Page:First course in biology (IA firstcourseinbio00bailrich).pdf/225



each producing a short branch with a little sporidium, s.

A most remarkable circumstance in the life history of the wheat rust is the fact that the mycelium produced by the sporidium can live only in barberry leaves, and it follows that if no barberry bushes are in the neighborhood the sporidia finally perish. Those which happen to lodge on a barberry bush germinate immediately, producing a mycelium that enters the barberry leaf and grows within its tissues. Very soon the fungus produces a new kind of spores on the barberry leaves. These are called æcidiospores. They are formed in long chains in little fringed cups, or æcidia, which appear in groups on the lower side of the leaf (Fig. 283). These orange or yellow æcidia are termed cluster-cups. In Fig. 284 is shown a cross-section of one of the cups, outlining the long chains of spores, and the mycelium in the tissues.

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,