Classification of Chaetophorales (With Diagram) | Algae

The following points highlight the four families under which chaetophorales has been classified. The families are: 1. Chaetophoraceae 2. Coleochaetaceae 3. Trentepohliaceae 4. Pleurococcaceae.

Family # 1. Chaetophoraceae:

Chaetophoralean algae are heterotrichous in habit frequently bearing terminal or lateral hair-like projections. In their heterotrichy both the erect and prostrate portions of the plant may be well developed (e.g., Stigeoclonium) or with an elaborate erect portion and feebly developed prostrate portion (e.g., Chaetophora, Draparnaldia, Draparnaldiopsis, Fritschiella).

Cells are with thin to gelatinous walls, usually uninucleate having a single more or less dissected plate-like or ring-like chromatophore with pyrenoids. Asexual reproduction is by biflagellate or qudriflagellate zoospores and sexual reproduction isogamous.

Genus Chaetophora:

This alga occurs abundantly in fresh-water being attached to some substratum preferably to submerged plant parts exhibiting a gelatinous macroscopic growth.

Outstanding characteristics lie in the form of the vegetative body known as the hetero­trichous filament being differentiated into feebly developed prostrate system often represented by loosely connected rounded cells, and a projecting system of branched filaments, apices of which are terminated by long hyaline hair-like structures known as setae (Fig. 48A & B).

The aerial branches are held together within a compact muci­lage forming a tough envelope. The prostrate system may be impregnated with calcium carbonate. Cell structure and the mode of reproduction being very similar to Ulothrix may suggest common ancestry and development being along two divergent lines.

Each cell contains a single banded chloroplast with several pyrenoids (Fig. 48C). Asexual reproduction is by quadriflagellate zoospores. Sexual reproduction is isogamous by the formation of biflagellate gametes.

Special Features of Chaetophoraceae :

1. Vegetative body heterotrichous being differentiated into feebly developed prostrate system and an elaborate projecting system of branched filaments, apices of which are terminated by long hyaline hair-like structures known as setae.

2. Banded chloroplast.

3. Isogamous sexual reproduction by the formation of biflagellate gametes.

Genus Draparnaldia:

Draparnaldia is a fresh-water alga. Its vegetative body indicates evolutionary tendency towards the development of elaborate aerial system as against a very insigni­ficantly developed prostrate portion. The plant body is macroscopic and is pale-green in colour (Fig. 49A). The projecting part of the alga is composed of a main axis consist­ing of large, more or less barrel-shaped cells with barrel-shaped chloroplast having toothed edges forming a girdle in the equatorial region (Fig. 49B).

The chloroplasts have several pyrenoids (Fig. 49D). The cells of the main axis are all alike and are uninucleate. From these cells arise short lateral branches in clusters, alternate, opposite, or verticillate, richly branched whose apices are drawn out into long hyaline setae.

These branches may be borne on any cell of the main axis except the lower region from which arise numerous multicellular rhizoidal branches serving the function of anchorage of the alga to its substratum (Fig. 49C).

Cells of the lateral, branches are cylindrical, much smaller than those of the main axis, and are uninucleate with a more or less parietal chloroplast with a single pyrenoid. The main axis is completely covered by lateral branches which grow very profusely. Formation of elements con­nected with reproduction is confined to the short branches.

Asexual reproduction is by means of quadriflagellate macro- and micro-zoospores and by aplanospores (Fig. 49E & F). The zoospores are pear-shaped.

They are pro­duced one to four in each cell. The zoospores arc formed simultaneously in all the cells of the lateral branches and are liberated through an aperture in the lateral wall (Fig. 49E). The zoospores swim around in water, lose their flagella and develop int6 new plants.

During unfavourable, conditions cell contents of the lateral branches may develop into thick-walled aplanospores instead of producing zoospores. The contents of each ceil recede from the cell wall and the whole structure comes to rest by developing a thick wall. On the return of favourable conditions the old wall is cast off and the new walls are developed. It then directly gives rise to a new individual.

Sexual reproduction is isogamous. The isogametes are quadriflagellate (Fig. 49G) like the zoospores, they are also produced in the cells of the lateral branches and are liberated out through an aperture of the lateral wall. Prior to union, the gametes lose their flagella and become amoeboid (Fig. 50). The amoeboid gametes fuse in pairs resulting in the formation of a zygote (Fig. 49H & I).

The zygote develops a wall (Fig. 49 J) and germinates into a new plant. Behaviour of the zygotic nucleus has not yet been confirmatively worked out. Gases of parthenospore development have been reported.

Some Indian species of Draparnaldia:

Draparnaldia distans Kg. var. elongata-, D. glomerata Agardh; D. groenlandica Bachmann; D. mutabilis (Roth.) Bory.; D. plumosa (Vauch.) Ag.

Special Features of Draparnaldia:

1. Heterotrichous macroscopic plant body having an elaborate aerial system and a very insignificantly developed prostrate system.

2. Barrel-shaped chloroplast with toothed edges.

3. Asexual reproduction by quadriflagellate macro- and microzoospores and by aplanospores.

4. Isogamous sexual reproduction by quadriflagellate gametes. Prior to union the gametes lose their flagella and become amoeboid and then fuse in pairs.

Genus Draparnaldiopsis:

Draparnaldiopsis is a fresh-water alga usually found in shallow pools and ponds. .It grows affixed to aquatic angiosperms. The heterotrichous plant body has a remark­ably elaborate aerial portion and an inconspicuous prostrate portion represented by rhizoidal branches with which the alga attaches itself to its substratum (Fig. 51A & E).

The main axis of the erect portion exhibits advanced characteristics of being composed of two types of cells: the short cells bearing branches of limited and unlimited growth developed in whorls, alternate with long cells in regular succession resembling nodes and internodes of higher plants (Fig. 51C, D & F).

In some species the branches have the tendency to coil around the main axis (Fig. 51 A) giving a somewhat corticated appearance. The entire plant body is enveloped by a thick gelatinous sheath. Cells are uninucleate having zonate chloroplasts with several pyrenoids (Fig. 5IB).

The alga reproduces asexually by three types of zoospores: quadriflagellate ellip­soidal to oval macrozoospores; quadriflagellate spherical microzoospores; and bi­flagellate spherical, ovoid to obovoid microzoospores (Fig. 52G to J). All the three types of zoospores are formed in the same plant and only from the cells of the lateral branches and never from the main axis.

All the cells of the lateral branches except the basal one are capable of producing zoospores. Each cell produces a single zoospore (Fig. 52E & F).

Both quadriflagellate macro- and micro-zoospores are developed out of the cells of the lateral branches of unlimited growth (Fig. 52 A to F). They are diploid in nature and produce diploid plants—the sporophytes. Whereas, the biflagellate micro-zoospores are formed from the cells of the lateral branches of limited growth, during which meiosis takes place.

These zoospores after being liberated swim around in water, lose their flagella, round up and produce new individuals resembling parent plant—the sporophyte. But these are haploid gamete producing plants—the gametophytes.

Sexual reproduction is usually isogamous, but cases of anisogamy have also been reported. Similar to the zoospores, biflagellate spherical to ovoid gametes (Fig. 53B) & C) are produced only from the cells of the lateral branches of limited growth borne on the gametophytic plants.

Prior to the development of the gametes the cells divide into a row of daughter cells. The contents of each daughter cell metamorphose into one to several gametes (Fig. 53A) which on being liberated out by the gelatinization of the cell wall of the parent plant swim in water.

The gametes from two different gametophytes pair together at the anterior end and finally fuse with each other resulting in the formation of a zygote (Fig. 53D to G).

The zygote germinates immediately into new plant without going to rest (Fig. 53H to L) and produces a diploid plant —the sporophyte which resembles the gametophytic plant morphologically (Fig. 54). The alga Drapamaldiopsis thus exhibits isomorphic alternation of generations (Fig. 54).

Draparnaldiopsis has four main features which suggest affinity with the Charo­phyceae:

(i) It is differentiated into nodes and internodes,

(ii) It has a cortex-like covering derived from the basal cells of the laterals,

(iii) The laterals have limited, growth and,

(iv) The reproductive organs are restricted to the laterals.

Some Indian species:

Drapanaldiopsis indica Bharadwaja; D. indica var. robusta Islam & Ahia.

Special features:

1. Heterotrichous plant body composed of upright thread and poorly developed rhizoidal branch performing the function of anchorage.

2. The upright thread is differentiated into main axis and lateral branches. The main axis again, is composed of long and short cells arranged alternately, and. the lateral branches are arranged in whorls originating from the short cells.

3. The alternate arrangement of short and long cells resembles nodes and inter- nodes of higher plants.

4. Zonate chloroplast.

5. Asexual reproduction by quadriflagellate macro- and micro-zoospores and by biflagellate micro-zoospores.

6. All the zoospores are developed from the cells of lateral branches.

7. Sexual reproduction is isogamous, cases of anisogamy have also been reported.

8. Gametes are formed from the cells of the lateral branches.

9. Isomorphic alternation of generations.

Genus Fritschiella:

Fritschiella grows gregariously on moist alkaline soil and on silt of drying pools and similar other habitats with a tendency of terrestrial habit.

It has a highly deve­loped heterotrichous plant body which is differentiated into a distinctly septate rhizoi­dal system consisting of downwardly directed rhizoid-like filament composed of elongated colourless cells, joined end to end; a prostrate system of rounded or irregularly swollen clusters of cells appearing to serve the purpose of perennation; a primary projecting system having short-celled branched threads; and a secondary projecting system being composed of a tuft of slightly elongated branches with longer cells (Fig. 55A-G).

In an immature plant, the prostrate system is very ill-developed in comparison with the projecting system. In a mature plant, only the secondary projecting system which arises as a tuft on the surface of the mud, is visible. The rest of the plant body remains buried in the damp soil. All the cells of the plant body are uninucleate.

The cells of the rhizoidal system have scanty contents. Whereas, the secondary projecting system has cells with dense contents having curved plate-like chloroplast containing two to eight pyrenoids in each (Fig. 55D & E).

The alga produces three types of swarmers. The formation of swarmers is strictly confined to the prostrate system portion. The swarmers are: the qudriflagellate macro- zoospores formed singly in each cell; the quadri-, biflagellate micro-zoospores developed two to four in each cell; and the biflagellate isogametes are formed in large numbers in each cell (Fig. 55F to J).

Both macro- and micro-zoospores (Fig. 55K & L and Fig. 56A & B) are formed simultaneously by the same plant—the sporophyte.

Whereas, the gametes are produced from different plants—the gametophytes. Both kinds of zoospores on liberation germinate to produce a new plants (Fig. 56C to N). The new plants so produced though morphologically indistinguishable, possess different nuclear constitution.

The ones produced by the quadriflagellate zoospores are diploid— sporophytic plants, and those by biflagellate zoospores are haploid—the gameto­phytic plants, since meiosis takes place during the formation of biflagellate zoospores.

Sexual reproduction is isogamous. The haploid gametophytic plants produce gametes. The gametes after being liberated from two different gametophytic plants, swim around in moisture for a while, meet in pairs (Fig. 56 O & P).

They then lose their flagella, gradually round off forming a zygote which germinates directly without any rest (Fig. 56 O &P) producing a diploid plant—the sporophyte which morpho­logically resembles the haploid gametophytic plants. The alga thus exhibits isomor­phic alternation of generations (Fig. 57).

The life history of the alga exhibits certain points of interest.

They are: the tendency of terrestrial habit accompanied with normal growth under highly alkaline condition; very clear distribution of labour indicated by photosynthetic sub-aerial portion, the prostrate system serving as an organ for the production of reproductive structures as well as perennation, and the underground rhizoidal portion performing the function of anchorage; the thallus resembles very well the gametophyte of some species of Equisetum.

Besides all these, the alga exhibits isomorphic alternation of generations and the sexual reproduction is isogamous.

Indian species of Fritschiella :

Fritschiella tuberosa Iyeng.

Special features of Fritschiella:

1. Highly developed heterotrichous plant body differentiated into septate rhizoidal system, prostrate system of rounded or irregularly swollen clusters of cells, and erect system of a primary projecting system composed of short-celled branched threads and a secondary projecting system being composed of a tuft of slightly elongated branches with longer cells.

2. Curved plate-like chloroplast.

3. Asexual reproduction by quadriflagellate macro- and microzoospores and by biflagellate micro-zoospores.

4. Isogamous sexual reproduction by biflagellate gametes.

5. All swarm cells are developed from the cells of the prostrate system.

6. Isomorphic alternation of generations.

Family # 2. Coleochaetaceae:

The family Coleochaetaceae is represented by a single genus. Coleochaete. This alga usually occurs as an epiphyte on fresh-water plants such as, Hydrilla and Potamogeton, and also occurs attached to the fronds of Lemna. It has also been reported to grow endophytically in the cells of Char a and Nitella.

Its plant body exhibits different degrees of heterotrichy like—a definite prostrate system giving rise to the projecting system, and only prostrate system present and the projecting system has disappeared.

The project­ing system grows by an apical cell and the prostrate system by means of marginal meristem. The vegetative cells are characterized by a laminate chloroplast and by the presence of sheathed bristles or hairs. Reproduction is both by asexual and sexual means. Asexual reproduction takes place by biflagellate zoospores. Sexual reproduction is oogamous.

The ovum is borne in an oogonium having a long neck, the trichogyne. It is fertilized by a biflagellate antherozoid produced in an antheridium. The zygote is retained within the enlarged basal portion of the oogonium which is surrounded by pseudoparenchymatous investment.

The first division of the zygotic nucleus is meiotic and by subsequent mitotic divisions 32 biflagellate zoospores are produced which give rise to new thalli.

Genus Coleochaete:

This is an epiphytic alga occurring on fresh-water plants, for example, Hydrilla, Potamogeton, Lemna, etc. Some species may be also endophytic inhabiting in the cells of Nitella and Chara.

There are quite a few heterotrichous species whilst others appear­ing either as hemispherical cushions composed of prostrate threads with branches dis­tinct from one another exhibiting apical growth or as one-layered parenchymatous discs formed out of laterally apposed branches showing growth by a marginal meristem (Fig. 58A & B).

The thallus is often enveloped in mucilage. In all these forms the thallus bears a single, long, unbranched cytoplasmic seta whose base is ensheathed by a gelatinous material (Fig. 58G). The cells are uninucleate with a large parietal chloro­plast containing one or two pyrenoids.

Asexual reproduction is effected by large ovoid biflagellate zoospores (Fig. 58H) produced singly in the ordinary cells of the thallus. The zoospore escapes through a small pore formed at the apex of a short papilla. On germination the zoospore divides either horizontally or vertically into two daughter cells, one of which divides to form hair-like structure whilst the other develops into creeping thread.

Sexual reproduction is a type of advanced oogamy. There are both homothallic or heterothallic species. The oogonium is a flask-shaped structure with a swollen base and colourless neck, the trichogyne (Fig. 581). Oogonia are borne terminally on short branches of the projecting system.

In the discoid forms, their position is also terminal (Fig. 58E to G). The antheridia are developed in clusters at the ends of the projecting branches from which they are cut off as small colourless cells (Fig. 58D).

In the discoid thallus, they are developed from marginal or midway cells. Each anthe­ridium produces a single oval or spherical biflagellate antherozoid which is set free by the breaking down of the wall at the apex of the anteridium. The liberated anterozoid then swims towards the opened neck of the oogonium, enters through the neck, and finally fuses with the egg within the oogonium.

After fertilization the neck of the oogonium is cut off by a septum, the basal part enlarges, the zygote secretes a heavy wall, and increases greatly in size (Fig. 58 J & K). At the same time branches arise from the underlying and other adjacent cells and unite to form a complete pseudoparenchy­matous envelope which more or less completely encloses the base of the oogonium(Fig. 58L).

The pseudoparenchymatous investment takes a red or reddish-brown colour. The entire structure is known as a spermocarp (Fig. 58B & M). The alga perennates in the spermocarp stage. With the advent of a favourable season, the zygote nucleus divides reductionally.

With further division as many as 32 daughter nuclei are formed. Cytoplasmic division by the production of wall perpendicular to the long axis of the original oogonium continues simultaneously with the nuclear division ultimately forming of 32 wedge-shaped cells. Each cell then gives rise to a zoospore by the bursting of the envelope.

There is no difference in form and behaviour between these zoospores and the ordinary zoospores. All the zoospores are liberated out by the breaking of the spermocarpic wall into two halves (Fig. 58M). The zoospores then develop directly into new thalli after swarming for a short period of time (Fig. 59).

The mode of development, structure, and behaviour of the spermocarp need particular discussion. During its development there is a clear mechanism to afford protection to the diploid oosporic nucleus against external adverse condition. That the first division of the diploid nucleus is meiotic has been clearly demonstrated by C.E. Allen in 1905.

The ultimate production of zoospores by reduction division clearly indi­cates that the spermocarp which behaves as a sporophyte alternates with the thalloid gametophyte to complete the life cycle of Coleochaete.

As such, there is somewhat indication of alternation of generations in the life cycle of this alga. Besides the development of spermocarp, the alga shows advancement in the thalloid gametophyte producing male and female gametes. The structure of the oogonium is also most advanced in the entire green algae for having long neck-like structure and a bulbous base.

Some Indian species of Coleochaetaceae:

Coleochaete orbicularis Pringsh.; C. scutata Breb.; C. soluta Pringsh.

Special features of Coleochaetaceae:

1. Heterotrichous plant body with well-developed projecting system and a prostrate system represented by a thallus which appears to be a parenchymatous disc.

2. Asexual reproduction by biflagellate ovoid zoospores.

3. Sexual reproduction oogamous.

4. The oogonium is a flask-shaped structure composed of enlarged basal portion and a long neck— the trichogyne.

5. The male gametes are biflagellate oval.

6. Elaborate post-fertilization stages.

7. Formation of specialized structure—spermocarp during post-fertilization stages.

8. The spermocarp is composed of post-fertilization oogonium enveloped, com­pletely by filamentous structures forming .a pseudoparenchymatous investment.

9. Meiosis takes place in the spermocarp.

10. The spermocarp produces biflagellate swarmer’s which give rise to new indivi­duals.

Family # 3. Trentepohliaceae:

Members of this family grow as epiphytes and parasites on and in the leaves of various angiospermic plants. Some constitute the algal component of a number of lichens, Cephaleuros virescens causes the red rust of tea plant. The widely distributed genus Trentepohlia occurs attached to rocks, tree-trunks, etc. The plant body is composed of branched threads comprising of heterotrichous conditions.

In the majority of cases the Trentepohlias appear as orange-yellow cushions due to the presence in the cells orange-red haematochrome dissolved in fat. Besides vegetative, reproduction is usually by the development of sessile, stalked, and funnel-shaped sporangia.

The sessile sporan­gia often behave as gametangia and produce isogametes, the stalked sporangia bear bi- or quadri-flagellate swarmer’s, and the behaviour of funnel-shaped sporangia is still obscure.

Genus Trentepohlia:

It is an aerial alga growing epiphytically as orange-yellow cushions or felt-like layer on bark of trees and other damp situations. Plants can withstand prolonged desiccation without any appreciable change. Some species also constitute the algal component of a number of Lichens.

The plant body is composed of heterotrichous filament. In the majority of the species, the thallus exhibits a differentiation of aerial as well as prostrate systems represented by richly branched threads. But there are some species where both prostrate and aerial systems are not equally well developed (Fig. 60D, E & L).

The branches of the aerial system are opposite, alternate, or unila­teral arising either from the tip of the parent cell or sub-terminally or even from the middle and are generally slightly attenuated (Fig. 60A to C). The cells are cylindrical or barrel-shaped and often length being twice the breadth (Fig. 60L). The cells have thick cellulose walls which are stratified with parallel or divergent layers (Fig. 60C & F).

The free ends of the apical cells are often covered by pectose caps (Fig. 60F to I). In some species, during the growth of the apical cells, the pectose caps get pushed aside and remain attached to the lateral walls of the older cells (Fig. 60J & K). The cells have numerous band-shaped or discoid chloroplasts without pyrenoids.

The characteristic orange-yellow colour of the cells is due to the presence of orange-red haematochrome dissolved in fat occurring as globules around the chloroplasts.

The function of this special pigment is not yet clearly known. According to some, the haema­tochrome is a kind of reserve food, whereas, others consider it as a light screen. Cells when young, are uninucleate and with maturity become multinucleate. Vegetative 8 [vol. ii] reproduction is through fragmentation.

Bi- or quadriflagellate swarmer’s are developed in three different kinds of sporangia:

(i) Sessile sporangia, these may be terminal, lateral, intercalary, or rarely axillary in position (Fig. 60D and Fig. 61A to G). They are formed by enlargement of cells and liberate biflagellate swarmer’s without being detached from the mother plant. These swarmer’s behave as isogametes (Fig. 61N to R). Sessile sporangia are quite commonly produced on the prostrate portion of the plant. They behave as gametangia.

(ii) Stalked sporangia are formed terminally or laterally by cutting off from a tubular outgrowth developed from enlarged supporting cell. The apical portion of the outgrowth swells to form sporangium and separates from the stalk cell underneath, which is often bent to assume a knee-shaped appearance (Fig. 61D & E).

These sporangia produce bi- and quadriflagellate swarmer’s which behave as zoospores. Stalked sporangia are usually developed on the projecting system of the plant (Fig. 61F to H).

(iii) Funnel-shaped sporangia formed from the apices of the terminal cells, become detached at maturity (Fig. 61 I to M). Their function has not yet been estab­lished.

Both sessile and stalked sporangia may be developed on the same plant or on separate plants. The development of sessile and stalked sporangia in the life cycle of the alga is suggestive of the possibility of alternation of generations.

Indian species of Trentepohliaceae:

Trentepohlia monilia de Wilderman.

Special features of Trentepohliaceae:

1. Epiphytic alga forming orange-yellow cushions or felt-like layer on damp substrate.

2. Plant body with adaptations to withstand prolonged desiccation.

3. Heterotrichous plant body, aerial as well as prostrate systems with or without being equally well developed.

4. The branches of the aerial system are opposite, alternate, or unilateral.

5. Cells with stratified wall, special pigment—orange-red haematochrome and band-shaped or discoid chloroplasts.

6. Free ends of the apical cells are covered by pectose caps.

7. Bi- or quadriflagellate swarmers are developed in specialized structures which according to the behaviour of the swarmer’s are gametangia or sporangia.

8. Biflagellate swarmer’s usually behave as isogametes and quadriflagellate ones as zoospores.

Family # 4. Pleurococcaceae:

This family is best known for its genus Pleurococcus, also called Protococcus. It includes terrestrial algae occurring in all kinds of damp situations as a thin incrustation of green coat on the windward side of stones walls, tree-trunks, etc. The cells can withstand prolonged desiccation without change.

The cells, which are globose in shape and occasionally branched, are single or else as many as four may be united into a group. Under certain cultural conditions branching may be copious. The sole method of reproduction is through vegetative division in three planes.

Genus Pleurococcus (or Protococcus):

This is a simple alga growing in moist situations on the bark of trees, old walls, flower pots, and similar other places. It may occur singly as isolated individuals (Fig. 62A), or as small compact masses or groups of two, four or eight individuals (Fig. 62B to E). When adhering together, the sides in contact are rather flattened.

This unicellular alga carries on all essential life processes within the single cell. From the physiological point of view it has the same characteristics as higher plants.

The cells can withstand prolonged desiccation. The resistance of the cells to desiccation is aided by a highly concentrated cell sap and a capacity to imbibe water directly from the air. Occasionally, when submerged under water, cell division may continue with the production of a colony or the daughter cells may grow out into short filaments or threads (Fig. 62F to H).

For this character Pleurococcus is considered to be reduced and simplified form originating from a filamentous one in which both vegetative structures and repro­ductive functions have retrogressed.

Each cell is spherical with a cellulose wall, and is densely filled with protoplasm containing a large lobed chloroplast occupying the peripheral portion of the cell, and a nucleus containing a nucleolus (Fig. 62A). The rest of the protoplasm is colourless. The only method of reproduction is by fission.

The cells divide freely into two, with successive divisions taking place in three directions and at right angles to each other (Fig. 62B & C). The daughter cells may either round themselves off and separate immediately after each division, or may remain adhered together (Fig. 62D & E) for a few generations and later they fall apart.

Some Indian species of Pleurococcaceae :

Pleurococctis cohaerens Kutz.; P. viridis Ag.

Special features Pleurococcaceae:

1. Unicellular alga occurring singly or in compact masses.

2. The cells can withstand prolonged desiccation.

3. Large lobed peripheral chloroplast.

4. The only method of reproduction is by fission. The daughter cells either may remain adhered together for a long time or may produce a short filamentous structure.

There is probably no other algal genus about which there has been so much con­fusion. Phycologists are not even in agreement as to whether the genus should be called Pleurococcus or Protococcus. This confusion is partly due to the fact that any unicellular aerial green alga has been considered a species of Pleurococcus.

Some authorities have even placed the genus Pleurococcus in the Chlorococcales, while others have treated it in a special group, the Pleurococcales.

In recent years it has been realized that the unicellular aerial green algae whose cells divide vegetatively and do not produce swarmer’s, belong to the Pleurococcaceae. Besides this, as the alga Pleurococcus can occasionally develop branched filaments there would seem to be evidence for regarding it as a much reduced member of the Chaetophorales.