Algae

Three groups of Algae - Rhodophyta (red algae), Phaeophyta (brown algae) and Chlorophyta (green algae) have been included in the Kingdom Metaphyta. As you have already learnt, the group cyanophyta (blue-green algae) has been placed in the Kingdom Monera while the group Bacillariophyta (golden-brown algae) has been placed in Kingdom Protista.

Rhodophyta

It is a group consisting of multicellular aquatic algae which exhibit a highly varied organisation of the plant body. They are generally found attached to rocky stones. Some of them occur as total or semi parasites on the other algal plants.

The members of this group are commonly called 'red algae' due to the presence of a characteristic red pigment called phycoerythrin (red), which masks the colour of other pigments like phycocyanin (blue) and chlorophyll (green).

fig. 8.25 - Examples of Red Algae

Vegetative reproduction occurs by fragmentation. Asexual reproduction involves the formation of different types of spores like monospores, neutral spores, corpospores and tetraspores. Sexual reproduction occurs by oogamy. The male sex organs are called spermatangia or antheridia and female sex organs are called carpogonia. The fertilized egg results in the formation of a new structure called carposporophyte. Life cycle may involve alternation of a haploid gametophytic generation with a diploid sporophytic generation.

Phaeophyta

It includes a much-advanced group of algae, which are mostly marine forms. They are commonly called as 'sea weeds'. The plant body may be filamentous or may show root like hold fast, stemlike stipe and leaf like blades. Many of them have air bladders which keep them afloat by providing buoyancy. The name brown algae comes from the fact that they contain large quantities of a brown coloured pigment fucoxanthin which masks the colour of other pigments.

fig. 8.26 - Examples of Brown Algae

Vegetative reproduction occurs in some examples by fragmentation. Asexual reproduction involves the formation of motile zoospores and non-motile spores such as monospores and tetraspores. Sexual reproduction may be isogamy or anisogamy or oogamy. Fertilization is external. The zygote develops into a diploid thallus. There is no zygotic meiosis.

Chlorophyta

It is a diverse group of algae, most of which are fresh water forms. A few are marine forms. The plant body may be unicellular and motile (as in Chlamydomonas) or colonial and motile (as in volvox) or colonial and non motile (as in hydrodictyon) or filamentous (as in Oedogonium and Cladophora). Shape of the individual cells may be spherical or oval or cylindrical. The cells possess one to many chloroplasts which occur in characteristic shapes in different examples.

Photosynthetic pigments are chlorophyll a, chlorophyll b, carotenes and xanthophylls.

fig. 8.27 - Examples of Green Algae

Vegetative reproduction occurs by cell division, fragmentation and stolon formation. Asexual reproduction occurs by the formation of akinites or zoospores or aplanospores. Sexual reproduction involves isogamy or anisogamy or oogamy. Zygote is generally the only diploid structure in the life cycle. There is a regular alternation of haploid gametophytic and diploid sporophytic generations in the life cycle.

According to Whittaker's five kingdom classification, the unicellular eukaryotes should be placed in the kingdom Protista. However, unicellular green algae such as Chlamydomonas, Chlorella etc. are placed in Kingdom Metaphyta along with other multicellular green algae only because of their evolutionary continuity with land plants. It is believed that primitive forms of life must have given rise to unicellular green algae and these in turn, must have given rise to multicellular forms. These forms must have undergone further evolutionary changes to give rise to land plants.