies (except for the formation of reproductive cells). The aforementioned similarity to the corresponding evolutionary series in Volvocales is therefore only superficial, not developmental. In Volvocales, colonies arise through repeated longitudinal division of their constituent cells, whereby THE POSITION OF each cell in the colony is determined from the very beginning. In Chlorococcales, all cells of a "brood" resulting from the division of the original protoplast can initially move freely relative to one another (inside the cells or mucilaginous vesicles) before secondary association occurs (Fig. 11.98, A, B).

Occurrence. Chlorococcales inhabit mainly freshwater plankton; some forms have transitioned to a terrestrial lifestyle. Such species inhabit damp soil and rocks. The soil alga Spongiochloris is resistant to high temperatures. Chlorococcales, along with other algae, are also very common components of green coatings on tree bark and walls. Some Chlorococcales are symbionts in lichens (but cf. Trebouxia, Trebouxiophyceae). Scenedesmus, Ankistrodesmus, and Hydrodictyon are often used in pure culture for physiological experiments.

As fossils, forms resembling modern Pediastrum have been described as early as the Permian. Forms resembling Chlorococcales (Caryosphaerioides) are among the oldest records of Eukaryotic cells (see 11.3.2).

3. Order: Chaetophorales. The thallus of algae belonging to this order consists of branched filaments composed of uninucleate cells with a single chloroplast. It is usually heterotrichous, i.e., it consists of two parts: a "prostrate system" of branched filaments creeping over the substrate, and more or less abundantly branched erect filaments bearing reproductive Organs (Fig. 11.99, A). Sexual reproduction in those species where it has been established is iso-, aniso-, or oogamy.

In species of some genera, the heterotrichous structure is not clearly pronounced because one of the two parts of the thallus is weakly developed or virtually absent. Some forms are aggregations of several individuals into colonies embedded in common mucilage. In Stigeoclonium (Fig. 11.99, A), alongside quadriflagellate zoospores (B), biflagellate isogametes occur. In the soil-dwelling alga Fritschiella (India, Africa; Fig. 11.99, C), vertical branched filaments rise into the air from underground creeping filaments. This demonstrates a functional differentiation highly pronounced in higher plants: one part of the thallus performs mainly an absorptive function, while the other performs an assimilatory one.

Fig. 11.99. Chlorophyceae, Chaetophorales: A — Stigeoclonium tenue (4x); B — Stigeoclonium subspinosum, zoospore (900x); C — Fritschiella tuberosa; a — eyespot; b — soil surface; p — pyrenoid; pa — primary vertical filament; pr — underground creeping filament; r — rhizoid; s — prostrate system (filaments creeping over the substrate); sf — secondary vertical filaments; v — contractile vacuoles; w — vertical filaments suspended in the water Column

Occurrence. Most species (often as epiphytes on algae and other aquatic plants) inhabit fresh waters (e.g., Chaetophora, Stigeoclonium).

4. Order: Oedogoniales. This is another order with a filamentous thallus organization. Although the filaments here are usually unbranched, their oogamous reproduction, as well as their unique mode of cell division and elongation, indicate that these are highly advanced forms that have followed an independent evolutionary path. The uninucleate cells each contain a single parietal, reticulate chloroplast with numerous pyrenoids (Fig. 11.100, A).

Unique cell division and elongation are associated with the formation of "caps" at the upper end of the cell (Fig. 11.100, J). Their formation is linked to the onset of nuclear division (prophase), when a ring-like cushion of fusing vesicles (Golgi?) forms at the upper end of the cell; it consists mainly of an amorphous, extensible fraction of the cell wall. After cell division is complete, a septum arises between the daughter nuclei within the phycoplast (see Chlorophyta, introduction), giving rise to the cell plate, which is initially unfixed—the future transverse wall. In the region of the upper annular cushion, the outer cell wall subsequently ruptures, forming a circular split, after which the annular cushion stretches into a cylinder. A characteristic cap remains at the site of the rupture. After repeated occurrences of this process, the number of caps at the upper end of the cell increases, and they appear nested within one another (Fig. 11.100, C).

The life cycle is haplontic. Relatively large zoospores arise singly from the entire contents of a cell. Near their anterior end, which lacks chloroplasts, they bear a characteristic crown of numerous flagella that are not grouped in pairs (Fig. 11.100, C). Elsewhere along the filament, individual cells swell like barrels into oogonia: their contents transform into a single large egg cell (E), which remains inside the oogonium. Certain Regions of the same or another filament (modificational sex determination) produce spermatozoids, usually two per cell, in cells that remain relatively short. These resemble zoospores but are smaller and yellowish in color.

Another pathway for The transfer of male reproductive cells occurs via so-called androspores and dwarf male plants ("dwarf males"). In cells resembling the aforementioned ♂ gametangia, slightly larger androspores are formed instead of spermatozoids and are attracted to the oogonia by chemotaxis. They cannot fertilize the egg cells directly; instead, they settle on or very close to them and germinate into tiny, few-celled plants—the so-called "dwarf males" (Fig. 11.100, E, F), whose upper cells then become gametangia and produce fertilizing spermatozoids. The simultaneous maturation of the oogonia is apparently regulated by Hormones secreted by the dwarf males. On the other hand, the spermatozoids are chemotactically attracted to the oogonia, which are now surrounded by mucilaginous sheaths. The spermatozoids penetrate the oogonium through a pore to reach the egg cell, and one of them fuses with it. Subsequently, a thick-walled, red hypnozygote develops within the oogonium. Upon germination (G), its contents divide into 4 large haploid meiozoospores (zygotic meiosis), which emerge and form new filaments (D).

Fig. 11.100. Chlorophyceae, Oedogoniales, Oedogonium; A — part of a filament (600x); B — D — O. concatenation, release of a zoospore and its germination (300x); E — G — O. ciliatum (350x): E, F — fertilization, G — germination of the zygote; H — L — cap formation during cell division (200x); a — dwarf male plant (nannandrium); c — chloroplast; k — nucleus; o — oogonium; p — pyrenoid; z — zoospore with reserve substances obscuring The Nucleus (see D)

Overview of Chlorophyta. Green algae exhibit great morphological diversity. The transition from unicellular forms to multicellular thalli proceeds through a series of organizational levels. This occurs convergently in different classes and, accordingly, in different evolutionary lineages. This applies not only to the classes of green algae but also manifests as a general evolutionary principle across various algal divisions. The cell "clad" only in a single (partially modified) Plasmalemma (e.g., Polyblepharides) underwent further development in green algae; in all more highly evolved taxa, the cells possess more or less thick cell walls. Thanks to these, some forms can even live out of water as soil or aerophilic algae.

Cell wall components are even more diverse: cellulose is already used as a polysaccharide material. Sporopollenin sometimes occurs as a protective substance (Trentepohliaceae, Chlorococcales). Cell division—the Separation of daughter cells by a transverse septum—goes through various evolutionary stages. In simple cases, nuclear division is followed by cleavage of the cytoplasm, and all parts are simultaneously enclosed by cell walls while still inside the mother cell. Centripetal cleavage of the mother cell starting from the lateral walls, due to the ingrowth of a septum like an iris Diaphragm between the separating daughter nuclei, is another method that can be considered ancestral. In more evolutionarily advanced cases, a phycoplast is involved in the formation of the new cell wall, and very rarely, a phragmoplast (Trentepohliophyceae). The originally cup-shaped chloroplast is in some cases divided into a network or individual discs, while in other cases, large chloroplasts are formed as plates or ribbons.

Sexual reproduction evolves from isogamy through anisogamy to primitive oogamy, and finally to its highest level, where the egg cell is no longer released but is fertilized within the oogonium. Some green algae, in addition to having already non-motile egg cells, have also lost flagella in their other reproductive cells. Thus, some species of Pleurococcus (Apatococcus, by contrast, has zoospores), having adapted to life out of water, disperse via non-flagellated aplanospores. In the life cycle, a trend toward an increasing share of the diplophase can be traced (Fig. 11.101). Green algae are typically haplonts with zygotic meiosis; in this case, only the zygotes are diploid (A). Due to a shift in the site of meiosis (mitotic nuclear division instead of meiosis), the zygote germinates into a diploid vegetative body. Consequently, an additional diploid phase is intercalated into the life cycle, which ends with meiosis shifted in time and space. Thus, an alternation of a haploid gametophyte and a diploid sporophyte arises, i.e., a heterophasic alternation of generations.

Fig. 11.101. Chlorophyta, alternation of generations and nuclear phases, diagram of the main types: A — Ulothrix; B — Cladophora; C — Halicystis-Derbesia. Dikaryotic phase — dark dotted line; diploid phase — dark solid lines; haploid phase — light lines; G — gametophyte; S — sporophyte; O — zygote; R! — reduction division

Alternation of generations (Fig. 11.101) can be isomorphic (some Cladophora species, B) or heteromorphic (with a predominant sporophyte, Halicystis-Derbesia, C). An exclusively diplontic cycle is not yet known in green algae¹. In general, the alternation of generations occurs on different individuals (diplobiontic). A haplobiontic alternation of generations on a single individual (which is the regular situation in bryophytes) is an extremely rare exception in green algae (Examples include Prasiola stipitata, Bryopsis). Alternation of generations should by no means be understood as an obligatory sequence of different phases. Through asexual reproduction, each generation can reproduce itself independently of the alternation of generations. Green algae with a simple life cycle (Ulothrix, A) usually reproduce vegetatively and asexually (e.g., by zoospores), whereas sexual reproduction occurs only under specific environmental conditions.

¹ The literature usually indicates that a purely diplontic cycle occurs in the species Cladophora glomerata, which is widespread in Europe, as well as in many (perhaps most) siphonous green algae. — Transl. note.

Chlorophyta is undoubtedly a very ancient group of lower plants. However, only the thalli of marine Dasycladales, which are resistant due to calcification, can be identified with certainty; these are known from the Cambrian. Since Dasycladaceae were already represented by a great diversity of forms in the Ordovician, they must have originated even earlier. Of the 120 genera that occurred over more than 500 million years, only 10 survive today.

• Algal divisions with complex plastids containing chlorophylls a and b. The following two divisions, presented in the respective Appendices, contain green-algae-type chloroplasts in their cells. However, the acquisition of these plastids occurred via secondary endocytobiosis (see 2.4).

1. Appendix to Chlorophyta: Chlorarachniophyta

This very small division, which includes only two genera with one species each¹, is of interest because their plastids, surrounded by four membranes, contain a nucleomorph, just like in cryptophytes (see 2.4). The latter can be regarded as the reduced nucleus of a photoautotrophic eukaryotic endosymbiont. Species of this division live in association with siphonous marine algae of warm seas as naked amoeboid cells, which are connected by thread-like cytoplasmic projections into reticulate plasmodia. The light-green chloroplasts contain chlorophylls a and b. Thylakoids in the plastids are grouped in stacks of 2 to 6. Chlorarachniophyta are believed to have evolved from filose amoebae through the acquisition of plastids.

¹ Currently, there are already about 8 species in this division. — Transl. note.

2. Appendix to Chlorophyta: Division Euglenophyta

This division comprises unicellular forms at the monadoid level of organization. Under certain conditions, they can sometimes transition into a palmelloid state. Reproduction occurs by longitudinal fission; sexual reproduction is unknown. The green chloroplasts contain the same set of pigments as green algae (chlorophylls a and b, β-carotene, traces of α-carotene), but they also contain a xanthophyll unknown elsewhere in the plant kingdom. As a reserve substance, along with Phospholipids, the polysaccharide paramylon is deposited in plasma vesicles in the form of granules or discs. This is a glucan with β-1,3-linkages that stains blue with iodine. The cells are often spirally twisted and almost always have a simple, predominantly proteinaceous envelope—the pellicle, which directly borders the plasmalemma (an exception is, for example, Trachelomonas with an iron-containing lorica). At the anterior end of the cell, There is a bottle-shaped invagination—the reservoir (ampulla), which is divided into a chamber and a canal. Adjacent to the reservoir is a contractile vacuole, which is surrounded by several accessory contractile vacuoles and serves as an organelle for osmoregulation. Two flagella almost always arise at the base of the reservoir, each from its own basal body: one long and one short, the latter not emerging from the reservoir and fusing at its tip with the long flagellum. At their point of fusion lies a light-sensitive organelle—the photoreceptor. Near the reservoir is a red eyespot (stigma) (Fig. 11.102, B). It consists of individual lipid globules, each surrounded by its own unit membrane (for The Role of the eyespot in phototaxis, see 8.2.1.2). The long flagellum, bearing hairs (see Fig. 11.20, C), traces The surface of a cone during movement. With simultaneous rotation around its longitudinal axis, the cell, for example in Euglena, moves forward every second by a distance equal to 2–3 times its body length.

Fig. 11.102. Euglenophyta: A — Euglena gracilis (600x); B — the same, anterior end (1000x); C — Colacium mucronatum (500x); D — Phacus triqueter (600x); c — chloroplast; g₁ — locomotor flagellum; g₂ — second flagellum; gs — flagellar swelling (photoreceptor); k — nucleus; p — free paramylon; py — pyrenoid; s — eyespot; st — mucilaginous stalk; v — contractile vacuoles

The ultrastructure of euglenoids shows the following features. In the interphase nucleus, Chromosomes can be seen in a condensed state. The chloroplasts have an envelope of three membranes, which is never connected to the nuclear envelope by The Endoplasmic reticulum. Within the chloroplasts, thylakoids are mostly grouped in lamellae of three.

Euglenoids comprise more than 800 species belonging to about 40 genera. Most of them live in freshwaters. Species of the genus Euglena are found mainly in stagnant water bodies rich in organic matter. Phacus species (Fig. 11.102, D), on the contrary, prefer nutrient-poor waters. Colacium (C) attaches itself by means of a mucilaginous stalk to free-swimming small organisms; its free movement by means of flagella is possible only during reproduction.

Although most species feed photoautotrophically, they tend to absorb organic substances in addition to the products of photosynthesis. A number of colorless forms feed entirely heterotrophically; some (e.g., Peranema) have a funnel-shaped invagination (cytostome) containing an ingestion apparatus and a rod-like organ, with which they are able to capture microorganisms such as Bacteria, algae, or Yeast cells. Thus, the boundaries between PLANT AND ANIMAL organization are blurred here.

In dark cultures, Euglena gracilis loses all chlorophyll and all thylakoids. The remaining bodies resemble proplastids; they retain The ability to divide during the dark phase, thereby maintaining the integrity of the plastidome. Upon subsequent illumination, the surviving colorless plastids develop into chloroplasts with thylakoids, and photosynthesis is resumed. Along with this, there are various strains of this species that lack chloroplasts altogether. Such forms, obtained under certain conditions (for example, during a very rapid succession of divisions), can never form chloroplasts again.

Euglenoids were initially a heterotrophic, probably very early-diverging group that acquired chloroplasts relatively late through endocytobiosis with green algae.