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

VI. PLANT KINGDOM SYSTEM

LOWER SPORE PLANTS

GENERAL CHARACTERISTICS AND FEATURES

Order Chlorococcales

It unites unicellular and colonial Algae characterized by coccoid, sarcinoid, and siphonaceous body structures, predominantly uninucleate or occasionally multinucleate. The Cell wall is strong and three-layered, with hydroxyprolines forming its skeletal framework. The chloroplast is parietal, sometimes central, with or without a pyrenoid. Mitosis is similar to that of Volvocales—specifically the Chlamydomonas type (so-called Ch-mitosis): the spindle is formed with the participation of basal bodies, daughter nuclei diverge in anaphase and then approach each other due to the contraction of interzonal spindle remnants in late anaphase and early telophase, while cytokinesis proceeds via a Cleavage furrow.

Reproduction is both Selection/8.html">Asexual and sexual: by means of biflagellate zoospores covered with a cell wall, as well as hemizoospores; under unfavorable conditions, via hypnospores or through isogamy. The basal bodies of zoospore flagella are shifted clockwise (the "1/7" position). The life cycle is haplontic with zygotic reduction, although representatives with a diplontic cycle are also known (e.g., Chlorococcum diplobionticum).

Typical Representatives of the soil algoflora, they are significantly less common in freshwater bodies.

The Structure and volume of the order have been substantially revised recently. Based on molecular biological studies, most taxa previously considered within Chlorococcales are now assigned to the order Sphaeropleales or to another Class—Trebouxiophyceae (including the orders Chlorellales and Trebouxiales). Refinement of the taxonomic placement of other taxa is ongoing and requires further study.

A typical representative of the order is the genus Chlorococcum.

Genus Chlorococcum (Fig. 31). Unicellular, spherical green algae with a single parietal cup-shaped chloroplast and a pyrenoid. Reproduction occurs via biflagellate zoospores (8-32), of the Chlamydomonas type, covered with a cell wall, or via hemizoospores. Under unfavorable conditions, hypnospores are formed—resting Cells covered with an additional thickened spiny wall. The sexual process is isogamous, and the life cycle is haplontic, or in some representatives diplontic with gametic reduction.

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Fig. 31. STRUCTURE OF THE vegetative cell and zoospore formation in Chlorococcum infusionum

A characteristic representative of soil algoflora, occasionally found in freshwater bodies and as a phycobiont in Lichens.