Fig. 36.—A, an insect that has decayed in water, and become attacked by a water mould (Saprolegnia), natural size. B, a ripe zoösporangium, × 100. C, the same discharging the spores. D, active. E, germinating zoöspores, × 300. F, a second sporangium forming below the empty one. G i–iv, development of the oögonium, × 100. H, ripe oögonium filled with resting spores, × 100.
On carefully removing a bit of the younger growth and examining it microscopically, it is found to consist of long filaments much like those of Vaucheria, but entirely destitute of chlorophyll. In places these filaments are filled with densely granular protoplasm, which when highly magnified exhibits streaming movements. The protoplasm contains a large amount of oil in the form of small, shining drops.
In the early stages of its growth the plant multiplies by zoöspores, produced in great numbers in sporangia at the ends of the branches. The protoplasm collects here much as we saw in V. sessilis, the end of the filament becoming club-shaped and ending in a short protuberance ([Fig. 36], B). This end becomes separated by a wall, and the contents divide into numerous small cells that sometimes are naked, and sometimes have a delicate membrane about them. The first sign of division is the appearance in the protoplasm of delicate lines dividing it into numerous polygonal areas which soon become more distinct, and are seen to be distinct cells whose outlines remain more or less angular on account of the mutual pressure. When ripe, the end of the sporangium opens, and the contained cells are discharged ([Fig. 36], C). In case they have no membrane, they swim away at once, each being provided with two cilia, and resembling almost exactly the zoöspores of the white rust ([Fig. 36], D, E). When the cells are surrounded by a membrane they remain for some time at rest, but finally the contents escape as a zoöspore, like those already described. By killing the zoöspores with a little iodine the granular nature of the protoplasm is made more evident, and the cilia may be seen. They soon come to rest, and germinate in the same way as those of the white rusts and mildews.
As soon as the sporangium is emptied, a new one is formed, either by the filament growing up through it ([Fig. 36], F) and the end being again cut off, or else by a branch budding out just below the base of the empty sporangium, and growing up by the side of it.
Besides zoöspores there are also resting spores developed. Oögonia like those of Vaucheria or the Peronosporeæ are formed usually after the formation of zoöspores has ceased; but in many cases, perhaps all, these develop without being fertilized. Antheridia are often wanting, and even when they are present, it is very doubtful whether fertilization takes place.[5]
The oögonia ([Fig. 36], G, H) arise at the end of the main filaments, or of short side branches, very much as do the sporangia, from which they differ at this stage in being of globular form. The contents contract to form one or several egg cells, naked at first, but later becoming thick-walled resting spores (H).
CHAPTER IX.
THE TRUE FUNGI (Mycomycetes).
The great majority of the plants ordinarily known as fungi are embraced under this head. While some of the lower forms show affinities with the Phycomycetes, and through them with the algæ, the greater number differ very strongly from all green plants both in their habits and in their structure and reproduction. It is a much-disputed point whether sexual reproduction occurs in any of them, and it is highly probable that in the great majority, at any rate, the reproduction is purely non-sexual.
Probably to be reckoned with the Mycomycetes, but of doubtful affinities, are the small unicellular fungi that are the main causes of alcoholic fermentation; these are the yeast fungi (Saccharomycetes). They cause the fermentation of beer and wine, as well as the incipient fermentation in bread, causing it to “rise” by the giving off of bubbles of carbonic acid gas during the process.
If a little common yeast is put into water containing starch or sugar, and kept in a warm place, in a short time bubbles of gas will make their appearance, and after a little longer time alcohol may be detected by proper tests; in short, alcoholic fermentation is taking place in the solution.
If a little of the fermenting liquid is examined microscopically, it will be found to contain great numbers of very small, oval cells, with thin cell walls and colorless contents. A careful examination with a strong lens (magnifying from 500–1000 diameters) shows that the protoplasm, in which are granules of varying size, does not fill the cell completely, but that there are one or more large vacuoles or spaces filled with colorless cell sap. No nucleus is visible in the living cell, but it has been shown that a nucleus is present.
If growth is active, many of the cells will be seen dividing. The process is somewhat different from ordinary fission and is called budding ([Fig. 37], B). A small protuberance appears at the bud or at the side of the cell, and enlarges rapidly, assuming the form of the mother cell, from which it becomes completely separated by the constriction of the base, and may fall off at once, or, as is more frequently the case, may remain attached for a time, giving rise itself to other buds, so that not infrequently groups of half a dozen or more cells are met with ([Fig. 37], B, C).