Classification of Ceramiales | Algae

The following points highlight the two families under which ceramiales has been classified. The families are: 1. Ceramiaceae 2. Rhodomelaceae.

Family # 1. Ceramiaceae:

Plants are usually bushy; branches usually terete, in some genera uniseriate, in others corticated; growth from the tip producing an axial row of cells, true colourless hairs often present, also hyaline hair-like extensions from the tips of branches; cordcation if present usually developed first about the nodes, consisting of single ring of cells, or spreading over the internodes in mote or less filamentous fashion, later appear­ing parenchymatous, or else consisting of an investment of rhizoidal filaments.

Spo­rangia are superficial or stalked on the branches, single or whorled at the nodes, or in corticated species in the cortex, ultimately dividing in tetrapartite or tetrahedral fashion.

Spermatangia are developed on special determinate branchlets, forming small colourless clusters, or covering the cortex portions of larger species. Carpogenic branches are of four cells, borne on supporting cells, which may also give rise to sterile cells and after fertilization to one or two auxiliaries near the carpogonium.

Cystocarp is composed of groups of gonimoblast filaments, the outer cells of which produce the carposporangia; cystocarps naked, enveloped in jelly, or partly enclosed by subtending filaments from below.

Genus Ceramium:

Ceramium occurs very commonly between tide levels and in deeper water being especially abundant along the Mediterranean coast. Some species may also thrive well in river water. The richly branched uniaxial plant body is recognizable from the banded appearance and the tong-like forking of the apical region of the branches, these are considered very valuable diagnostic characters (Fig. 122B and Fig. 123A, C & F).

The thallus, when young, is attached to the substratum by a cushion-like structure, which with maturity develops into rhizoids. The typical dichotomous branching is initiated by the forking of the dome-shaped apical cell (Fig.122A & C).

The plant body is made up of a single row of large cylindrical or barrel-shaped cells arranged one upon the other. The banded appearance of the filaments is developed as a result of the formation of an envelope of small-celled cortical cells cut out from the central axial cell (Fig. 122D).

This development of envelope is not continuous throughout the entire surface for which there results the banded appearance of the filaments. The colour of the plant body is extremely variable, may be red, yellowish-green, or reddish- brown, depending on whether a particular filament is inhabiting in deep or shallow water. Ceramium possesses specialized diplobiontic forms in the life cycle.

Ceramium has both monoecious and dioecious species. The antheridia occur in clusters on the upper side of the lateral branchlets. The antheridial mother cells arise by division from the antheridial protuberances which are formed subterminally from the cortical cells of the branchlets (Fig. 123B).

Each antheridium is spherical to oblong and unicellular-in structure. A single male cell—spermatium is developed from the entire contents of each antheridium. The spermatium is liberated through a narrow apical split on the antheridial wall. The spermatium is a highly vacuolated uninucleate structure.

The female organ or carpogonium is borne terminally on a short lateral branch known as carpogonial branch. The carpogonium consists of a swollen basal portion in which lies the female nucleus and a narrow elongated structure, the trichogyne which serves as a receptive organ for the spermatium.

An open passage for the male nucleus is developed due to dissolution of the walls at the point of contact of the spermatium and the trichogyne. The male nucleus travels down to the female nucleus in the basal portion of the carpogonium. In the mean- time the trichogyne becomes separated from the rest of the carpogonium by the development of a membrane at its base.

After fertilization an auxiliary cell with dense content is formed. Connection between the carpogonial base and auxiliary cell is maintained by the development of a connecting cell which is very short. The entire structure is known as pro-carp.

The diploid nucleus now passes in the auxiliary cell. A gonimoblast initial originates from the upper side of the auxiliary cell which gives rise to several tufts of threads in which all the cells, except the basal one, develop into carposporangia. The carposporangia are sur­rounded by a loose envelope of sterile branches, the entire structure developing into a cystocarp (Fig. 123C to E).

A carpospore on liberation from a carposporangium germinates to produce a diploid tetrasporic plant which morphologically resembles the gametophytic individual except that it bears tetrasporangia forming tetraspores (Fig. 123F). In the tetrasporic plants, tetrasporangia are developed in clusters from the enlarged cells of the cortical bands within which they remain partly embedded (Fig. 123G to J).

In each tetrasporangium four tetraspores are formed as a result of the meiotic division of the diploid nucleus of the tetraspore mother cell (Fig. 123K to M). The tetraspores germinate to produce gametophytes.

The life cycle of Ceramium includes a haploid gametophytic phase which bears sex organs and two diploid asexual phases—the carposporangia producing carpospores and the tetrasporic plant. These are the three successive phases that follow in regular sequence comprising isomorphic alternation of generations exhibited by the sexual and the tetrasporic individuals.

In some species the development of special spores—polyspores inside poly- sporangia has been recorded. They bear polyspores numbering mostly four or multi­ple of four. The polysporangia appear as large spherical structures projecting from the cortical bands.

Some Indian Species of Genus Ceramium:

Ceramium cruciatum Collins et Harv.; C. elegans Duel.; C. maryae Web.; C. strictum Grev. et Harv.; C. subdichotomum Web.; C. transversale Collins et Harv.; C. truncatum H. & P..

Special Features of Genus Ceramium:

1. Richly branched uniaxial plant body possessing banded appearance.

2. Tong-like forking of apical region of the branches.

3. Isomorphic alternation of generations.

4. Sexual reproduction advanced oogamy.

5. Elaborate post-fertilization stages.

6. Carposporangium and carpospores diploid.

7. Meiosis during the development of tetraspores in the tetrasporangium borne on sporophytic tetrasporic plant.

8. Triphasic life cycle.

Genus Callithamnion:

This is a marine alga possessing a delicate body of richly branched tufts of fila­ments (Fig. 124A). It attaches itself to the mud-covered rocks near the low water- level with the help of multicellular rhizoidal filaments emerging from the lower part of the main axis. A similar condition is encountered in epiphytic species.

There are species which persist more than one season, others are mostly annual. The main axis of the plant body is composed of thick-walled elongated cells bearing branches. (Fig. 124B). The branches develop from just below the septa of the cells of the main axis.

In a mature plant the development of the branches shows somewhat pseudo- dichotomous arrangement which may be due to some irregularity in the development of the cells of the main axis. Certain species at times produce a profusion of long slender hairs on the ultimate branches (Fig. 124B).

The filaments of the plant body are monosiphonous or at times may be corticated by the rhizoidal filaments in the mature region.

The cells of the vegetative body may be uninucleate or multinucleate having ribbon- shaped chromatophores. Callithamnion exhibits isomorphic alternation of generations.

Bodi antheridia and carpogonia are borne on separate plants. Antheridia form hemispherical or ellipsoidal tufts arising as lateral appendages on the branches of the male plant (Fig. 124C). The lateral appendages may produce crops of antheridia two or three times successively from the same place (Fig. 124D & E). The entire content of each antheridium develops into a spermatium (Fig. 124D).

The carpogonia are borne on female plants in combination with auxiliary cells, exhibit a primitiveness in character. The combination of a carpogonial branch and the two auxiliary mother cells is known as procarp. Of the two auxiliary mother cells only one bears the carpogonial branch. The carpogonial branch is four-celled, the apical cell of which is the carpogonium with long hair-like trichogyne (Fig. 125A & B).

A floating spermatium in water comes in contact with the trichogyne (Fig. 125A) and rests for a while when the walls between dissolve and the spermatial nucleus moves down the trichogyne and fuses with the carpogonial nucleus. Simultaneously with the process of fertilization the two auxiliary mother cells divide, each one forming a basal cell and an auxiliary cell.

Along with the development of the auxiliary cells a cytoplasmic connection is maintained from cell to cell. The carpogonial base along with the diploid nucleus divides producing two daughter cells, each one bearing one diploid daughter nucleus (Fig. 125C to G).

The two daughter cells extend laterally and ultimately fuse with the adjacent auxiliary cells, and thus the two diploid daughter nuclei are distributed in the auxiliary cells (Fig. 125H to J). Ultimately the two auxi­liary cells give rise to two gonimoblasts of the cystocarp. Cystocarps are usually developed in pairs (Fig. 125K and Fig. 126A) on branches of the female plant and are enveloped by mucilage.

Carposporangia are spherical bearing one carpospore each which germinates into a tetrasporophyte. The tetrasporophyte bears tetrasporangia (Fig. 126B). Each tetrasporangium ultimately produces four tetraspores (Fig. 126C) by the meiotic division of the diploid nucleus. The tetrasporangium dehisces by a lid and the tetraspores are liberated out, which on germination produce haploid gameto­phytes.

In the life cycle of Callithamnion there is a regular alternation of gametophytic and sporophytic individuals represented by haploid male and female gametophytes and tetrasporic plant respectively.

Special Features of Genus Callithamnion:

1. Plant body comprising richly branched tufts of filaments.

2. Development of branches from just below the septa of the cells of the main axis.

3. Pseudodichotomous development of branches.

4. Isomorphic alternation of generations.

5. Sexual reproduction advanced oogamy.

6. Development of cystocarp.

7. Cystocarp, carposporangia and carpospores all are diploid.

8. Meiosis takes place during the development of tetraspores in the tetrasporangia borne in the tetrasporic plant.

9. Triphasic life cycle.

10. Diploid phase partly dependent on the gametophyte and partly represented by an independent tetrasporic plant.

Family # 2. Rhodomelaceae:

Plants are usually bushy, sometimes sparingly branched; branches often delicate, usually terete, occasionally flat; growth from persisting apical cells producing an axial cell row, the principal branches of the thallus developed from successive segments; branched colourless hairs (trichoblasts) often present.

Axial cells are generally surround­ed, at least in the fruiting portions, by a series of pericentral cells cut off from them by longitudinal walls, producing a typically polysiphonous structure, and sometimes fur­ther corticated either by subsequent divisions of these to several degrees or by appressed rhizoidal down growths; tetrahedral sporangia formed from internal segments of the pericentral cells, the branchlets bearing them usually little modified, but in extreme cases like stichidia.

Spermatangial clusters are developed from trichoblast rudiments in the form of colourless tufts, cones, or plates of spermatangia. Procarps are developed from polysiphonous basal trichoblast segments, the fertile pericentral cell as a support­ing cell producing sterile cells and the four-celled carpogenic branch; from the sup­porting cell beside the carpogonium an auxiliary is cut off after fertilization, from which,
by means of a fusion cell, the sympodially branched gonimoblasts are produced; outer cells of the gonimoblasts alone forming carpospores. Cystocarps become enclosed by an ostiolate pericarp.

Genus Polysiphoni:

This marine alga is relatively common along the coasts of the United States and British seas. In India, it has been reported from West coast. Most species prefer quiet waters. Its plant body is attached to the substrate, may be submerged stones, rocks, shells, wood, or other algal body, by means of flattened discoid holdfasts.

The plant body is only a few to several inches in length (Fig. 127A). The plant body is filamentous branched laterally or dichotomously with brownish-red to purple-red in colour.

There are species of thick bushy habit. Both annual as well as perennial forms are present with regular shedding and regeneration of branches which is influenced by the seasonal variation. The alga is readily recognizable from the arrangement of its cells placed end to end in tiers.

The main axis and its branches possess a polysiphonous appearance as the central axial cell (siphon) is surrounded by pericentral cells (siphon) of variable number (Fig. 127G).

The cells have prominent cytoplasmic connections between cell to cell and each cell has one nucleus and many discoid plastids embedded in dense cytoplasm. The central axial cells are comparatively large, elongated and uninucleate. They are surrounded by pericentral cells (Fig. 127D). Growth of the thallus is initiated by a dome-shaped apical cell (Fig. 127B).

In many species the main axis produces two kinds of branches, for example, some are polysiphonous like the parent axis and others uniseriate structures of unlimited growth. These uniseriate structures are known as trichoblasts (Fig. 127B).

There is well-developed alternation of generations in the life cycle of Polysiphonia. The gametophytic generation is represented by morphologically identical male and female filaments with which alternately occurs asexual or tetrasporic plant which produces tetrasporangia and tetraspores.

The lateral branches of the male plant bear antheridia known as spermatangia, in dense clusters (Fig. 127E). The sperma­tangia are short-stalked colourless, spherical to oval, structures. Each spermatangium produces a single male cell or a spermatium from its entire contents. The wall of the mature antheridium splits open through which the spermatium is liberated.

The carpogonium is developed terminally on a lateral carpogonial branch of the female filament. The carpogonium has a swollen base in which lies the female nuc­leus, and a narrow elongated process, the trichogyne (Fig. 127F) constituting the re­ceptive organ for the spermatia. The spermatia are carried by water currents to the carpogonium and finally one of them adheres to the trichogyne.

Both the membrane of the spermatium and the wall of the trichogyne dissolve at the point of contact pro­ducing an open passage for the male nucleus which travels down the trichogyne to the female nucleus, where fusion between the two nuclei takes place.

The sporophytic generation has its beginning in the zygote. After fertilization the supporting cell of the carpogonial filament cuts off an auxiliary cell from its apex (Fig. 128A). Soon a tubular connection is established between the auxiliary cell and the carpogonium. The thread connecting the auxiliary cell with the carpogonium is known as a procarp.

The diploid nucleus of the carpogonium divides mitotically and one of the two daughter nuclei passes in the auxiliary cell through the connecting filament.

The carpogonium gradually shrivels and finally disappears. A small lateral outgrowth, known as gonimoblast initial, arises from the auxiliary cell (Fig. 128B). This is repeated with the ultimate production of a set of gonimoblast initials. The diploid nucleus of the auxiliary cell further divides and each gonimoblast initial receives a single nucleus.

Now each gonimoblast initial gives rise to a short gonimoblast filament, with its cells bearing a diploid nucleus. The apical cell of each gonimoblast filament develops into a carposporangium, the contents of which are transformed into a single carpospore. Simultaneously with the development of carposporangia and carpospores lateral sterile cells closer to the carpogonium grow out to form sterile filaments.

These sterile filaments form one- to two-layered walls surrounding the carposporangia forming an urn-shaped structure open at its tip through which the carpospores are liberated. This wall affords protection to the carposporangia. This entire urn-shaped structure is known as cystocarp (Fig. 128C & D).

The diploid carpospores, after being released, form new plants—tetrasporophytic plants, which look like the gametophytic individuals except that a few cells of which become sporangia—tetrasporangia (Fig. 128E). The tetrasporangia are borne on the central cells of the axis (Fig. 128F).

The diploid nucleus of a tetrasporangium divides meiotically to form four haploid nuclei around which four tetraspores are developed. After liberation, the tetraspores germinate to form male and female gametophytes.

The life cycle of Polysiphonia is a complex one, it comprises of three phases. There is a haploid sexual phase or the gametophytic generation. It is represented by game­tophytic filaments which bear sex organs and gametes. After fertilization, the female gametophyte bears carposporangia and carpospores in a cystocarp. The carposporangia, carpospores and cystocarp all are diploid structures.

The cystocarp is entirely depen­dent on the female gametophyte. This phase is followed by a diploid independent tetrasporophyte bearing tetrasporangia and tetraspores. It starts along with the germination of carpospore and terminates with the formation of haploid tetraspores by meiosis.

This phase is concerned with asexual reproduction. Thus, Polysiphonia exhibits triphasic life cycle. It also exhibits distinct alternation of generations represented by independently alternating well-developed sexual (gametophytic) and asexual (sporophytic) plants (Fig. 129).

Some Indian species of Genus Polysiphoni:

Polysiphonia platycarpa Boergs.; P. sertularioides (Gvat.) J. Ag.; P. suotilissima Mont.; P. tuticorinensis Boergs.; P. unguiformis Boergs.

Special features of Genus Polysiphoni:

1. Polysiphonous plant body.

2. Growth in length by a single doing-shaped apical cell.

3. Cytoplasmic connections between adjoining cells.

4. Isomorphic alternation of generations.

5. Sexual reproduction advanced oogamy.

6. Elaborate post-fertilization stages.

7. Presence of well-developed cystocarp.

8. Cystocarp, carposporangium and carpospores are diploid.

9. Meiosis taking place during the development of tetraspores in the tetra­sporangium borne on tetrasporic plant.

10. Triphasic life cycle.

11. Sporophyte comprising of two phases, an early phase dependent on the game­tophyte represented by cystocarp which is followed by an independent phase exhibited by the tetrasporic plant.