Growth Forms and Life Histories in Green Algae

Abstract

Green algae are predominantly diverse in growth forms. Plant body in green algae shows evolutionary progression from simple to more complex types of construction. It is fascinating to trace the increasing complexity in the algal structure through certain members of the Chlorophyceae, beginning from simple unicellular motile forms (Chlamydomonas) through multicellular flagellated colonies (Volvox), unicellular non-motile forms (Chlorella), filamentous forms (Ulothrix, Oedogonium, Spirogyra), heterotrichous forms (Coleochate), siphonous forms (Acetabularia), well developed parenchymatous thalli (Ulva) to a thallus which bear leaf and stem like structures and resembles land plants (Chara). The perpetuation of these algae passes through a cycle of events involving more than one phase for a species and every phase must be able to survive or reproduce for the species to persist. In its life cycle, an alga usually cycles through different ploidy stages and this cycle is characterized by the occurrence of sporophyte and gametophyte. Sporophyte is diploid and is concerned with the production of haploid meiospores after meiosis, whereas gametophyte is haploid and is responsible for sexual reproduction. Gametophyte produces the haploid gametes. Sporophyte and gametophyte usually follow each other in a single life cycle and this progression is called alternation of generations. These stages face varying environmental conditions and are closely linked to each other. Study of these life cycles is immensely important because of their usefulness for both ecological and evolutionary studies. Most of the unicellular and filamentous forms of Chlorophyceae show haplontic life cycle (e.g. Chlamydomonas, Volvox, Chlorella, Oedogonium, Ulothrix, Spirogyra, Coleochate and Chara), some members show diplontic life cycle (e.g. Acetabularia, Codium) while a few others show isomorphic alternation of generations between haploid and diploid plants (e.g. Ulva) called diplohaplontic life cycle. The aim of this chapter is to present a systematic framework of the information on structure, ultrastructure, reproduction and life histories of selected taxa in Chlorophyceae.