Plant Kingdom Test - 4

Nearly 50% of total carbondioxide fixation or photosynthesis of world is carried out by algae Photosynthesis by algae releases oxygen in the immediate aquatic environment. It is essential for respiration of aquatic life. Algae are primary producers of food in large bodies of fresh, brackish and sea water.

Chlorella (Green alga), a future hope of modern world as food supplement is rich in protein and carbohydrates. It yields about 30% proteins, 15% lipids, 30% carbohydrates and 5% ash. Spirulina platensis (Blue-green alga) is one of the richest sources of protein and vitamin B₂. In long range space exploration, scientist have turned to Chlorella ard Spirulina to use them as portable oxygen generators and food sources.

Classes of algae and their main characteristics have been summarized in the table given below:

Algae are usually aquatic, either marine or fresh, water. Only a few algae occur in moist terrestrial habitats - like tree trunks, wet rocks, moist soil, etc. Plant body, thallus (a form without any differentiation into root, sterm and leaves) which may be unicellular, colonial, filamentous, parenchymatous or pseudoparenchymatous. Sexual, asexual and vegetative modes of reproduction are abundant. Form, and size of algae range from microscopic unicellular forrns like Chlamydomonas, to colonial forms like Volvox and to filamentous forms like Ulothrix and Spirogyra.

The algae shown in figure are Chlamydomonas, Volvox and Chara respectively, belonging to class Chlorophyceae.

Cell wall of green algae is thin, transparent andd form and consists of outer pectic and inner cellulosic layers. It is smooth but in most of species it gets thickened at the anterior end to form an apical papilla.

Pyrenoids are cellular micro-compartments, and are not membrance bound organelles. Pyrenoids are found within chloroplast. Chloroplasts generally contain one to many pyrenoids for storage of starch. 

A single cup-shaped chloroplast is characteristic of Chlamydomonas. It occupies the major portion of cell and is thick at the base while its sides are relatively thin and projected upward.

In isogamy both the fusing gametes are morphologically and physiologically similar. They may be flagellate of nonflagellate. In anisogamy the fusing gametes differ in size and behaviour. One of the two gametes is larger and is called macrogamete or female gamete; the other is smaller and is termed microgamete or male gamete. In oogamy, there is a large food laden nonflagellate female gamete called egg or oosphere whereas the male gamete or antherozoid is smaller and motile. Oogamy is the familiar form of sexual reproduction, it is a form of anisogamy (heterogamy).

Dictyoia is a ribbon shaped dichotomously branched marine brown alga that grows in shallow waters. Its frond is flat and dichotomously branched. Polysiphonia is a small, upright, bushy marine red alga, fixed to the substratum by rhizoids, hold fast, etc.

Batrachospermum is a fresh water red alga widely distributed in tropical, sub-tropical and temperate areas. Gelidium amansll is an economically important species of red algae commonly found in shallow cost of many East and Southeast Asian countries.

Volvox is colonial green alga. The cells are like unicelled alga chlamydomonas and are definitely organized in the form of a colony called coenohimn. The coenobia of Volvox are largest, highly differentiated and well evolved among motile forms. Each coenobium is motile, the movement is brought about by the joint action of the flagella of individual call.

Batrachospermum is a freshwater filamentous red alga, commonly called frog sprawn alga.

Volvox shows advanced oogamy which takes place by the formation of antheridia and oogonia. They may be formed on the same plant (monoeclous) or on different plants(dioecious). The sex-organs are produced fewer in number.

Seaweeds are large multicellular algae living in the sea or in intertidal zone. They are common species of the Chlorophyta, Rhodophyta and Phaeophyta. The seaweeds are a polyphyletic group. Seaweeds, especially brown algae are used as food for domestic animals in different parts of the world. Pelvelia canaliculata is used as food for younq live-stocks. Sargassum, Fucus and Laminaria are eaten by cattle in Scotland, Ireland and Iceland. Macrocystis spp and other kelps being rich in vitamins A and E are served as cattle fodder.

Isogamous type of sexual reproduction is found in Ulothrix and in majority, the plants are heterothallic. The gametes are found in large number, I.e. 32 to 64 in each gametangium.

Nostoc and Anabaena are nitrogen fixing blue-green algae, Azotobacter is free-living nitrogen fixing bacterium, while Spirogyra is a green alga which does not fix atmospheric nitrogen.

Agar is dried gel-like non-nitrogenous extract obtained from red algae. It is used as a medium in the cultures of bacteria, fungi and other micro-organisms and also in numerous industrial processes, e.g., food, bakery, cosmetics, leather, textile, industries, in confectionary, etc.

Seaweeds are rich in minerals such as iodine, calcium sodium, magnesium, potassium, iron, zinc, copper. Seaweeds also provide fibre, vitamins, enzyme and high quality protein.

Brown algae or Phaeophyta include about 2000 species. Most of the them are marine except few e.g., Pleuroction. Heribandiella and Bodanella, which are found in fresh water in European countries.