Page:EB1911 - Volume 19.djvu/122

 spore formation they present an essentially similar appearance. There is, however, great variety in the degree of concentration or expansion presented by plasmodia, in relation with food supply, moisture and other circumstances. The plasmodia move slowly about over or in the substratum, concentrating in regions where food supply is abundant, and leaving those where it is exhausted.

On examining under the microscope a film which has spread over a cover-slip, the channels are seen to be streams of rapidly moving granular protoplasm. This movement is rhythmic in character, being directed alternately towards the margin of an advancing region of the plasmodium, and away from it. As a channel is watched the stream of granules is seen to become slower, and after a momentary pause to begin in the opposite direction. In an active plasmodium the duration of the flow in either direction varies from a minute and a half to two minutes, though it is always longer when in the direction of the general advance over the substratum. When the flow of the protoplasm is in this latter direction the border becomes turgid, and lobes of hyaline protoplasm are seen (under a high magnification) to start forward, and soon to become filled with granular contents. When the flow is reversed, the margin becomes thin from the drainage away of its contents. A delicate hyaline layer invests the plasmodium, and is apparently less fluid than the material flowing in the channels. The phenomena of the rhythmic movement of the protoplasm are not inconsistent with the view that they result from alternating contraction and relaxation of the outer layer in different regions of the plasmodium, but any dogmatic statement as to their causation appears at present inadvisable.

Minute contractile vacuoles may be seen in great numbers in the thin parts of the plasmodium between the channels. In stained preparations nuclei, varying (in Badhamia utricularis) from 2·5 to 5 micromillimeters in diameter, are found abundantly in the granular protoplasm (fig. 6, 𝑏). They contain a nuclear reticulum and one or more well-marked nucleoli. In any stained plasmodium some nuclei may be found, as shown in the figure 𝑏, which appear to be in some stage of simple (amitotic) division, and this is, presumably, the chief mode in which the number of the nuclei keeps pace with the rapidly growing plasmodium. There is, however, another mode of nuclear division in the plasmodium which has hitherto been observed in one recorded instance (19, p. 541), the mitotic (fig. 6, 𝑐-𝑓), and this appears to befall all the nuclei of a plasmodium simultaneously. What the relation of these two modes of nuclear division may be to the life-history is obscure.

That the amitotic is the usual mode of nuclear division is indicated by the very frequent occurrence of these apparently dividing nuclei and also by the following experiment. A plasmodium of Badhamia utricularis spreading over pieces of the fungus Auricularia was observed to increase in size about fourfold in fourteen hours, and during this time a small sample was removed and stained every quarter of an hour. The later stainings showed no diminution in the number of nuclei in proportion to the protoplasm, and yet none of the sample showed any sign of mitotic division (20, p. 9). It would appear therefore that the mode of increase of the nuclei during this period was amitotic.

Prowazek (28) has recently referred to nuclear stages, similar to those here regarded as of amitotic division, but has interpreted them as nuclear fusions. He does not, however, discuss the mode of multiplication of nuclei in the plasmodium.

In the group of the Calcareae, granules of carbonate of lime are abundant in the plasmodia, and in all Mycetozoa other granules of undetermined nature are present. The colour of plasmodia varies in different species, and may be yellow, white, pink, purple or green. The colouring matter is in the form of minute drops, and in the Calcareae these invest the lime granules.

Nutrition.—The plasmodium of Badhamia utricularis, advancing over the pilei of suitable fungi, feeds on the superficial layer dissolving the walls of the hyphae (17). The protoplasm may be seen to contain abundant foreign bodies such as spores of fungi or sclerotium cysts (vide infra) which have been taken in and are undergoing digestion. It has been found experimentally (11) that pieces of coagulated proteids are likewise taken in and digested in vacuoles. On the other hand it has been found that plasmodia will live, ultimately producing sporangia, in nutrient solutions (9). It would appear therefore that the nutrition of plasmodia is effected in part by the ingestion of solid foodstuffs, and in part by the absorption of material in solution, and that there is great variety in the complexity of the substances which serve as their food.

Sclerotium.—As the result of drought, the plasmodium, having become much denser by loss of water, passes into the sclerotial condition. Drawing together into a thickish layer, the protoplasm divides up into a number of distinct masses, each containing some 10 to 20 nuclei, and a cyst wall is excreted round each mass (fig. 7). The whole has now a hard brittle consistency. In this state the protoplasm will remain alive for two or three years. On the addition of water the cyst walls are ruptured and in part absorbed, their contents join together, and the active streaming condition of the plasmodium is resumed. It is to be noted, however, that the sclerotial condition may be assumed under other conditions than dryness, and sclerotia may even be formed in water.

The existence of the sclerotial stage affords a ready means of obtaining the plasmodium for experimental purposes. If a cultivation of the plasmodium of Badhamia utricularis on suitable fungi (Stereum, Auricularia) is allowed to become partially dry the plasmodium draws together and would, if drying were continued, pass into the sclerotial stage on the fungus. If now strips of wet blotting-paper are placed so as to touch the plasmodium, the latter, attracted by the moisture, crawls on the blotting-paper. If this is now removed and allowed to dry rapidly, the plasmodium passes into sclerotium on it. By this means the plasmodium is removed from the partially disintegrated and decayed fungus on which it has been feeding, and a clean sclerotium is obtained, which, as above stated, remains alive for years (21, p. 7). An easy method for obtaining small plasmodia for microscopic examination is to scatter small fragments, scraped from a piece of the hard sclerotium, over cover-slips wetted with rain-water and kept in a moist atmosphere. In twelve to twenty-four hours small plasmodia will be seen spreading on the cover-slips and these may be mounted for observation.

The plasmodial stage ends by the formation of the sporangia. The plasmodium withdraws from the interstices of the material among which it has fed, and emerges on the surface in a diffuse or concentrated mass. In the case of Badhamia utricularis it may withdraw from the fungus on which it has been feeding, or change into sporangia on it. The mode of formation of the sporangia will be described in the case of Badhamia, some of the chief differences in the process and in the structure of the sporangia in other forms being subsequently noticed.

When the change to sporangia begins the protoplasm of the plasmodium becomes gradually massed in discrete rounded lobes, about a half to one millimeter in diameter and scattered in clusters over the area occupied by the plasmodium. The reticulum of channels of the plasmodium becomes meanwhile less and less marked. When the whole of the protoplasm is drawn in to the lobes, the circulation ceases. The lobes are the young sporangia. Meanwhile foreign bodies, taken in with the food, are ejected, and the protoplasm secretes on its outer surface a pellicle of mucoid, transparent substance which dries as the sporangia ripen. This invests the young sporangia, and as they rise above the substratum falls together at their bases forming the stalks; extended over the substratum it forms the hypothallus, and in contact with the rounded surface of the sporangium it forms the sporangium-wall. While the sporangium-wall is formed externally a secretion of