Elements of Structural and Systematic Botany. Douglas Houghton Campbell
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.
Fig. 7.—Forms of Cyanophyceæ. A, Nostoc. B, Glœotrichia, × 1. C, individual of Glœotrichia. D, Chroöcoccus. 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.
Fig. 8.—Bacteria.
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.
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.
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 (Volvocineæ).
This group of the protophytes is unquestionably closely related to certain low animals (Monads or Flagellata), with which they are sometimes united. They are characterized by being actively motile, and are either strictly unicellular, or the cells are united by a gelatinous envelope into a colony of definite form.
Of the first group, Euglena (Fig. 9), may be selected as a type.
This organism is found frequently among other algæ, and occasionally forms a green film on stagnant water. It is sometimes regarded as a plant, sometimes as an animal, and is an elongated, somewhat worm-like cell without a definite cell wall, so that it can change its form to some extent. The protoplasm contains oval masses, which are bright green in color; but the forward pointed end of the cell is colorless, and has a little depression. At this end there is a long vibratile protoplasmic filament (c), by means of which the cell moves. There is also to be seen near this end a red speck (e) which is probably sensitive to light. A nucleus can usually be seen if the cell is first killed with an iodine solution, which often will render the flagellum (c) more evident, this being invisible while the cell is in motion. The cells multiply by division. Previous to this the flagellum is withdrawn, and a firm cell wall is formed about the cell (Fig. 9, B). The contents then divide into two or more parts, which afterwards escape as new individuals.
Fig. 10.—Volvox. A, mature colony, containing several smaller ones (x), × 50. B, Two cells showing the cilia, × 300.
Of the forms that are united in colonies[2] one of the best known is Volvox (Fig. 10). This plant is sometimes found in quiet water, where it floats on or near the surface as a dark green ball, just large enough to be seen with the naked eye. They may be kept for some time in aquaria, and will sometimes multiply rapidly, but are very susceptible to extremes of temperature, especially of heat.
The colony (Fig. 10, A) is a hollow sphere, the numerous green cells of which it is composed forming a single layer on the outside. By killing with iodine, and using a strong lens, each cell is seen to be somewhat pear-shaped (Fig. B), with the pointed end out. Attached to this end are two vibratile filaments (cilia or flagella), and the united movements of these cause the rolling motion of the whole colony. Usually a number of young colonies (Fig. x) are found within the mother colony. These arise by the repeated bipartition of a single cell, and escape finally, forming independent colonies.
Another (sexual) form of reproduction occurs, similar to that found in many higher plants; but as it only occurs at certain seasons, it is not likely to be met with by the student.
Other forms related to Volvox, and sometimes met with, are Gonium, in which there are sixteen cells, forming a flat square; Pandorina and Eudorina, with sixteen cells, forming an oval or globular colony like Volvox, but much smaller. In all of these the structure of the cells is essentially as in Volvox.
CHAPTER IV.