Читать книгу Elements of Structural and Systematic Botany - Douglas Houghton Campbell - Страница 9

These curious organisms are among the most puzzling forms with which the botanist has to do, as they are so much like some of the lowest forms of animal life as to be scarcely distinguishable from them, and indeed they are sometimes regarded as animals rather than plants. At certain stages they consist of naked masses of protoplasm of very considerable size, not infrequently several centimetres in diameter. These are met with on decaying logs in damp woods, on rotting leaves, and other decaying vegetable matter. The commonest ones are bright yellow or whitish, and form soft, slimy coverings over the substratum (Fig. 5, A), penetrating into its crevices and showing sensitiveness toward light. The plasmodium, as the mass of protoplasm is called, may be made to creep upon a slide in the following way: A tumbler is filled with water and placed in a saucer filled with sand. A strip of blotting paper about the width of the slide is now placed with one end in the water, the other hanging over the edge of the glass and against one side of a slide, which is thus held upright, but must not be allowed to touch the side of the tumbler. The strip of blotting paper sucks up the water, which flows slowly down the surface of the slide in contact with the blotting paper. If now a bit of the substance upon which the plasmodium is growing is placed against the bottom of the slide on the side where the stream of water is, the protoplasm will creep up against the current of water and spread over the slide, forming delicate threads in which most active streaming movements of the central granular protoplasm may be seen under the microscope, and the ends of the branches may be seen to push forward much as we saw in the amœba. In order that the experiment may be successful, the whole apparatus should be carefully protected from the light, and allowed to stand for several hours. This power of movement, as well as the power to take in solid food, are eminently animal characteristics, though the former is common to many plants as well.

After a longer or shorter time the mass of protoplasm contracts and gathers into little heaps, each of which develops into a structure that has no resemblance to any animal, but would be at once placed with plants. In one common form (Trichia) these are round or pear-shaped bodies of a yellow color, and about as big as a pin head (Fig. 5, D), occurring in groups on rotten logs in damp woods. Others are stalked (Arcyria, Stemonitis) (Fig. 5, J, K), and of various colors—red, brown, etc. The outer part of the structure is a more or less firm wall, which breaks when ripe, discharging a powdery mass, mixed in most forms with very fine fibres.

When strongly magnified the fine dust is found to be made up of innumerable small cells with thick walls, marked with ridges or processes which differ much in different species. The fibres also differ much in different genera. Sometimes they are simple, hair-like threads; in others they are hollow tubes with spiral thickenings, often very regularly placed, running around their walls.

The spores may sometimes be made to germinate by placing them in a drop of water, and allowing them to remain in a warm place for about twenty-four hours. If the experiment has been successful, at the end of this time the spore membrane will have burst, and the contents escaped in the form of a naked mass of protoplasm (Zoöspore) with a nucleus, and often showing a vacuole (Fig. 5, v), that alternately becomes much distended, and then disappears entirely. On first escaping it is usually provided with a long, whip-like filament of protoplasm, which is in active movement, and by means of which the cell swims actively through the water (Fig. 5, I i). Sometimes such a cell will be seen to divide into two, the process taking but a short time, so that the numbers of these cells under favorable conditions may become very large. After a time the lash is withdrawn, and the cell assumes much the form of a small amœba (I ii).

The succeeding stages are difficult to follow. After repeatedly dividing, a large number of these amœba-like cells run together, coalescing when they come in contact, and forming a mass of protoplasm that grows, and finally assumes the form from which it started.

Of the common forms of slime moulds the species of Trichia (Figs. D, I) and Physarum are, perhaps, the best for studying the germination, as the spores are larger than in most other forms, and germinate more readily. The experiment is apt to be most successful if the spores are sown in a drop of water in which has been infused some vegetable matter, such as a bit of rotten wood, boiling thoroughly to kill all germs. A drop of this fluid should be placed on a perfectly clean cover glass, which it is well to pass once or twice through a flame, and the spores transferred to this drop with a needle previously heated. By these precautions foreign germs will be avoided, which otherwise may interfere seriously with the growth of the young slime moulds. After sowing the spores in the drop of culture fluid, the whole should be inverted over a so-called “moist chamber.” This is simply a square of thick blotting paper, in which an opening is cut small enough to be entirely covered by the cover glass, but large enough so that the drop in the centre of the cover glass will not touch the sides of the chamber, but will hang suspended clear in it. The blotting paper should be soaked thoroughly in pure water (distilled water is preferable), and then placed on a slide, covering carefully with the cover glass with the suspended drop of fluid containing the spores. The whole should be kept under cover so as to prevent loss of water by evaporation. By this method the spores may be examined conveniently without disturbing them, and the whole may be kept as long as desired, so long as the blotting paper is kept wet, so as to prevent the suspended drop from drying up.