Elements of Structural and Systematic Botany | Page 8

with a rotary motion of the free ends of the filaments, which are often twisted together like the strands of a rope. If the filaments are entirely free, they may often be observed to move forward with a slow, creeping movement. Just how these movements are caused is still a matter of controversy.
The lowest of the Cyanophyce? are strictly single-celled, separating as soon as formed, but cohering usually in masses or colonies by means of a thick mucilaginous substance that surrounds them (Fig.?7, D).
The higher ones are filaments, in which there may be considerable differentiation. These often occur in masses of considerable size, forming jelly-like lumps, which may be soft or quite firm (Fig.?7, A, B). They are sometimes found on damp ground, but more commonly attached to plants, stones, etc., in water. The masses vary in color from light brown to deep blackish green, and in size from that of a pin head to several centimetres in diameter.
[Illustration: FIG.?7.--Forms of Cyanophyce?. A, Nostoc. B, Gloeotrichia, ��?1. C, individual of Gloeotrichia. D, Chro?coccus. E, Nostoc. F, Oscillaria. G, H, Tolypothrix. All ��?300. y, heterocyst. sp. spore.]
In the higher forms special cells called heterocysts are found. They are colorless, or light yellowish, regularly disposed; but their function is not known. Besides these, certain cells become thick-walled, and form resting cells (spores) for the propagation of the plant (Fig.?7, C. sp.). In species where the sheath of the filament is well marked (Fig.?7, H), groups of cells slip out of the sheath, and develop a new one, thus giving rise to a new plant.
The bacteria (Schizomycetes), although among the commonest of organisms, owing to their excessive minuteness, are difficult to study, especially for the beginner. They resemble, in their general structure and methods of reproduction, the blue-green slimes, but are, with very few exceptions, destitute of chlorophyll, although often possessing bright pigments,--blue, violet, red, etc. It is one of these that sometimes forms blood-red spots in flour paste or bits of bread that have been kept very moist and warm. They are universally present where decomposition is going on, and are themselves the principal agents of decay, which is the result of their feeding upon the substance, as, like all plants without chlorophyll, they require organic matter for food. Most of the species are very tenacious of life, and may be completely dried up for a long time without dying, and on being placed in water will quickly revive. Being so extremely small, they are readily carried about in the air in their dried-up condition, and thus fall upon exposed bodies, setting up decomposition if the conditions are favorable.
A simple experiment to show this may be performed by taking two test tubes and partly filling them with an infusion of almost any organic substance (dried leaves or hay, or a bit of meat will answer). The fluid should now be boiled so as to kill any germs that may be in it; and while hot, one of the vessels should be securely stopped up with a plug of cotton wool, and the other left open. The cotton prevents access of all solid particles, but allows the air to enter. If proper care has been taken, the infusion in the closed vessel will remain unchanged indefinitely; but the other will soon become turbid, and a disagreeable odor will be given off. Microscopic examination shows the first to be free from germs of any kind, while the second is swarming with various forms of bacteria.
[Illustration: FIG.?8.--Bacteria.]
These little organisms have of late years attracted the attention of very many scientists, from the fact that to them is due many, if not all, contagious diseases. The germs of many such diseases have been isolated, and experiments prove beyond doubt that these are alone the causes of the diseases in question.
If a drop of water containing bacteria is examined, we find them to be excessively small, many of them barely visible with the strongest lenses. The larger ones (Fig.?8) recall quite strongly the smaller species of oscillaria, and exhibit similar movements. Others are so small as to appear as mere lines and dots, even with the strongest lenses. Among the common forms are small, nearly globular cells; oblong, rod-shaped or thread-shaped filaments, either straight or curved, or even spirally twisted. Frequently they show a quick movement which is probably in all cases due to cilia, which are, however, too small to be seen in most cases.
[Illustration: FIG.?9.--Euglena. A, individual in the active condition. E, the red "eye-spot." c, flagellum. n, nucleus. B, resting stage. C, individual dividing, ��?300.]
Reproduction is for the most part by simple transverse division, as in oscillaria; but occasionally spores are produced also.
CLASS III.--GREEN MONADS (Volvocine?).
This group of the protophytes is unquestionably closely related to certain low animals (Monads or Flagellata), with which they