Phylum Thallophyta - The Algae - CYANOPHYCEAE (Myxophyceae)

Phylum Thallophyta - The Algae - CYANOPHYCEAE (Myxophyceae) 
The Cyanophyceae are Algae in which the thallus may be either unicellular or filamentous. It is always enclosed in a gelatinous sheath. The cells possess phycocyanin (blue) and carotin (orange) in addition to chloro­phyll, and the varying proportions of these pigments not only mask the green colour but cause a wide variation of tint from almost pure blue to brown. They occur chiefly in fresh water and on damp surfaces; some are found in the soil, whilst a minority are marine. They constitute the principal vegetation of hot springs, some being able to live in water as high as 85° C.
Apart from their interest as the simplest of the Algae they are remarkable for their cytological structure. The cell wall is usually very thin and is composed either of pure cellulose or a hemi-cellulose. External to the wall is the gelatinous sheath, composed of pectic compounds.* It is sometimes thin and tough, sometimes massive and diffuse. It may also consist of multiple lamellae, and not infrequently forms a common investment round a group of filaments, or trichomes as they are called in this family. These sheaths may become very tough, and it is undoubtedly due to them that the plants can withstand high temperatures and considerable desiccation.
The protoplast is remarkably simple. It is more or less uniform, without any vacuole or plastids. The central region is almost colourless, and scattered in it are chromatin granules which represent all there is of a nucleus. These granules can divide, and they separate into two groups prior to cell division, but in some instances there appears to be little, if any, co-ordination between the grouping of these granules and the division of the cells. There is certainly no regular mitosis. In the external zone of the protoplasm the pigments are diffused throughout its substance and there are also present numerous minute granules, including oil drops, glycogen and others which are probably volutin and are called metachromatic granules.
Many of the filamentous species develop either terminal or intercalary heterocysts, which are easily recognized by their larger size, thick wall and homogeneous contents. The function of these bodies is not known.
Reproduction may take place by the breaking of the trichome, within the sheath, into short filaments of cells which are termed hormogones. The points of disjunction are marked by collapsed biconcave cells. The hormo­gones are set free by a breakdown of the sheath at the end of the parent filament. They are capable of slow movement though they have no flagella, and finally settle down and form fresh filaments. In one or two species reproduction may be eft'ected by gonidia, which are tiny non-motile cells derived by the division of one or more cells of the parent plant. These are liberated and finally grow into fresh filaments. Resting spores may also be produced. These bodies arise from vegetative cells which increase in size, become gorged with reserve material, and are often yellow or brown in colour. They are frequently developed in association with heterocysts. The unicellular species generally multiply only by simple cell division.
Many species of the Cyanophyceae are world-wide in distribution. They are among the first organisms to colonize new ground and they were the first organisms found on the island of Krakatau after the eruption of 1883. They are found in the Arctic, even though they may be covered for months on end with snow and ice. On the other hand, as we have seen, they occur very commonly in hot springs. Some species live within the tissues or even within the cells of higher plants. Species of Anabaena are found in cavities in the fronds of the water-fern Azolla, and also in the roots of the gymno­sperm Cycas. Species of Nostoc form colonies in the underground stem of the giant angiospermic marsh plant Gunnera manicata. Many as we shall see later, are concerned in the formation of Lichens. Their wide distribution, coupled with their peculiar structure, seems to indicate that they are probably the most ancient and primitive of the Algae.
The Cyanophyceae are divided into several orders of which we shall consider only the Hormogonales, which includes most of the filamentous types.
Hormogonales 
The Hormogonales are Cyanopqyceae in which the filaments have no individual membrane on the vegetative cells. The trichomes are simple, free and sometimes coiled. They are septate, but the septa are frequently obscure. The cells are very uniform, short and discoid or rectangular in shape, with a uniform granular cytoplasm, divisible into two regions, a peripheral one containing the pigment together with glycogen and oil­drops, and a central region which is colour less and contains metachromatic granules. The trichomes may be free-floating, or may form a more or less mucilaginous mass on solid surfaces. Heterocysts occur only in certain families.
Reproduction is by means of hormogones, short lengths cut off from the parent filament which are capable of growing into mature trichomes. Thick­walled resting spores or akinetes are often developed, usually next to the heterocysts. No method of sexual reproduction occurs.
We shall consider one common type-Nostoc.·
Nostoc 
There are a number of common species which occur either terrestrially or as aquatics in mucilaginous masses          (" moon-spit "), living either attached to a substratum or free floating. Each mass is composed of a more or less firm jelly or mucilage, with a denser surface layer, inside which are numerous twisted trichomes, which are yellowish-green in colour. These gelatinous masses may be either solid or hollow and as much as several centimetres in diameter; they frequently break open to form flattened expanses with lacerated margins.
The trichomes are much contorted and form an intricately woven mass which is more crowded towards the exterior. Each is composed of nearly spherical cells which are only loosely joined together. The heterocysts are intercalary and solitary, and are oval in shape and slightly larger than the vegetative cells.
Reproduction in CYANOPHYCEAE (Myxophyceae) 
Hormogone formation is very common and results from the rupture of the trichome at points where a vegetative cell and a heterocyst join. The large numbers of trichomes in a colony is probably due to the development of the hormogones into fresh trichomes without being liberated from the colony.
When the colony is mature akinetes may be formed from any or all of the vegetative cells, and it is not unusual to find all the cells between two heterocysts converted into akinetes. Each germinates to form a new trichome.
While many species grow on bare soil among mosses, others are sub­terranean, and have been found as much as three feet deep in the soil. Many are aquatic and live free-floating, attached to vegetation or on the bottom of the pond. A few species favour running water, particularly fast-flowing mountain streams.
Nostoc frequently forms the algal component of Lichens, while one species is found in symbiotic association with Anthoceros.

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