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

VI. PLANT KINGDOM SYSTEM

LOWER SPORE PLANTS

GENERAL CHARACTERISTICS AND FEATURES

Class Zygnematophyceae - Zygnematophyceae

It unites unicellular coccoid and filamentous unbranched Algae, The life cycle of which lacks any motile flagellate stages and is characterized by a sexual process of the conjugation type. These are predominantly microscopic algae inhabiting freshwater bodies, with only a small number of species known from saline waters and aerophytic or edaphic habitats. This is one of the most numerous classes of algae, numbering about 10,000 species in the global flora and over 1,100 species and infraspecific taxa in the algal flora of Ukraine. Together with charophyte algae, the algae of this Class form the phragmoplast evolution line of green plants, which, through their ancestors, gave rise to higher land plants.

Cells of coccoid forms are solitary or form chain-like colonies, often constricted in the central part and consisting of two semicells. Filamentous forms appear as uniseriate unbranched filaments consisting of elongated or short cylindrical cells. The Cell wall is 3-layered — pectic-Cellulose (the outer layer is mucilaginous and pectic; the middle and inner layers are cellulosic) and porous. Characteristic features include central star-shaped or plate-like METABOLISM/14.html">Chloroplasts, as well as parietal spiral-ribbon-like ones. Cytokinesis occurs through the simultaneous growth of an annular furrow and a cell plate, which is continuous, without pores and plasmodesmata between daughter cells. Vegetative Reproduction (Cell Division in half in unicellular forms, fragmentation of filaments in filamentous forms, and sometimes The formation of akinetes) and sexual reproduction via conjugation are typical. The zygote germinates into 1-4 seedlings. Mitosis is semi-open, centrioles are absent, and the spindle persists throughout telophase. The life cycle is haplontic, without Morphology/12.html">ALTERNATION OF GENERATIONS.

Biochemically, it is analogous to other classes of algae belonging to the "green" evolutionary line, showing no differences in pigment composition, assimilation products, etc.

The Classification scheme of the class is currently undergoing refinement and harmonization of classical morphological-cytological features with modern molecular-genetic ones. Based on the type of morphological body Structure, Cell Differentiation into semicells, The Nature of The Cell wall and its ornamentation, as well as the number of seedlings developing from the zygote, 2-4 orders are distinguished; however, based on the aggregate of classical and modern data, recognizing only two is justified: Desmidiales (unicellular, coccoid, occasionally pseudofilamentous algae with heavily ornamented walls, often formed of two symmetrical halves — semicells, and a complex pore apparatus: Closterium, Cosmarium, Gonatozygon, Micrasterias, Staurastrum) and Zygnematales (coccoid forms with smooth walls not divided into semicells: Cylindrocystis, Mesotaenium, and filamentous algae with a continuous, smooth wall, often covered with a layer of mucus: Mougeotia, Spirogyra, Zygnema).

Representatives of Desmidiales are characteristic components of algocenoses in standing, swampy Water bodies with an acidic environment and calcium-depleted waters, occasionally occurring under aerophytic conditions and in the soil. Species of the order Zygnematales are found in various types of streams and water bodies, frequently forming large masses of green, mucilaginous algal mats.

According to the syllabus and objectives of the course, we will consider only the main Representatives of the order Zygnematales.