Describe in brief occurrence, structure and reproduction of Algae.

Ans. Occurrence : Algae, an important group of Thallophyta, are cosmopolitan in a distribution and universal in occurrence. Algae are found on the trunk of trees, on telephone wires, on rocks in hot springs and in several other unusual habitats. Algae thrive well in snow and geysers. The most favourable haunt in salt water of algae.

Structure: In the organization of the plant algae exhibit a great diversity. The size of the algae is very much variable. A huge size laminariales occurs from a microscopic unicellular Chlamydomonas. Chlamydomonas is the most primitive algae. This is microscopic volvox and unicellular, which is a colonial forms. Cladophora is branched and Ulothrix and spirogyra are unbranched among the filamentous forms.

Reproduction:

(i) Vegetative reproduction : This is the processes of propagation in which portions of the plant body are separated to give rise to new individuals without any change in the protoplast.

(ii) Asexual reproduction : This reproduction takes place by the following methods.

(a) By Zoospores: These are the common occurrence in algae, they are formed in the favourable conditions. They are practically similar to parent cell and are liberated by breaking up of the mother cell wall. Zoospores are formed from certain older cell of filaments. To form Zoospores, the older cytoplasm is divided to form zoospores which escaped from the mother cell wall. Zoospores are always motile.

(b) By Akinetes: In this case the protoplast of a single cell converts in a single akinete. Sometimes they are formed in chains. Each akinete may develop into a new plant. e.g. oedogonium, ulothrix, and members of myxophyceae.

(c) By Hypnospores: When the aplanospores undergo a long resting period and become thick walled, they are said to be hypnospores e.g. Pediastrum & spharella.

(d) Palmella Stage: Sometimes the zoospores do not come out of the mother cell but they remain surrounded by a mucilaginous sheath inside the mother cell. On division, they form colony shaped structure called the Palmella. On liberation they form new plants e.g. chlamydomonas.

(e) By Endospores : They are formed within the cells in many blue green algae and bacillariophyceae. Each endospore develops in a new individual on the approach of favourable conditions.

(f) By Aplanospores: Aplanospores are the eliminated bodies of motile phase of Zoospores. They develop in unfavourable conditions.

(g) By Cysts: The plant body of certain algae viz vaucheria becomes septate by the development of transverse septa during unfavourable conditions. e of s4 30

(h) By Autospores: In some chlorococcales and some plants viz Scenedesmus, the resting spores develop all the structure of the parent cell in which they are formed and are exactly similar to the later except size and these spores are termed as autospores.

(iii) Sexual Reproduction: Sexual Reproduction takes place by gametes, which are produced in cells known as gametangia. Following are the ways of fusion.

(a) The fusing gametes are different in size in certain algae viz chlamydomonas braunii. The smaller gamete is active (macrogametes) while the large gamete is passive. This type of sexual union is known as Anisogamous reproduction.

(b) In some algae the young gametes are similar morphologically. These gametes are called isogamous and the sexual process is described as isogamous reproduction.

(c) In some algae the young gametes are different in size and behaviour i.e. one of the gametes is small and motile, while other is large and nonmotile. The former is described male gamete or sperm and the later a female gamete, egg or oospore. This is known as oogamous type and the product is called oospore or Zygote.

On germination it may behave in either of the three ways-

(a) Zygote divides mitotically to give rise to a large diploid plant or small diploid plant which produces the meióspore e.g. cladophora.

(b) The outer wall brusts and contents surrounded by the inner layer grow into a new plant e.g. spirogyra.

(c) The contents within the wall divided by meiosis to form a number of motile or non-motile meiospores each of which on a liberation develop into a new plant e.g. Ulothrix.

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