Discuss briefly the general features and classification of Chlorophyceae.

Ans. Occurrence : The class consists of 350 genera and 5,500 species. It corresponds to the Isokontae of the older classification. It is the most widely distributed group of algae. It contains a very large number of forms which are found almost everywhere. They are present in moist places, on tree-trunks, on flower-pots, in stagnant and running water and in the sea. Some are epiphytes while a few occur as symbionts with plants and animals. Some are cryophytes while there is a long list of those which occur in the most unusual places.

Form and Shape or Range of thallus structure: The members of Chlorophyceae exhibit an extraordinary variety in form and shape of their thallus.

Important features: Some of the important features of Chlorophyceae can be summarised as follows:

(a) Presence of chlorophyll a and b.

(b) Presence of pyrenoid or pyrenoids in the chloroplast, though they may be completely lacking in Siphonales.

(c) Starch as food material (Oil is Siphonales).

(d) Cell wall made up of cellulose like substance.

(e) Presence of equally oriented cilia in motile forms and swarmers (gametes and zoospores). They are generally present at the anterior end and help in movement. Their number varies from two to many.

Structure of cell: The cell structure may be discussed under the following heads :

(a) Shape

(b) Cell wall

(c) Nucleus

(d) Chloroplast

(e) Pyrenoids

(f) Stigma or eye spot

(g) Cilia

(h) Contractile vacuoles

(a) Shape: In the primitive algae, the shape of the cell is spherical or ovoid. The colony is also made up of such cells. In the filamentous forms, the cells are nearly rectangular which are placed end to end and thus form a filament. In Siphonales there are no septa and the thalli are profusely branched and multinucleate. Such a structure is called coenocyte.

(b) Cell wall: All the cells are surrounded by a definite cell wall meant for the protection of protoplast (few primitive forms have naked protoplast). The wall has two distinct layers. The inner one is made up of cellulose and the outer one is of pectose. Sometimes, the latter has a deposit of chitin over it and in such cases the wall becomes three- layered. Occasionally, mucilage is present as in Chlorococcales and Conjugates. The advantage of mucilage is two-fold. First, it keeps the individuals in group and secondly, it is a protection against desiccation and drought. Sometimes, it helps in movement. Its origin is obscure.

(c) Nucleus : Generally, the condition in Chlorophyceae is uninucleate but multinucleate condition is also not nmon. The latter is prevalent in Siphonales. Though algae are the lowermost group of the plant kingdom, yet the nucleus here resembles the nucleus of the higher plants. Sometimes, it may be slightly insignificant as in the case of Ulothrix, where the whole cell takes up a nearly uniform stain.

(d) Chloroplast: It is the important part of the protoplast. It is universally present. In fact, it forms the chief basis for generic differentiation in this group. The different genera may be recognised simply by seeing the shape of chloroplast, for example, it is horse-shoe shaped in Chlamydomonas; in Hydrodictyon it is said to be in a diffused state; spirally coiled in Spirogyra; band-shaped in Ulothrix; reticulate in Oedogonium, discoid in Vaucheria and stellate in Zygnema. Generally the chloroplast is situated in the middle and in this position it is said to be axile, but when it occupies peripheral region it becomes parietal.

(e) Pyrenoids: These are generally embedded in the chloroplast. They are present where the food is in the form of starch and, therefore, pyrenoids are absent in Siphonales, where the food material is oil.

A pyrenoid consists of a central body, the pyreno-crystal around which a single or more layers of starch are present. Pyreno-crystal may be angular or round. The starch sheath round the pyreno-crystal possesses a greater degree of permanency than the starch of stoma, because during drought it is the starch of stoma which is consumed first.

Pyrenoids may increase or decrease in number. Usually, they multiply with the cells and sometimes they completely disappear during this process. But in the daughter cell they reappear again.

(f) Eye spot or stigma: In many motile forms as well as in the reproductive bodies, there is a brownish, blackish or reddish dot-like structure called eyespot of stigma. Generally, it is situated in the thickening of the plasma membrane at the anterior end. In position, it may slightly differ in different species. Its structures and function are a matter of controversy but it is presumed to be a photoreceptive organ.

(g) Cillia: These are thin, long, thread-like structures situated at the anterior end of the motile forms as well as in the swarmers of many algae. Their number varies from two to many. They are flexible, pointed and help in movement.

(h) Contractile vacuoles: These are present mostly in the motile forms and occasionally in the sedentary ones. Their number varies from one to many. Though constant in position, they are never constant in their activity. Sometimes, they enlarge in size but suddenly they diminish or completely disappear. When they are two in number, they contract and expand alternately. They are supposed to be the organs for excretion but it is still a debatable point.

Reproduction: The reproduction in this class takes place by all the three methods, i.e., vegetative, asexual and sexual.

(a) Vegetative: It may be brought about either by ordinary cell division as in Pleurococcus and Desmids or by fragmentation. The latter may occur in many filamentous forms, for example Spirogyra, Caulerpa, etc.

(b) Asexual: It takes place mostly by zoospores. They are generally naked or many possess a cell-wall, but only occasionally. They are formed either in the ordinary vegetative cells as in Ulothrix or in special cells called the sporangia as in Vaucheria. Their number may vary from one (Oedogonium) to many (Ulothrix). They are produced in large numbers in Cladophora and Hydrodictyon.

They are generally biflagellate as in Chlamydomonas but may be quadri flagellate as in Ulothrix or multiflagellate as in Oedogonium. In Vaucheria, they are more or less complex in structure and are called synzoospores. Each synzoospore is multinucleate and multiflagellate.

The zoospores are entirely lacking in Conjugales and many Chlorococcales.

In addition to zoospores, asexual reproduction may take place by Akinetes (Cladophora), Aplanospores (ulothrix) Hypnospores (Sphaeroplea) and Autospores (Pediastrum).

(c) Sexual: It is brought about with the help of gametes. They are similar to zoospores in their structure and mode of formation. They are produced in large numbers. They are generally naked though occasionally a thin wall may be present around them. They may be equal or unequal in size. Accordingly, the condition is known as isogamous and anisogamous.

Isogamy is common in Chlamydomonas, Cladophora and ulothrix. It is regarded to be a primitive condition.

Anisogamy is more advanced than isogamy. It is common in Eudorina, Pleodorina, Bryopsis and some species of Chlamydomonas.

Oogamy is most advanced than isogamy. It is common in oedogonium, vaucheria, chara and some species of Chlamydomonas.

Oogamy is the most advanced stage in sexual reproduction and is commonly prevalent among the advanced forms of Chlorophyceae. Volvox, Oedogonium, Vaucheria and Chara exhibit clear cases of oogamy.

Classification of chlorophyceae : Chlorophyceae has been divided into nine Orders according to Fritsch :

1. Volvocales

2. Chlorococcales

3. Ulotrichales

4. Cladophorales

5. Chaetophorales

6. Oedogoniales

7. Conjugales

8. Siphonales

9. Charales

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