Here is a list of four commonly found algae:- 1. Oscillatoria 2. Nostoc 3. Spirogyra 4. Ulothrix.
Oscillatoria is a very common blue-green alga growing abundantly in muddy places, ditches, drains and similar damp situations forming colonies. It belongs to a very primitive group of algae, there the cells are prokaryotes, and a blue pigment, phycocyanin, is present besides the photosynthetic ones diffused in the protoplast.
The plant body is an un-branched filament, with a row of cells attached end on end. The terminal cell is rounded in shape. A distinct swaying or oscillating movement is exhibited by the filament, what can be clearly seen under the microscope—and hence the name Oscillatoria. Each filament has a gelatinous sheath as envelope.
The cells are square in shape and shorter than length. Protoplast is differentiated into two distinct portions—the peripheral coloured portion, called chromoplasm, and the colourless central portion named central body.
The coloured portion is made of chlorophyll and associated pigments in form of granules with diffused blue pigment phycocyanin. Absence of plastids is a notable feature. Various reserve materials—usually glycogen (not starch), protein and fat globules occur in the chromoplasm.
The colourless central body is regarded as the ‘incipient’ or degenerate nucleus without a nuclear membrane and nucleolus. Only chromatin materials are present. It has no power of meiotic cell division. The two portions of protoplast may be considered corresponding to the cytoplasm and nucleus of normal cells.
Reproduction takes place by vegetative method only. Some dead cells, called separation discs, occur here and there along which the filament breaks up into small pieces or fragments, called hormogonia. Each of them may be made of one or a few cells, and may grow into a new filament by cell-division in one plane only. The new filaments also exhibit swaying movement.
Nostoc is a blue-green alga very commonly growing in masses or colonies embedded in a thick mucilaginous matrix, in ponds, ditches and similar damp places. Some of them may also occur endophytically in the intercellular cavities of plants like Anthoceros and Lemna.
The plant body is an un-branched filament made of rounded cells, attached end on end, attaining a distinct beaded appearance. A pretty good number of them, each with its own gelatinous sheath, remain embedded in a mass of mucilage, looking like a number of beaded chains interwoven together.
The cells are prokaryotic like those of Oscillatoria. The protoplast is differentiated into peripheral coloured portion, chromoplasm, with green chlorophyll and blue pigment phycocyanin in form of granules, and reserve food globules; and the colourless central portion, the central body, representing the ‘incipient’ nucleus which has no nuclear membrane and nucleolus.
Here and there on the filament some vegetative cells enlarge, become colourless which may occur either terminally or in intercalary position.
These cells are known as heterocysts. Each of them has two pores at the poles, through which cytoplasmic strands pass to adjacent vegetative cells. The poles are ultimately closed by button-like thickening of the wall forming the so-called polar nodules. In case of a terminal heterocyst there is a single pole and polar nodule.
Nostoc reproduces vegetatively by fragmentation. The filament breaks up into pieces—the hormogonia, at the point of heterocysts. The fragments grow in one plane and remain in a tangled mass in the mucilaginous matrix.
Some vegetative cells singly or in a row often enlarge and become full of stored food to serve as resting spores. These bodies, called akinetes, are formed mainly for tiding over unfavourable periods. In course of time they germinate and give rise to new filaments.
Spirogyra is a very common free-floating fresh-water alga. It occurs in stagnant water in ponds and ditches and also in springs and slow-running streams. A large number of them usually grow in tangled masses. It is commonly called pond-scum.
The plant body is a long filament of cylindrical cells attach: end to end, hardly showing any difference between base and apex. Colourless holdfasts or attaching organs are rarely formed in those growing in running water.
The cell wall is composed of cellulose and-pectin, the latter dissolves in water to produce a gelatinous sheath surrounding the filament which, in fact, gives Spirogyra the characteristic slimy nature. Every cell is long and cylindrical with lining cytoplasm and large central vacuole (Fig. 186).
The nucleus remains suspended in the vacuole by fine cytoplasmic threads. One or more chloroplasts remain arranged in the form of spiral bands with smooth or toothed margins. The name Spirogyra has been derived from the peculiar spiral chloroplasts.
Some shiny refractive bodies, called pyrenoides, are present here and there on the spiral bands. These nodule-like bodies are stored food matters, each having a protein-centre surrounded by starchy layers. The filaments grow in length by cell- division.
Reproduction takes place by vegetative and sexual methods. Spirogyra reproduces vegetatively by simple fragmentation, when the filament breaks up into one- or more-celled pieces which grow and multiply by cell-division. The splitting of the filament into pieces may be accidental or due to softening of the middle lamella between two cells.
Sexual reproduction takes place by conjugation, an isogamous method, which involves union of similar gametes. Conjugation may occur between cells of two different filaments, or rather rarely, between the adjacent cells of the same filament. In the former case, two filaments of Spirogyra approach each other and lie side by side touching at a number of points. They are surrounded by a common sheath.
Short protuberances come out at the points of contact which elongate, naturally pushing the two filaments away from each other. On dissolution of the end walls, pathways or tubes are established connecting the two cells of different filaments.
These are called conjugation tubes. In the meantime, protoplast of every cell loses water, contracts and forms rounded mass at the centre. Contracted mass of protoplast of every cell function as the gamete, and so every cell is a gametangium.
The gametes of one filament move through the conjugation tubes in amoeboid fashion to unite with the gametes of the corresponding filament. By union of gametes zygotes, called zygospores, are formed. It is often found that all the cells of one filament are empty and all the cells of the corresponding filament contain zygotes. This type of conjugation is called scalariform conjugation (Fig. 187).
In some species of Spirogyra another type of conjugation, called lateral conjugation, takes place between the gametes of the adjoining cells of the same filament. In this case also there is a formation of outgrowth, accompanied by partial dissolution of the side wall, to produce the conjugation tube.
Gametes are formed in the same fashion. A gamete of a cell moves through the conjugation tube to unite with the gamete of the adjacent cell. After conjugation, it is often found that alternate cells contain zygotes and alternate cells are empty (Fig. 188).
Though Spirogyra produces similar gametes or isogametes, there is differentiation of sex. The moving gametes are male and stationary ones are female. Thus the .two filaments involved in scalariform conjugation are unisexual, and the one in lateral conjugation is bisexual.
The zygospore formed by union of two gametes, soon secretes a tough wall around itself. The wall, mainly made of cellulose, has three coats, of which the second one contains chitin. Nuclear fusion is considerably delayed, and chloroplasts contributed by the male gamete are supposed to disintegrate, and those of the female gamete persist in the zygote.
It is liberated by rupture of the wall of the cell and falls to the bottom of the tank or ditch wherever the filaments may grow. After a period of rest, usually covering the unfavourable season, the zygospore germinates to produce a new filament.
The diploid zygote nucleus undergoes reduction division to produce four haploid nuclei; three of them disintegrate and only one survives (Fig. 189). Now the outer wall bursts and the innermost wall with protoplasmic contents comes out in the form of a tube. The cells divide and ultimately give rise to a new Spirogyra filament (Fig. 190).
Often conjugation fails in Spirogyra, when one of the gametes surrounds itself by thick wall and behaves like a zygote, without actual union with the corresponding one. Such, a spore is called azygospore, and the process is parthenogenesis.
Ulothrix is a common green filamentous alga growing abundantly in fresh-water of ponds, reservoirs and slow-running streams. A few species are marine.
The thalloid plant body is a long un-branched filament composed of short cylindrical cells attached end to end. Only the colourless basal cell elongates and serves as the holdfast to attach the filament to convenient substratum. Every short cell has cytoplasm, a single nucleus and chloroplast in the form of a parietal band with many pyrenoids (Fig. 191, 1). The filaments grow in length by cell division.
Reproduction takes place by all the three methods—vegetative, asexual and sexual. Vegetative reproduction occurs by simple fragmentation, when the filament breaks up into fragments, each of which grows in length by cell division and carries on normal activities.
Asexual reproduction takes place by the formation of ciliated spores called zoospores. The protoplast of any cell, excepting the basal holdfast, may produce one or more naked uninucleate pear- shaped bodies; each of them develops into a zoospore. As many as 32 zoospores may be formed in a cell. The zoospore is provided with four cilia, and has a conspicuous eye-spot near the cilia (Fig. 191, 2).
The lateral wall of the cell breaks forming an opening or pore, through which the zoospores come out. They swim freely for some time, a day or more, and ultimately settle down by getting themselves attached to any object by the ciliated end. Cilia are withdrawn and cell wall is formed around each of them. Now they germinate and by cell division develop into new filaments.
Sexual reproduction takes place by conjugation usually at the end of the growing season, when conditions are less favourable. Gametes are formed in a large number and liberated just like the zoospores. They are bi-ciliate and smaller in size. Coming out of the cells, gametes produced by different plants unite in pairs to form quadriciliate zygote or zygospore (Fig. 191, 3 & 4).
Cilia are withdrawn and a thick wall is formed around the zygote. After a period of rest, the diploid zygote nucleus undergoes reduction division and ultimately four or more zoospores are produced. Each of them develops into a new plant.