The æcidiospores are formed in the spring, and after they have been set free, some of them lodge on wheat or other grasses, where they germinate immediately. The germ-tube enters the leaf through a stomate, whence it spreads among the cells of the wheat plant. In summer one-celled reddish uredospores (“blight spores,” red-rust stage) are produced in a manner similar to the teleutospores. These are capable of germinating immediately, and serve to disseminate the fungus during the summer on other wheat plants or grasses. Late in the season, teleutospores are again produced, completing the life cycle of the plant. [Illustration: FIG. 285.--HOW A PARASITIC FUNGUS WORKS. Anthracnose on a bean pod entering the bean beneath. (Whetzel.)] 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. _Puffballs_, _mushrooms_, _toadstools_, _and shelf fungi_.--These represent what are called the _higher fungi_, because of the size and 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 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 puffballs 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 Chap. XIV.