Many rusts besides _Puccinia graminis_ produce different spore forms on different plants. The phenomenon is called _heterœcism_, and was first shown to exist in the wheat rust. Curiously enough, the peasants of Europe had observed and asserted that barberry bushes cause wheat to blight long before science explained the relation between the cluster-cups on barberry and the rust on wheat. The true relation was actually demonstrated, as has since been done for many other rusts on their respective hosts, by sowing the æcidiospores on healthy wheat plants and thus producing the rust. The _cedar apple_ is another rust, producing the curious swellings often found on the branches of red cedar trees. In the spring the teleutospores ooze out from the “apple” in brownish yellow masses. It has been found that these attack various fruit trees, producing æcidia on their leaves. Fig. 285 explains how a parasitic fungus works. [Illustration: FIG. 285.—HOW A PARASITIC FUNGUS WORKS. Anthracnose on a bean pod entering the bean beneath. (Whetzel.)] [Illustration: FIG. 286.—PART OF GILL OF THE CULTIVATED MUSHROOM. _tr_, trama tissue; _sh_, hymenium; _b_, basidium; _st_, sterigma; _sp_, spore. (Atkinson.)] * * * * * _Puffballs, mushrooms, toadstools, and shelf fungi._—These represent what are called the _higher fungi_, because of the size and the complexity of the plant body as well as from the fact that they seem to stand at the end of one line of evolution. The mycelial threads grow together in extensive strands in rotten wood or in the soil, and send out large complex growths of mycelium in connection with which the spores are borne. These aërial parts are the only ones which we ordinarily see, and which constitute the “mushroom” part (Fig. 131). Only asexual spores (_basidiospores_) are produced, and on short stalks (_basidia_) (Fig. 286). In the puff-balls the spores are inclosed and constitute a large part of the “smoke.” In the mushrooms and toadstools they are borne on _gills_, and in the shelf fungi (Fig. 134) on the walls of minute pores of the underside. The mycelium of these shelf fungi frequently lives and grows for a long time concealed in the substratum before the visible fruit bodies are sent out. Practically all timber decay is caused by such growth, and the damage is largely done before the fruiting bodies appear. For other accounts of mushrooms, see Chapter XIV. LICHENS [Illustration: FIG. 287.—LICHEN ON AN OAK TRUNK. (A species of _Physcia_.) ] Lichens are so common everywhere that the attention of the student is sure to be drawn to them. They grow on rocks, trunks of trees (Fig. 287), old fences, and on the earth. They are thin, usually gray ragged objects, apparently lifeless. Their study is too difficult for beginners, but a few words of explanation may be useful.