In this article we will discuss about:- 1. General Features of Chlorophyta 2. Distribution of Chlorophyta 3. Plant Body 4. Reproduction.
General Features of Chlorophyta:
Chlorophyta is the largest of the eight divisions of algae. Members of the Chlorophyta, or grass-green algae are similar to higher plants being characterized by a well-defined nucleus, photosynthetic pigments localized in chloroplastids in which usually pyrenoids are present, the food reserve is commonly stored as starch, and the possession of cell walls in which cellulose is usually a clearly recognizable ingredient.
Most of the grass- green algae are fresh-water forms with a few exception of marine species. They exhibit a considerable range of variation in the form and structure of the plant body. In short, the diversity of habit and habitat is very striking and in this respect the Chlorophyta surpass any other algal taxon.
Motile cells both zoospores and gametes may be bi-, quadri-, or multiflagellate with the exception of forms where motility is lacking altogether. Gametic reproduction is common within the entire taxon.
Moreover, clear division of labour indicated by some heterotrichous forms, and special adaptations of the terrestrial species to survive against external unfavourable conditions have been quite intriguing to some phycologists to trace the algal ancestry of land plants.
Distribution of Chlorophyta:
The green algae enjoy a wide range of distribution in aquatic (both fresh-water and marine) and terrestrial habitats.
Some of the fresh-water plankton forms form greenish scum on the surface of quiet, stagnant water (Volvox, Chlamydomonas, Spirogyra, Zygnema, Cosmarium) or grow firmly attached to submerged rocks, plant parts, and similar objects in free-flowing streams (Oedogonium, Cladophora, Chaetophora, Coleochaete).
Some of the marine green algae are: Bryopsis, Caulerpa, Acetabularia, Codium; and some of the common ones which prefer moist terrestrial habitats, such as damp soil and shaded sides of rocks and bark of trees are: Trentepohlia, Pleurococcus.
Several are designated as snow or ice algae (Scotiella spp.) because of their habitats. Whereas, others are able to live in concentrations of salts many times higher than occurring in the oceans.
Some of the green algae have lost their green pigments turning out to be parasites upon other plants causing serious-diseases, for example, of tea and pepper plants (Cephaleuros), of Sphagnum (Phyllobium sphagnicolum), of Ambrosia (Rhodochytrium spilanthidis).
Others merely grow upon other plants as epiphytes. Some are epizooic forms growing on small crustacians (Chlorogonium spp.). Again, some are space parasites, of which most common is Chlorochytrium lemnae growing inside the tissue of Lemna. Still others form algal component of most lichens (Pleurococcus, Trentepohlia, Cladophora). Again Zoochlorella and Carteria live in association with Hydra and Convoluta.
Plant Body of Chlorophyta:
Members of the green algae are well known for the extraordinary variety of forms and shapes and organization of plant body. They are, on the whole simple algae and many of the unicellular forms belong here. But simple as they are in structure, some of them in their mode of development and in external complex form approach nearer to the higher plants than any other algae.
Major forms of green algae are outlined below.
i. Unicellular Motile Forms:
The simplest structure is seen under this category where unicellular spherical, oval or pear-shaped motile plant body may be isolated individual or colonial form being not a mere assemblage of flagellated cells, but is a coordinated whole behaving as a well-organized unit with a definite number of cells arranged in a:specific manner known as a coenobium.
ii. Unicellular Non-Motile Palmellate Forms:
A loose assemblage of cells which lie embedded in a common gelatinous matrix secreted by the several cells forming a colony. Colonies are generally amorphous, may often have definite shapes.
iii. Unicellular Non-Motile Coccoid Forms:
The plant body is small isolated unicellular non-motile spherical cell having sedentary habit.
iv. Filamentous Forms:
The filamentous condition is an important advancement over the unicellular condition. In a large number of higher forms, the plant body is filamentous which may be un-branched or branched.
v. Parenchymatous Thalloid Forms:
Plant body is parenchymatous thallus of simple to complex in form produced as a result of division of cells in more than one plane.
vi. Heterotrichous Forms:
This is the most advanced type. Plant body is composed of two portions, a prostrate portion consisting of creeping threads attached to the substratum —the prostrate system, and a more or less richly branched erect portion growing from the prostrate portion—the erect system.
In some forms both the systems are fairly well developed, whereas in others, one of these systems is developed better, the other being poorly developed, or very much reduced, or even completely suppressed.
vii. Siphonaceous Forms:
In a number of green algae considerable enlargement of the plant body may take place without the occurrence of any septation producing a large multinucleate structure known as a coenocyte.
Reproduction in Chlorophyta:
The methods of reproduction encountered in the green algae show great diversity which may be considered under three headings vegetative, asexual and sexual.
i. Vegetative Reproduction:
It is one of those processes in which portions of the plant body become separated off to give rise to new individuals without any obvious changes in the protoplasts. Common examples are afforded by the fragmentation into two or more pieces or through an accidental or natural separation of its parts.
Each portion may then grow into an independent individual. Besides this, the process of multiplication by ordinary cell division is also characteristic of some unicellular algae.
ii. Asexual Reproduction:
Most common method of asexual reproduction is by the formation of zoospores. The zoospores are flagellate spores that are formed singly or in numbers either in a vegetative cell or in a specialized part of the plant body known as a sporangium. The zoospores vary in size, shape, and nature of flagellation.
The formation of zoospores is a complicated process and may take place in a variety of ways. Each zoospore, under favourable conditions develops into a new plant. In some species non-motile spores are formed which are called aplanospores, but if these should then secrete a thick wall they are known as hypnospores. All these spores produce new plants.
iii. Sexual Reproduction:
Sexual reproduction is well established among the most primitive forms and persists in the highest members of the green algae with a few exception of autosporic forms. Gametic union ranges from isogamy to anisogamy, and even oogamy with a relatively advanced stage of evolution having a tendency of retention of the products of gametic fusion in the mother plant and to develop a mechanism to afford protection against unfavourable conditions.
Depending on species, union of gametes both of which may be non-flagellate (aplanogamy) or flagellate (piano- gamy). Normally the two gametic nuclei fuse immediately after fusion of gametes, but in many cases they do not fuse until just before germination of the zygote.
Germination of the zygote may take place soon after its formation, but in the great majority delayed as the time interval is necessary for the ripening of the zygote. Meiosis generally takes place during the germination of the zygote and as such practically in all fresh-water algae the diploid phase in the life cycle is restricted to the zygospore or oospore which is comparable with the sporophyte of the higher plants.