The division Chlorophyta (Gr. chloros=green + phyton=plant) includes a single class Chlorophyceae (Gr. phykos=seaweeds).
Characteristics of Chlorophyta
- The members of this division are characterized by presence of grass-green chromatophores.
- The green algae are widespread and occur both in fresh and marine waters, on moist wood and rocks, and on the surface of and within soil. A number of species are epiphytic on other algal plants, flowering plants and animals. Most of the marine species are attached to the rocks or larger algae, or growing on sandy bottoms.
- Species like Trebauxia, Chlorella, Coccomyxa, etc partner fungi to form lichens.
- The thallus ranges from a motile unicell to complex heterotrichous filamentous forms.
- The cell is typically eukaryotic and contains pyrenoids which are thought to be concerned with starch formation.
- The cell is bounded by a two-layered cell wall usually. In some forms, the cell wall is absent. The outer layer is made up of pectin and the inner is formed of cellulose, pectin and in some cases polysaccharides are also present.
- The cytoplasm is distinct and contains typical eukaryotic organelle, the chloroplasts, mitochondria, Golgi bodies, endoplasmic reticulum, and ribosomes.
- The nucleus is bounded by a definite nuclear membrane and encloses nucleoli, chromosomes and karyolymph. Usually a single nucleus is present but in some genera the cells are coenocytic (multinucleate due to absence of cross walls).
- The chloroplasts are grass-green and vary in number and shape from species to species. They contain chlorophyll a and b, alpha- and beta-carotenes and certain xanthophylls. In majority of species pyrenoids (Gr. pyren=fruit or stone) are present in the chloroplasts.
- The reserve food is starch and stored in the plastids. The chloroplasts of motile algae and motile reproductive cells of non-motile green algae contain a reddish spot called eyespot or stigma. It is thought to be sensitive to light and directs the movement of the swimming cells.
- Green algae reproduce by vegetative, asexual or sexual reproductive methods.
- Majority of forms exhibit sexual reproduction ranging from isogamous to oogamous type.
- Most of the forms have a haploid dominant phase but a few exhibits a regular alternation of generations.
- More than 450 genera and 7500 species have been described so far.