BOTANY VOLUME 3 - EVOLUTION AND SYSTEMATICS - 2007

11. SYSTEMATICS AND PHYLOGENY

11.2. Bacteria, Fungi, Plants

5. Class: Brown Algae (Phaeophyceae)

Brown Algae are a group represented by highly diverse forms (Fig. 11.80, A; see Figs. 5.5; 11.83; 11.85). These can be small, branched filaments divided into Cells (heterotrichous type), pseudoparenchymatous thalli, or multi-meter plants with highly pronounced differentiation into Organs and Tissues (tissue thalli! — see 5.3.2). Brown algal macrophytes are often divided into organs (phylloid, cauloid, rhizoid) that resemble the leaf, stem, and ROOT of higher plants. Unicellular forms are absent, meaning that monadoid and coccoid Levels of Organization are not represented here. Along with red and some green (Chara) algae, brown algae are considered among the most highly organized forms.

Fig. 11.80. Phaeophyceae, Ectocarpales: A — D — Ectocarpus siliculosus: A — gametophyte branch with plurilocular gametangium (380x), B — D — Fertilization (B — 1,200x; C, D — 1,600x), E, F — Asperococcus bullosus, zygote and nuclear fusion (2,000x); G — Nemacystis divaricatus, germination (780x); H — J — Ectocarpus: H — Ectocarpus lucifugus, unilocular meiosporangium on a diploid sporophyte (400x), J — Ectocarpus globifer, zoospore, hairs of the tinsel flagellum are not shown

Plastids. In addition to the assimilation pigments characteristic of the entire division, brown chromatophores contain primarily fucoxanthin as an accessory pigment, whose color masks that of all other components.

Cell wall consists of solid and mucilaginous fractions; the former is composed of Cellulose fibrils and alginate, the latter of alginate and fucoidan. Alginates are salts of alginic acid (a polymer of two sugar acids: β-D-mannuronic and β-L-guluronic) with various cations (such as Ca²⁺, Mg²⁺, Na⁺ ions). Motile stages (zoospores and Gametes) range in shape from pyriform to spindle-shaped and usually bear 2 flagella of unequal length (Fig. 11.80, A, B, I)¹.

¹An important feature of the motile stages of brown algae is considered to be the lateral insertion of flagella. — Translator's Note.

Near the flagella, There is a red-brown eyespot in the brown chloroplast (usually one, occasionally several). The hairs of the tinsel flagellum are formed in vesicles of The Endoplasmic reticulum or the vesicular PARTS OF THE nuclear endoplasmic reticulum. The smooth flagellum is swollen at its base; this Swelling is located near the eyespot and probably acts as a photoreceptor. The flagella terminate (the smooth one always, and the tinsel one sometimes) in a thin, Hair-like appendage. This feature is not found anywhere else except in brown and yellow-green algae.

The life cycle proceeds as an Morphology/12.html">ALTERNATION OF GENERATIONS, where meiospores are always formed in unilocular sporocysts, and gametes, as a rule, in plurilocular (= multilocular) gametangia. The heterophasic alternation of generations ranges from isomorphic to heteromorphic or extremely heteromorphic, with (almost) complete reduction of the haploid gametophyte. The increasing dominance of the diploid sporophyte in The life cycle, which emerged among the Ectocarpales, is considered an evolutionary trend.

Occurrence and lifestyle. Most of the approximately 1,500 to 2,000 species of brown algae, distributed across 250 genera, are marine, reaching their most intensive development in the waters of temperate and high latitudes. These are benthic algae (see Box 11.7): they live as lithophytes, firmly attached to rocks, stones, pilings, etc. Some of them are exposed to the air at low tide. Brown algae often live as epiphytes on other algae. In the intertidal zone of rocky shores, they form luxuriant stands with a characteristic zonal distribution of species (see Fig. 11.102). An impressive sight on the Pacific coast of America is the underwater forests formed by brown algae of the genera Lessonia, Macrocystis, and Nereocystis with multi-meter thalli. Tiny filamentous or discoid brown algae, though less conspicuous, are also widespread, occurring on rocky substrates, barnacles, snails, bivalves, and epiphytically on larger brown algae. Small brown algae can, to some extent, live endophytically within larger algae. Only a few species, belonging to about five genera, are found in freshwaters.

The class is divided into 11 orders, of which the following, being of lesser importance, will not be discussed further: Chordariales (with the genera Chordaria and Leptonema; Elachista — an epiphyte on Fucus with vegetative diploidization in gametophytes); Desmarestiales (characterized by the fusion of filaments into a pseudoparenchymatous cortex; heteromorphic alternation of generations), Dictyosiphonales (parenchymatous thallus), Scytosiphonales (pseudoparenchymatous crustose microthalli (sporophytes) alternate in the life cycle with parenchymatous megathalli — gametophytes?), Sporochnales and Sphacelariales (for example, Halopteris is distinguished from Ectocarpales and others by the presence of an apical cell).

1. Order: Ectocarpales. This order includes the majority of brown algae. Species of the genus Ectocarpus are very common. Their bushy thalli, consisting of branched filaments, resemble green algae of the genus Cladophora in appearance (see Fig. 11.90), but they are brown in color. They inhabit the shallow waters of our seas, attaching to the substrate (rocky substrate, larger algae) by creeping filaments. The filaments grow intercalarily; an apical cell is absent, and only some cells are capable of transforming into reproductive organs. The life cycle as a whole represents an isomorphic (or weakly heteromorphic) alternation of generations.

Gametophyte. The haploid, bushily branched filamentous thallus of the gametophyte bears plurilocular gametangia laterally and at the tips of the filaments, in which by no means every cell is capable of forming a gamete. Gametes are released upon The breakdown of the outer walls of the gametangium, escaping at its apex. Although the gametes are morphologically identical, many species of the genus Ectocarpus exhibit physiological anisogamy: shortly after release from the gametangium, female ("-") gametes settle and shed their flagella, while male ("+") gametes, attracted by the pheromone ectocarpene, cluster around them. The male gametes attach to the resting female gamete with the tips of their longer flagella, and one of them fuses with her (Fig. 11.80, B).

Sporophyte. After fertilization, the zygote grows without a resting period into a diploid sporophyte, which consists of several stiffer and less branched filaments. Numerous ovoid unilocular sporangia develop on it, in which, after Meiosis, A large number of meiozoospores are formed, giving rise to a new gametophyte generation. Sex Determination is haplogenotypic.

This typical — isomorphic and heterophasic — alternation of generations can be complicated by numerous deviations, which are usually difficult to explain: the alternating generations are not always linked to a specific nuclear phase. Each generation can directly reproduce itself again. Sex is not strictly fixed; organisms can also do without it.

While the reproductive structures in Ectocarpus arise at the tips of lateral branches, in Pylaiella they differentiate intercalarily.

In some epiphytic species of the genera Ectocarpus and Pylaiella, gametophytes and sporophytes occur not on the same, but on different host plants (phorophytes) (for example, in Pylaiella litoralis, the sporophyte is on Fucus, and the gametophyte is on Ascophyllum).

2. Order: Cutleriales. The alternation of generations in Cutleria is heteromorphic, with a clearly dominant gametophyte generation (Fig. 11.81, A). The gametophyte is an erect, dichotomously branched plant consisting of ribbon-like lobes, cleft at the tips. In Cutleria multifida — an alga from warmer European seas — the gametophyte lives near the Water surface, reaches a size of about 40 cm, and produces small (♂) and larger (♀) flagellated gametes in micro- and megagametangia on male and female plants (Fig. 11.81). Male gametes are attracted to female gametes by the pheromone multifidene, after which copulation (anisogamy) occurs. The sporophyte, previously described as a representative of a separate genus (Aglaozonoia), is significantly smaller (several centimeters in size); it is flat, lobed, prostrate, and crustose, living on rocks and mollusk shells at depths of 8 to 10 m. On the upper side of the parenchymatous thallus, there are sori consisting of unilocular sporocysts. After meiosis, they release zoospores. Zanardinia has an isomorphic alternation of generations.

Fig. 11.81. Phaeophyceae, Cutleriales, Cutleria multifida: A — ♂, B — ♀ plurilocular gametangia (400x); C — ♂ and ♀ gametes, hairs of the tinsel flagella are not shown (1,200x)

3. Order: Dictyotales. The flat parenchymatous thalli of Dictyota, about the size of a hand, are repeatedly dichotomously branched (Fig. 11.82). Growth and dichotomous branching are based on the division of a large apical cell (see Fig. 5.12, B), which cuts off basal segments basipetally. The latter divide further into many cells that form tissues (see Fig. 5.12, B — D). They differentiate into peripheral assimilatory and central storage cells (see Fig. 11.82). From time to time, the original apical cell is divided into two by a septum running along the longitudinal axis of the thallus; the adjacent cells, continuing to grow, cause the dichotomous branching of the thallus. The alternation of generations is isomorphic (see Fig. 11.87, B).

Fig. 11.82. Phaeophyceae, Dictyotales, Dictyota dichotoma: A — transverse section through a ♂ thallus with a group of antheridia (surrounded by an envelope of sterile cover cells, 200x); B — transverse section of a ♀ thallus with a group of oogonia (200x); C — egg cell with three spermatozoids (400x); D — transverse section of a thallus with tetrasporangia (one of which has emptied) and "hairs" (200x)

Gametophyte. Sexual reproduction is oogamous. Plurilocular spermogonia and oogonia are borne on different plants and are always clustered in groups (sori) (see Fig. 11.82, A, B).

Each oogonium contains a large, non-motile, brown egg cell, which is released into the water and fertilized by a spermatozoid (C). The pear-shaped male gametes have a single, highly reduced chloroplast and only one lateral, tinsel flagellum; the second, reduced flagellum, with its own basal body, remains inside The Cell as a tiny stub. Gametangia develop only during the summer months; the release of sperm is dependent on THE POSITION OF the Sun and Moon and occurs only two days a month, always During the first hour after sunrise.

Sporophyte. The haploid gametophytes are morphologically identical to the diploid sporophyte (Fig. 11.87, B). The meiospores, which arise in groups of four in the unilocular tetrasporocysts of the sporophytes (Fig. 11.82, D), are relatively large and lack flagella. Colorless hairs project between the tetrasporocysts.

The fan-shaped Padina, common in warmer seas, grows by means of a marginal meristem. Dictyopteris grows by means of a group of specialized apical cells.

4. Order: Laminariales. The alternation of generations is heteromorphic, with a clear dominance of the diploid sporophyte (see Fig. 11.87, C). The sporophytes are highly differentiated morphologically and histologically, and sometimes reach considerable sizes (Fig. 11.83).

Fig. 11.83. Phaeophyceae, Laminariales: A — Laminaria saccharina (1/40x); B — Laminaria hyperborea, top with remnants of the previous year's blade (1/40x); C — Nereocystis luetkeana (1/200x); D — Lessonia flavicans (1/30x); E — Macrocystis pyrifera (1/250x); F — the same Organism, apex of the thallus (1/20x)

Gametophytes of all Laminariales, by contrast, are microscopically small. The ♂ and ♀ forms are morphologically distinct, i.e., they exhibit clear secondary sexual characteristics. Male gametophytes are relatively highly branched, grow rapidly, consist of many but small cells (Fig. 11.84, G), and bear unicellular spermogonia at the tips of the branches, each containing a single spermatozoid. Female gametophytes (F) have significantly larger cells but grow more slowly and consist of fewer cells; in extreme cases, they consist of only a single sac-like cell and form oogonia, each with a single egg cell. The naked egg cell emerges through an opening at the apex of the oogonium, where it usually remains (F: e), and after fertilization (oogamy) develops into a diploid sporophyte (F: s₁— s₃).

Fig. 11.84. Phaeophyceae, Laminariales: A — E — Chorda filum: A — meiozoospores, (A') rounded off prior to germination (1,200x); B — E — development of a unilocular sporangium (1,000x): B — uninucleate, C — 4-nucleate, D — 16-nucleate, E — with nearly mature zoospores; F, G — Laminaria (300x): F — ♀ gametophyte, G — ♂ gametophyte; a — spermogonia (a₁ — released); e — egg cell; s₁—s₃ — young sporophytes still attached to the emptied oogonium

The sporophyte generation represents the macroscopic phase in the life cycle. On the surface of the sporophyte, in addition to elongated sterile cells (paraphyses), an extensive layer of club-shaped unilocular sporocysts is formed (Fig. 11.84, D). Within each of these, following reduction division and simultaneous genotypic sex determination, a large number of biflagellate zoospores develop.

Sporophytes of Macrocystis pyrifera (see Fig. 11.83, E) in the cold seas of the Southern Hemisphere reach lengths of over 50 m; their thallus, anchored at depths of 2–25 m by a claw-like holdfast, bears long, drooping blades (phylloids, see 5.3.2) along one side of its axes (cauloids, see 5.3.2), each of which has a large air bladder at its base. These bladders keep the alga afloat near the sea surface. Antarctic species of Lessonia (D), which have trunk-like main axes as thick as a human thigh and up to 5 m long, develop long, drooping phylloids on their lateral branches, externally resembling

palms. In Nereocystis (Pacific coast from California to Alaska), a long (up to 25 m) rope-like cauloid bears a large air bladder at its tip (with a high carbon monoxide content!), from which a cluster of phylloids (C) arises. Chorda filum ("sea lace") has a cord-like unbranched thallus reaching several meters in length. In addition to the terminal phylloid, Alaria also has smaller lateral "leaflets"¹.

¹ The division of labor among these phylloids is interesting: the large central phylloid with a prominent midrib is sterile, while the sori of zoosporangia develop only on the small lateral phylloids, which are therefore also called sporophylls. — Translator's note.

Laminaria species (cf. Fig. 11.102) are distributed along the coasts of the North Atlantic, forming extensive underwater kelp beds ("meadows") just below the low-water mark. Their thalli, up to 5 m long, consist of a perennial stipe with a claw-like holdfast (see Fig. 11.83, B) and a blade-like lamina (A) composed of many cell layers. This blade (phylloid) is replaced annually because an intercalary growth zone at its base gives rise to a new "blade"; the old blade is pushed further away and gradually dies off (B). The phylloid in L. saccharina (A) is simple, while in L. digitata and morphologically similar species (B) it is palmately divided. Sporophytes of L. hyperborea can live for 10–20 years.

Tissue differentiation. A transverse section through the cauloid of Laminariales reveals pronounced differentiation from the periphery to the center. On the outside is the meristoderm (outer limiting layer). Its cells, capable of dividing in several planes, form tangential, radial, and horizontal walls. The deeper layers of the meristoderm are primarily responsible for growth in thickness. Growth occurs seasonally, so that older cauloids exhibit well-defined annual rings. The Cells of the cortex become progressively larger from the periphery to the center. Due to the mucilaginization of cell walls, loose rows of cells arise, running partly longitudinally and partly radially, and in older cauloids, wide-lumened mucilage ducts are formed. The cortical layer provides mechanical strength to the cauloid, and its outer parts, consisting of small cells with METABOLISM/14.html">Chloroplasts, function as assimilatory tissue; growth in thickness also occurs partly here. The medulla serves for the storage and conduction of nutrients. It consists of multicellular filaments (so-called hyphae) that flare out like trumpets at the transverse walls. In species of some genera (e.g., Nereocystis and Macrocystis), the cross-walls of these filaments are perforated like sieves. The transport function of these elements has been demonstrated using radiolabeled carbon compounds. These sieve tubes are similar in Structure and function to the sieve tubes of vascular plants discussed earlier (see Fig. 5.10).

5. Order: Fucales. Due to the extreme reduction of the gametophyte, these algae can be regarded practically as pure diplonts (see Fig. 11.87, D). Reproduction occurs via oogamy. The alternation of nuclear phases is gametic, i.e., meiosis occurs during gamete formation. The diploid sporophyte (Fig. 11.85) forms the only vegetative body in the life cycle, in the form of a thallus that sometimes reaches 1 m or more in length. In perennial Fucus species, the leathery, ribbon-like, dichotomously branched thalli are reinforced by a kind of "midrib". They are attached to the rocky substrate by a holdfast. In some Fucus species, the tips of the thallus branches (apical cell — see Fig. 5.12, B) are somewhat swollen and bear closely spaced, pitcher-like cavities called conceptacles¹ (Fig. 11.86, A), in which the receptacles of ♂ and ♀ sex cells (spermogonia and oogonia, respectively) are located among sterile hairs (paraphyses).

¹ In Russian literature, another name for conceptacles, "scaphidia", is also widely encountered. — Translator's note.

Fig. 11.85. Phaeophyceae, Fucales: A — Sargassum bacciferum, B — Himanthalia lorea; C — Ascophyllum nodosum; D — Fucus_vesiculosus; E — Fucus serratus, tip of the thallus (A — E — 1/4x)

In some species, spermogonia and oogonia are located in the same conceptacles (monoecy, e.g., in Fucus spiralis, Fig. 11.86, A); other species are dioecious (e.g., F. serratus and F. vesiculosus).

Fig. 11.86. Phaeophyceae, Fucales: A — Fucus spiralis, bisexual (monoecious) conceptacle with oogonia (o) of various ages (25x); B — F — Fucus vesiculosus: B — branch with spermogonia (a) (200x), C — release of sperms from the spermogonium (250x), D — young oogonia, E — the Contents of the oogonium divide into eight egg cells after its release from the wall, F — liberation of egg cells (D — F — 120x); p — paraphyses

The parts of the thallus bearing conceptacles are shed annually. A varying number of mitoses occur within the reproductive organs. These organs can be regarded as unilocular, sexually differentiated meiosporocysts, and the primary products of meiosis as meiospores. The cells formed in these unilocular organs As a result of mitotic nuclear divisions represent, to some extent, extremely reduced gametophytes that have lost all independence; they appear to be completely integrated into the corresponding (oogonia or spermogonia) meiosporocysts (reproductive organ = meiosporocyst = gametangium). From the 4 haploid cells formed after meiosis, 8 egg cells develop in the oogonium after a further mitosis, and 64 spermatozoids develop in the spermogonium after four mitoses.

Oogonia (Fig. 11.86, A: o, D) are large, rounded structures seated on a unicellular stalk. The oogonial wall consists of three layers. Upon maturation, initially only the outer layer of the wall ruptures, so that the 8 egg cells remain enclosed by the two inner layers when they leave the conceptacle (E). In seawater, the innermost layer of the wall finally ruptures as well, so that the 8 egg cells (♀) float freely in the water Column, independent of each other (♂).

Spermogonia are oval cells closely appressed to abundantly branched short filaments (Fig. 11.86, A: a; B). The wall of the spermogonium consists of two layers. The inner-

wall is retained and encloses 64 spermatozoids (♂) when the entire packet is extruded from the conceptacle upon maturation due to mucus secretion. The spermatozoids consist predominantly of nuclear material and a single rudimentary chloroplast containing an eyespot; they are equipped with two flagella (in contrast to other brown algae, the forward-directed tinsel flagellum is shorter).

The released spermatozoids (C) are attracted to the egg cells by the pheromone fucoserratene (cf. 8.2.1.1). The initially naked zygote becomes surrounded by a cellulose-containing wall, attaches to the substrate, and, undergoing cell divisions, grows into a new diploid sporophyte (see Fig. 7.23).

The Fucales represent the pinnacle of an evolutionary Lineage characterized by the progressive reduction of the gametophyte. This is clearly visible when examining the features of certain Laminariales. In these, the male gametophyte can be reduced to a single cell, while the contents of the settling meiozoospore emerge and directly transform into an egg cell. The diploid Fucus plant can thus be interpreted as a sporophyte whose meiospores directly become an almost completely vanished gametophyte.

In many species of Fucales (as in most larger Laminariales), the thallus is also kept upright in the water by air bladders; these allow it to sway in the waves without being swept along the bottom. Fucus species form stunted stands in the shallow waters of Northern European seas; at low tide, they are temporarily exposed to the air but are protected by the mucus (fucoidan) secreted by the plants, thereby even retaining their capacity for Photosynthesis.

Fucus serratus has a thallus with serrated margins; the thallus of F. vesiculosus features rounded, gas-filled air bladders. Ascophyllum nodosum, found in the same localities, also possesses air bladders (see Fig. 11.85, C). In Himanthalia (B), the thallus consists of a top-shaped base, from which one to several strap-like, dichotomously branched vertical branches arise. More abundant branching of the thallus is observ-

ed in Representatives of the genus Sargassum (250 species!), which are widespread in warm seas. Due to the presence of air bladders, some of its species float freely in countless clumps in the Sargasso Sea of the Atlantic Ocean (from the Gulf Stream and the West Indies to the Azores). Here, they reproduce exclusively vegetatively by fragmentation of the thalli (A). This group also includes the genera Cystoseira and Halidrys; Pelvetia (see Fig. 11.107), Coccophora, and Durvillaea (Box 11.7, p. 199).

In brown algae, a progressive transition from isogamy to oogamy via anisogamy can be traced. In ancestral forms (Ectocarpus, see Fig. 11.80, A), the gametangia of both sexes are pluricellular and identical in appearance. In more highly organized forms (e.g., Cutleria), the increase in the size of the ♀ gametes is accompanied by an increase in the size of the gametangia, with a simultaneous decrease in the number of locules. In Dictyota and Laminaria, this leads to The formation of only a single egg cell. While the spermogonia in Dictyota are still pluricellular, in Laminaria they are, like the oogonia, already unicellular, containing a single spermatozoid. However, such character development does not allow any Conclusions to be drawn about the actual phylogeny. The alternation of generations (Fig. 11.87) is either isomorphic (Isogeneratae) or heteromorphic (Heterogeneratae), with the similarity of generations being regarded as the ancestral state, whereas the dominance of the sporophyte (first observed in Ectocarpales) is considered a more advanced state.

Phylogeny. Brown algae apparently shared a common ancestor with golden algae and diverged into separate evolutionary lineages early on. Despite their sometimes very substantial size, brown algae have generally been poorly preserved in the fossil record compared to the calcareous forms of green algae. It is highly probable that they already existed in the Silurian and Devonian. Certain Lower Devonian and Silurian 'trunks' (Nematophycus = Prototaxites) as thick as a human thigh, which consisted of an interwoven mesh of tubular filaments and terminated in massive clusters of lobes resembling Laminaria blades, were also likely brown algae.

Fig. 11.87. Phaeophyceae. Alternation of generations and nuclear phases in some brown algae (A–D). The haploid phase is shown by light lines, the diploid by dark lines; G — gametophyte; S — sporophyte; O — zygote; R! — reduction division

Overview of Heterokontophyta. Broadly speaking, the Heterokontophyta reflect the entire diversity of organizational forms found in algae. In this sense, this division appears highly heterogeneous. On the other hand, the relationships among various forms are evident through a series of shared 'conservative' characters, which are likely either selectively neutral and stable, or absolutely essential for the continued survival of the species. Based on The structure of motile cells and chloroplasts, as well as the composition of chlorophylls, accessory pigments, and storage products, the Heterokontophyta can be characterized as an evolutionarily coherent group of algae.

Possessing parenchymatous thalli, the brown algae are among the most highly developed marine plants. In the differentiation of their vegetative body and the presence of conducting elements analogous to sieve tubes, they resemble the structure of higher plants. The Increasing complexity of the vegetative body in response to adaptation to diverse environmental conditions is an evolutionary principle realized across very different plant groups. In this context, it is consistently observed that a single evolutionary lineage (a group of related forms) develops through the diverse differentiation of various adaptive forms, and conversely, different evolutionary lineages, in adapting to similar living conditions, can evolve similar organizational forms (convergence!). An example of the first premise is the various levels of organization (Heterokontophyta. The second premise is supported by several parallel evolutionary lineages among the Heterokontophyta and green algae, which are discussed below.

A distinctive feature of the Heterokontophyta is the frequent use of silicic acid in the formation and Structuring of a rigid cell wall. Plates,

scales, and silicic acid deposits are not characteristic of all representatives. For instance, they are completely absent in the naked cells of the Chloromonadophyceae and occasionally in other classes. On the other hand, as an exception, they occur in other algal divisions (e.g., Pediastrum among the Chlorobionta). Nevertheless, a clear tendency toward the formation of these structures can be traced here. Cellulose is clearly not the preferred material for cell wall construction. In the Chloromonadophyceae, a rigid cellulose wall is absent, and in other classes, with the exception of the brown algae, it is not widely represented.

Adaptation to terrestrial life. Inhabitants of moist soil include, for example, various diatoms, and among the yellow-green algae, Botrydium (see Fig. 11.73, C). To the same class belongs the aerophilic alga Capitulariella, which disperses by wind-dissemination of entire sporocysts (which can be functionally equated to aplanospores) (see Fig. 11.72, C). The male reproductive cells of the Heterokontophyta possess chloroplasts (when anisogamy or oogamy occurs), which are often highly reduced.

Progressive evolutionary development toward pure diplonts occurs independently in different advanced evolutionary lineages. Thus, all diatoms are diplonts, and furthermore, the Pennales no longer possess flagellated gametes. Among the brown algae, there is an increasing dominance of the sporophyte generation in the life cycle and an evolution toward virtually pure diplonts. In oogamous representatives, the female reproductive cells have become non-motile egg cells.