Fig. 158.—Penicillium.
An aërial hypha with its
terminal spore-brushes.
(× 360).
Blue mould (Penicillium).—It is generally found that the fleecy hyphae of Mucor, which first cover the damp bread, are crowded out in a short time by the growth of another mould, which is known to botanists as Penicillium. This also consists of a mycelium, which penetrates the nutritive substance, and of aërial hyphae which produce spores. Here, however, the spores are not borne in cases, but break off from the ends of brush-like branches of the aërial hyphae ([Fig. 158]). The spores of Penicillium are greenish-blue in colour, and it is from this circumstance that the mould receives its common name. The colour is, however, quite different from the leaf-green which gives the higher plants the power, in sunlight, of decomposing the carbon dioxide of the air and building up their own carbonaceous food—a power which neither Penicillium nor any other fungus possesses.
The smallest plants.—The description of microscopic organisms is beyond the scope of this book, but the student ought to realise that by far the greater number of plants are quite invisible to the naked eye. Some of these—to be found in every pond and ditch—are green, and, though they are of very simple structure, they obtain their food in a manner substantially resembling that adopted by a flowering-plant or a fern. Others, like the yeast plant, are fungi, and are subject to the limitations in food-supply which are characteristic of that class. The smallest of all plants are the bacteria; they are almost inconceivably minute, yet they possess an influence upon the health and wellbeing of mankind which it is impossible to over-estimate.
EXERCISES ON CHAPTER XI.
1. Explain in what respects a moss plant resembles and differs from the prothallus of a fern.
2. What structure in a moss corresponds, as regards reproduction, to an ordinary fern plant?
3. What do you mean by the term “alternation of generations”? Explain your answer by references to ferns and mosses.
4. How does a mushroom resemble and differ from a green plant in its method of obtaining food?