The classification of the rusts is based mainly upon the size and shape of the teleuto spores where they are known, as the cluster-cup and red-rust stages are pretty much the same in all. Of the commoner genera Melampsora, and Uromyces ([Fig. 47], L i), have unicellular teleuto spores; Puccinia (ii) and Gymnosporangium, two-celled spores; Triphragmium, three-celled; and Phragmidium (iii), four or more.
The rusts are so abundant that a little search can scarcely fail to find some or all of the stages. The cluster-cup stages are best examined fresh, or from alcoholic material; the teleuto spores may be dried without affecting them.
Probably the best-known member of the group is the wheat rust (Puccinia graminis), which causes so much damage to wheat and sometimes to other grains. The red-rust stage may be found in early summer; the black-rust spores in the stubble and dead leaves in the autumn or spring, forming black lines rupturing the epidermis.
Probably to be associated with the lower Basidiomycetes are the large fungi of which Tremella ([Fig. 51], A) is an example. They are jelly-like forms, horny and somewhat brittle when dry, but becoming soft when moistened. They are common, growing on dead twigs, logs, etc., and are usually brown or orange-yellow in color.
Of the higher Basidiomycetes, the toadstools, mushrooms, etc., are the highest, and any common form will serve for study. One of the most accessible and easily studied forms is Coprinus, of which there are several species growing on the excrement of various herbivorous animals. They not infrequently appear on horse manure that has been kept covered with a glass for some time, as described for Ascobolus. After two or three weeks some of these fungi are very likely to make their appearance, and new ones continue to develop for a long time.
Fig. 48.—A, young. B, full-grown fruit of a toadstool (Coprinus), × 2. C, under side of the cap, showing the radiating “gills,” or spore-bearing plates. D, section across one of the young gills, × 150. E, F, portions of gills from a nearly ripe fruit, × 300. sp. spores. x, sterile cell. In F, a basidium is shown, with the young spores just forming. G, H, young fruits, × 50.
The first trace of the plant, visible to the naked eye, is a little downy, white speck, just large enough to be seen. This rapidly increases in size, becoming oblong in shape, and growing finally somewhat darker in color; and by the time it reaches a height of a few millimetres a short stalk becomes perceptible, and presently the whole assumes the form of a closed umbrella. The top is covered with little prominences, that diminish in number and size toward the bottom. After the cap reaches its full size, the stalk begins to grow, slowly at first, but finally with great rapidity, reaching a height of several centimetres within a few hours. At the same time that the stalk is elongating, the cap spreads out, radial clefts appearing on its upper surface, which flatten out very much as the folds of an umbrella are stretched as it opens, and the spaces between the clefts appear as ridges, comparable to the ribs of the umbrella ([Fig. 48], B). The under side of the cap has a number of ridges running from the centre to the margin, and of a black color, due to the innumerable spores covering their surface (C). Almost as soon as the umbrella opens, the spores are shed, and the whole structure shrivels up and dissolves, leaving almost no trace behind.