Algae are simple, chlorophyll bearing and photosynthetic non vascular plants whose body is a simple thallus without differentiation into roots, stems and leaves. There are about 35000 species of algae.They include planktons, pond scums, stone worts, sea weeds, sea lettuce etc. Phycology is the branch in botany which deals with the study of algae.
Characteristics of Algae
- Body is a relatively simple simple unicellular or multicellular thallus, not differentiated into roots, stems and leaves.
- Unicellular thallus may be non motile, rhizopodial or coccoid.
- Multicellular thallus may be colonial, palmelloid, dendroid, filametous siphonous and so on.
- Algae cells exhibit three levels of organization, namely prokaryotic (eg: Myxophyceae), mesokaryotic (eg: Dinophyceae) and eukaryotic (other groups).
- Cells are covered by a rigid cellulose cell wall.
- Cells contain plastids and three classes of pigments, namely chlrophyll(a, b, c,d, and e), caroteneoids (alpha, beta, gamma and the theta carotenes, lycopene, leutin, flvicine, fucoxanthin, violaxanthin, astaxanthin, zeaxanthin, myxoxanthin), and phycobilins or biliproteins(phycocyanin, phycoerythrin, allophycocyanin).
- The reserve food includes mostly starch and oils (in Chlorophyceae starch; in Xanthophyceae and Bacillariophyceae chrysolaminarin and oils; in Phaeophyceae laminarin, mannitol and oils, in Rhodophyceae floridean starch and galactan; in Cyanophyceae cyanophycean satarch)
- Absence of conducting (vascular) and mechanical tissues; the entire thallus is formed of only parenchyma cells.
- Presence of holdfast, stipe and lamina. Holdfast is for attachment, stipe forms the axis, and lamina serves as the leaf like photosynthetic part.
- Algae flagella have typical 9+2 pattern of arrangement of microtubules.
- Reproduction occurs by vegetative, asexual and sexual methods
- Vegetative reproduction is by fragmentation, hormogonia, akinetes etc.
- Asexual reproduction is by motile zoospores, or by non motile apalnospores, autospores, hypnospores, exospores, endospores, carpsospores etc. Spores are produced in sporangia.
- Sexual reproduction may be isogamous, anisogamous or oogamous. Oogamous species possess antheridia and oogonia.
- Sex organs are usually unicellular and non jacketed; multicellular sex organs are rare and in them each cell is fertile without a jacket of sterile cells.
- Embryo is not formed after gametic fusion.
- In most genera the only diploid stage in the life cycle is the zygote, which immediately undergoes meiosis.
- Sporophytic and gametophytic genetations, which present in the life cycle, are independent. So, there is no alga with a sporophyte parasitic on gametophyte.