CHAPTER XIII. ABOUT FUNGI. FUNGI are an important class of flowerless plants, belonging to the division called Thallogens, those plants which have no distinction between stem and leaf. They consist wholly of cells, and are distinguished from other plants by the entire absence of chlorophyll (see Chapter II.) from their cells, which are also devoid of starch. Instead of absorbing carbon from the atmosphere, as do green plants, they absorb oxygen and give off carbon, in this respect α b. с FIG. 134. resembling animals. Some of the lower forms were described in Chapter I.; we shall have occasion again to refer to several of these. The Yeast-plant (Torula) we may take as a type of the fungus cell. Here, in fig. 134, we have enlarged representations of it. At A we have a single plant,a simple cell, consisting of a cellulose wall (a) and a central mass of protoplasm (b), with a clear space or vacuole (c). In this it does not differ from ordinary vegetable cells, but its difference may be seen on subjecting it to an experiment. Examined by the microscope, we note the absence of any green or red colouring matter. It has no chlorophyll. If we run in a little solution of iodine on the slide we shall observe that the protoplasm is stained brown; the cell-wall remains uncoloured. If starch were present it would be stained blue. Torula, therefore, has no starch, and this absence of starch is a characteristic of fungi. If we place the slightest particle of yeast in a saccharine solution, we shall observe that the liquid, previously clear, has in a day or two become turbid. If we take up a small quantity of the liquid on the point of a pencil or a glass rod, and place it under the microscope, we shall find that the whole of the fluid. is teeming with millions of Torula, which have been produced by the multiplication of the few we added to our liquid. We now take two bottles and half fill them with fresh saccharine fluid, then add the slightest drop of the turbid liquor to each, cork them both up, and place one in complete darkness, the other in the light. On examining them in a few days, we shall find that they are equally turbid. Therefore Torula is not dependent upon light for the power of growth; this also is a characteristic of fungi. On loosening the cork after a day or two, we shall notice a terrific rush of air or gas from the bottle, or if we fail to loosen or remove the cork the pressure from within will do it for us with considerable vehemence. But we can test this gas, and shall then find it to be carbonic anhydride, which is the same as the gas exhaled by animals. Here we have another characteristic of fungi, for we have seen (Chapter II. ante) that plants possessing chlorophyll give off oxygen. Fungi are essentially either parasites or scavengers, or both. We have seen (Chapter II.) that green plants have the power of constructing protein out of a few elementary substances. Not so fungi; their food must be organised,—that is, the elementary substances must be chemically combined to form a part of some previously existing animal or plant. Thus sugar is a vegetable product which consists of the chemical elements Carbon, Hydrogen, and Oxygen. Torula can grow and flourish in a solution of sugar and water, though it cannot live in a mixture. of Carbon, Hydrogen, and Oxygen, unless they have been elaborated into a compound by being taken into the vegetable economy. The reproduction of Torula is effected in the simplest manner possible. It either develops transverse partitions across the cell and thus divides itself into two or more cells; or, what is more common, it produces a swelling or bud at some point outside the cell-wall, and this bud grows into a full-sized cell identical in every respect with that from which it originated. Fig. 134, B, shows how this process of budding goes on. All fungi are composed of cells similar in every respect to Torula, though in the higher species these cells are combined in a variety of ways to produce forms varying greatly from each other. As an advance in organisation upon Torula we have Penicillium (fig. 135). Here we have simply a number of cells like Torula, a little drawn out and placed end to end. At its summit it bears a string of round ⇒ FIG. 135. mass formed by their growth is known as the mycelium. These hyphæ send off branches above and below; those above, which are erect, are the aerial hypha, whilst those below are the submerged hypha, and serve the purpose of roots. The aerial hypha bear upon their summits the conidia, which also are formed by the transverse division of the cells. It should be noted that, neither in this nor in any other form of fungus, do cells multiply by longitudinal division. Another form very similar to Penicillium is Mucor, but in this the hypha consists of an undivided tube, or a cell drawn out to a very great length. The aerial hypha bear a large globular cell (the sporangium), which contains a large number of smaller cells (ascospores). These ascospores are set free by the bursting of the sporangium (or ascus, as we shall have to call it hereafter), and, germinating, repeat the process described in Penicillium. But in this species there is an alternative method of reproduction shown in figs. 137 and 138. Two aerial hyphæ (H) in the same vicinity throw out a branch each. These branches have dilated ends which ultimately come into contact with each other. A septum or division is formed across the branch just below the dilated end, so that the branch becomes terminated by a cell. After these two cells come in contact, their applied faces become attached, the intervening cell-walls become absorbed, and the protoplasm of the two cells mingle and form one large cell-the zygospore. It is very different in FIG. 136. nature to the ascospore, for the latter, on germinating, gives rise directly to a perfect Mucor; but the zygospore produces a short hypha, which gives off an erect |