Monday, March 7, 2011

The Algae - Archimycetes - Myxomycetales


Phylum Thallophyta - The Algae - Archimycetes - Myxomycetales
With the exception of one genus, Ceratiomyxa, to which we shall refer later, all the members of this order live on the surface of dead wood, leaves and the like.
The diploid organism formed as a result of the fusion of swarm c~lls is termed a myxamoeba and resembles a true Amoeba. It can migrate by means of pseudopodia, that is, by extending a part of its protoplast followed by a flowing movement of the whole body in the direction of the pseudopodium thus formed. In this way it can move at a not inconsiderable pace. The myxamoeba is devoid of any cell wall, and the protoplast only possesses a rather thicker and tougher ectoplasm through which food material in solution is absorbed. The protoplast can, however, engulf, by flowing around them, small objects such as bacterial cells, which it appears able to digest. In this way the body grows and increases in size, becoming a multi­nucleate structure termed the plasmodium. Several myxamoebae may contribute to the formation of a plasmodium and any unconjugated swarm cells may also be incorporated into the plasmodium, which may thus, in time, both by feeding and accretion, become a body of considerable size; in fact, plasmodia containing as much as a litre of living protoplasm have been found quite frequently. During this period the plasmodium may become coloured, and, though white plasmodia are common, sulphur yellow or pink are characteristic of some species. The plasmodium now contains a very large number of nuclei produced by the division of pre-existing ones.
When the plasmodium is mature, sporangium formation begins. This may result in the conversion of the whole plasmodium into a single giant sporangium termed an aethalium, or a number of separate sporangia may be formed. In the latter case the sporangia may be sessile but are more usually stalked.
During the differentiation of the sporangium an elaborate system of internal threads is laid down as a result of the filling up of cleavage furrows in the protoplasm of the young sporangium with chitinous material. This system of threads is called the capillitium, and it is sometimes attached to a prolongation of the stalk. The sporangium also secretes a hard brittle wall. Granules of calcium carbonate frequently cover the surface of the wall, and are also formed in the meshes of the capillitium. During the formation of the spores a meiosis occurs, so that the mature spores are monoploid.
The spores are liberated by the bursting of the wall of the sporangium. This also exposes the capillitium, which is hygroscopic and twists about as it absorbs or loses water, thus breaking up the spore mass. The spores are minute, round bodies, often with beautifully sculptured walls.
On germination the spore gives rise to a single spindle-shaped swarm cell, which is provided with two apical flagella, one of which is longer than the other. The time taken for the spores to germinate varies with' the species, but laboratory tests by growing them in a sugar solution show that while some species, e.g., Reticularia lycoperdon, may germinate in a few hours, others take two to three weeks to produce swarm cells. After liberation from the spores the swarm cells fuse in pairs at once and give rise to binucleate zygotes or myxamoebae, from which a new plasmodium will arise in which the nuclei fuse in pairs, a process called karyogamy
In the genus Ceratiomyxa mentioned above the development is somewhat different, for the plasmodium lives internally within dead or decaying wood and only produces fine processes termed sporangiophores externally. These bear minute sporangia which are oval in shape and pure white in colour. It is said that each sporangium, which is frequently referred to as a spore, gives rise to about eight Swarm cells. The individual spores in other genera produce only one.
Species of the Myxomycetales are very common among dead wood and leaves in damp woods and are extremely beautiful objects. Though the individual sporangia are often no larger than a pin's head they are generally produced together in sufficient numbers to be seen easily. A careful search of any damp wood or damp pile of sticks or straw will be certain to yield a number of different species.

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