As the spores are forming, an abnormal growth is set up in the cells of the part attacked, which in consequence becomes enormously enlarged ([Fig. 38], A), single grains sometimes growing as large as a walnut. As the spores ripen, the affected parts, which are at first white, become a livid gray, due to the black spores shining through the overlying white tissues. Finally the masses of spores burst through the overlying cells, appearing like masses of soot, whence the popular name for the plant.
The remaining Mycomycetes are pretty readily divisible into two great classes, based upon the arrangement of the spores. The first of these is known as the Ascomycetes (Sac fungi), the other the Basidiomycetes (mushrooms, puff-balls, etc.).
Class II.—Ascomycetes (Sac Fungi).
This class includes a very great number of common plants, all resembling each other in producing spores in sacs (asci, sing. ascus) that are usually oblong in shape, and each containing eight spores, although the number is not always the same. Besides the spores formed in these sacs (ascospores), there are other forms produced in various ways.
There are two main divisions of the class, the first including only a few forms, most of which are not likely to be met with by the student. In these the spore sacs are borne directly upon the filaments without any protective covering. The only form that is at all common is a parasitic fungus (Exoascus) that attacks peach-trees, causing the disease of the leaves known as “curl.”
All of the common Ascomycetes belong to the second division, and have the spore sacs contained in special structures called spore fruits, that may reach a diameter of several centimetres in a few cases, though ordinarily much smaller.
Among the simpler members of this group are the mildews (Perisporiaceæ), mostly parasitic forms, living upon the leaves and stems of flowering plants, sometimes causing serious injury by their depredations. They form white or grayish downy films on the surface of the plant, in certain stages looking like hoar-frost. Being very common, they may be readily obtained, and are easily studied. One of the best species for study (Podosphæra) grows abundantly on the leaves of the dandelion, especially when the plants are growing under unfavorable conditions. The same species is also found on other plants of the same family. It may be found at almost any time during the summer; but for studying, the spore fruits material should be collected in late summer or early autumn. It at first appears as white, frost-like patches, growing dingier as it becomes older, and careful scrutiny of the older specimens will show numerous brown or blackish specks scattered over the patches. These are the spore fruits.
Fig. 39.—A, spore-bearing filaments of the dandelion mildew (Podosphæra), × 150. B, a germinating spore, × 150. C–F, development of the spore fruit, × 300. ar. archicarp. G, a ripe spore fruit, × 150. H, the spore sac removed from the spore fruit, × 150. I, spore-bearing filament attacked by another fungus (Cicinnobulus), causing the enlargement of the basal cell, × 150. J, a more advanced stage, × 300. K, spores, × 300.