Classification of Nemalionales | Algae

In this article we will discuss about the classification of nemalionales. 

Family Batrachospermaceae:

This family includes algae whose plant body is of soft, thick, gelatinous branched thallus differentiated into structures resembling nodes and internodes. Branches arise in whorls from the nodes. Asexual reproduction is by monospores and sexual repro­duction oogamous by the development of carpogonia and spermatangia.

Zygotic nucleus, divides meiotically immediately after development. This is followed by the formation of gonimoblast filaments from which carposporangia and carpospores are developed in a cystocarp.

Genus Batrachospermum:

Batrachospermum, commonly known as ‘frog-spawn’ alga, has a very wide range of distribution both in temperate and tropical regions. Generally it prefers cool, shady and well-aerated situations in comparatively slowly flowing streams and margins of lakes. There are species which occur in rapidly flowing streams being attached to stones and similar other submerged objects.

Most of the species are annuals but a few are perennial. The plant body is of gelatinous texture appearing as chains of delicate beads (Fig. 119A). The primary axis which grows from an apical cell consists of a large number of elongated cylindrical cells placed end to end in a single row exhibiting an example of simple uniaxial construction.

Numerous branches of both limited and unlimited growth arise in whorls from the upper ends of the cells just below the septa. This peculiar arrangement of the branches has resulted the main axis to be differentiated into nodes and internodes (Fig. 119B). The basal cells of the lateral branches grow into narrow, threads (Fig. 119F).

These threads grow downward forming an envelope around the main axis which is completely concealed assuming a corticated appearance. Cells are uninucleate containing a number of parietal, lobed chromatophores with pyrenoids, Batrachospermum possesses haplobiontic life cycle.

Batrachospermum reproduces both asexually and sexually.

Flagellate cells are completely absent in the life cycle of this alga. Asexual reproduction is brought about by means of non-flagellate uninucleate spores known as monospores, developed singly within monosporangia which are again developed on the erect portion of the heterotrichous filament called Chantransia stage that is produced during the post- fertilization stage of the sexual reproduction (Fig. 119C to E).

Sexual reproduction is a case of advanced oogamous condition. Thallus may be monoecious or dioecious depending on species. The carpogonia (female sex organs) are borne terminally on small branches of the thallus. The carpogonium at its upper end is prolonged into a trichogyne which shrivels away after fertilization (Fig. 120A and F).

The nucleus, chromatophores and the reserve food material are located in the lower swollen portion of the carpogonium.

Antheridia or spermatangia (male sex organs) are single-celled spherical struc­tures borne in clusters at the tip of the lateral branches of the thallus (Fig. 120B). Con­tents of each antheridium become metamorphosed into a non-motile, spherical male gamete or spermatium, which is liberated through a narrow apical slit of the antheri- dial wall.

The spermatium thus liberated floats in water and is finally carried away by water current to come in contact with the trichogyne of the carpogonium.

The walls dissolve at the point of contact of the spermatium and the trichogyne and the contents of the spermatium pass through the open passage and move down the trichogyne into the base of the carpogonium where both male and female nuclei fuse together (Fig. 120G to E).

After fertilization the trichogyne is separated from the carpogonium by a mucilage plug and finally shrivels and disappears (Fig. 120F). The diploid carpogonial nucleus divides meiotically into two haploid nuclei.

Simultaneously with the nuclear division a lateral protuberance is developed from the carpogonium. One of the two daughter nuclei moves into this protuberance, the other remaining in the carpogonium. The protuberance is cut off from the carpogonium by a wall and is known as gonimoblast initial (Fig. 120G).

Then the daughter nucleus of the carpo­gonium divides mitotically along with the formation of another protuberance on the other side of the carpogonium in which the daughter nucleus migrates and ultimately a second gonimoblast initial is produced (Fig. 120H) and like this several gonimoblast initials are produced which by repeated divisions form branched or unbranched filaments known as gonimoblast filaments (Fig. 120 I & J).

The terminal cells of the gonimoblast filaments become enlarged, each one developing into a carposporangium, the contents of which become metamorphosed into a single, non-motile carpospore. Along with the development of the carposporangium and carpospore numerous sterile threads are developed from the cells below the carpogonium which ultimately envelope the gonimoblast filaments.

These threads are known as the enveloping threads (Fig. 120 I).

The structure so formed with gonimoblast filaments surrounded by the enveloping threads is the cystocarp. The carpospore is liberated from the carpo­sporangium (Fig. 120J) and germinates into a heterotrichous filament which morpho­logically is quite different from the main thallus.

The heterotrichous filament is known as Chantransia stage or juvenile stage. For long time this heterotrichous filament was considered to be a new genus Chantransia.

But later on it was found that the apical cells of the lower branches of the erect threads develop into new Batrachosper­mum thallus and the so-called Chantransia is nothing but a stage in the life cycle of the alga Batrachospermum. Finally it was linked up in the life cycle of Batrachospermum naming as Chantransia stage (Fig. 121).

The life cycle of Batrachospermum consists of two gametophytic phases alternating with one sporophytic phase which is, however, confined in the zygotic stage. The early gametophytic phase—Chantransia stage, is formed by the germination of the carpospore which is the product of post-fertilization stages. The late gametophytic phase, the main Batrachospermum plant is developed from the Chantransia stage.

Special Features of Genus Batrachospermum:

1. Complete absence of any flagellate cell.

2. Structure of the carpogonium, the process of fertilization.

3. Advanced oogamy followed by elaborate post-fertilization stages with the production of carposporangium and carpospore in a special structure called cystocarp. All these structures are haploid.

4. The alga has attained great advancement but the zygotic meiosis has at the same time kept it far down in the evolutionary level.