BOTANY WITH THE BASICS OF HYDROBOTANY (AQUATIC PLANTS OF UKRAINE) - B.Ye. Yakubenko - 2011

VI. PLANT SYSTEM

LOWER SPORE PLANTS

GENERAL CHARACTERISTICS AND FEATURES

Class Chlorophyceae - Chlorophyceae

This Class unites unicellular, colonial, and multicellular forms with monadoid, palmelloid, coccoid, filamentous, and heterotrichous, and occasionally siphonal and siphonocladal thallus structures. They grow predominantly in fresh continental Water bodies, though they also occur in terrestrial habitats and occasionally in aerophytic ones. The Earth's flora comprises about 4,000 species, while the flora of Ukraine includes approximately 900 taxa of species and infraspecific rank.

The Cells of the vast majority of representatives of this class possess a typical three-layer Cellulose-pectin Cell wall; however, monadoid forms are characterized by a glycoprotein envelope, which is absent in some of them, being represented solely by the Plasmalemma (Dunaliella). Occasionally, an additional sporopollenin layer of The Cell wall is present, the outgrowths of which form a diverse surface ornamentation of the cell wall and contribute to its strength and durability. Motile green Algae possess 2–4 flagella that are isocont and isomorphic.

Characteristic Features of the class include a cruciate flagellar ROOT system with basally positioned bodies either opposite each other or shifted clockwise (systems with "12/6" and "1/7" positions), cytokinesis occurring via The formation of a partition involving the phycoplast, the presence of closed mitosis and a collapsing spindle, and the presence of centrioles in non-motile forms alongside their absence in motile ones.

All types of Selection/8.html">Asexual and sexual reproduction are known. Vegetative Reproduction of motile forms lacking a cell wall occurs via schizogony through binary fission. At the same time, reproduction via fragmentation of filaments and colonies is observed, as well as the formation of akinetes and other resting stages. Among the algae of this class, species with a haploid life cycle and zygotic reduction, without Morphology/12.html">ALTERNATION OF GENERATIONS, predominate. Species with a diplophasic cycle (Chlorococcum) and haploid ones with a heteromorphic alternation of generations (Sphaeroplea) are occasionally encountered. Agamic species are characterized by complex cyclomorphoses. The most widespread is spore formation with The production of zoospores (with or without a wall) and aplanospores, and more rarely hemizoospores and autospores. Sexual reproduction in naked forms is represented by hologamy as well as iso-, hetero-, and oogamy.

Based on the aggregate of classical morphological and cytological characters—primarily centered on the type of MORPHOLOGICAL Structure OF the body and its characteristic features—and supported by the results of modern molecular-biological approaches, a Classification Scheme for the class has been developed, distinguishing the following orders: Volvocales, Tetrasporales, Chlorococcales, Sphaeropleales, and Chaetophorales.