General Characteristic features
The algae show a great diversity in size, shape and structure. A wide range of thallus organisation is found in algae. Unicellular motile (Chlamydomonas), unicellular non-motile (Chlorella), Colonial motile (Volvox), Colonial non motile (Hydrodictyon), siphonous (Vaucheria), unbranched filamentous (Spirogyra), branched filamentous (Cladophora), discoid (Coleochaete) heterotrichous ( Fritschiella), Foliaceous (Ulva) to Giant Kelps (Laminaria and Macrocystis). The thallus organization in algae is given in Figure 2.3.
Algae are Eukaryotes except blue green algae. The plant body does not show differentiation into tissue systems. The cell wall of algae is made up of cellulose and hemicellulose. Siliceous walls are present in diatoms. In Chara the thallus is encrusted with calcium carbonate. Some algae possess algin, polysulphate esters of polysaccharides which are the sources for the alginate, agar agar and Carrageenan.
The cell has a membrane bound nucleus and cell organelles like chloroplast, mitochondria, endoplasmic reticulum, golgi bodies etc., Pyrenoids are present. They are proteinaceous bodies found in chromatophores and assist in the synthesis and storage of starch. The pigmentation, reserve food material and flagellation differ among the algal groups.
Algae reproduces by vegetative, asexual and sexual methods (Figure 2.4). Vegetative reproduction includes fission (In unicellular forms the cell divides mitotically to produce two daughter cells Example: Chlamydomonas); Fragmentation (fragments of parent thallus grow into new individual Example: Ulothrix ) Budding (A lateral bud is formed in some members like Protosiphon and helps in reproduction) Bulbils, (a wedge shaped modified branch develop in Sphacelaria) Akinetes (Thick walled spores meant for perennation and germinates with the advent of favourable condition Example: Pithophora). Tubers (Structures found on the rhizoids and the lower nodes of Chara which store food materials).
takesplace by the production of zoospores( Ulothrix,
Oedogonium) aplanospore(thin walled non motile spores Example: Vaucheria);
(spores which look similar to parent cell Example: Chlorella ); Hypnospore (thick walled aplanospore – Example: Chlamydomonas nivalis); Tetraspores (Diploid thallus of Polysiphonia produce haploid spores
Sexual reproduction in algae are of three types 1. Isogamy (Fusion of morphologically and Physiologically similar gametes Example: Ulothrix) 2. Anisogamy (Fusion of either morphologically or physiologically dissimilar gametes Example: Pandorina) 3. Oogamy (Fusion of both morphologically and physiologically dissimilar gametes.
Example: Sargassum). The life cycle shows distinct alternation of generation.