The _resting-spores_ of wheat rust are produced in late summer, when they may be found in black lines breaking through the epidermis of the wheat stalk (black-rust stage). They are formed in masses, called _sori_ (Fig. 280), from the ends of numerous crowded mycelial strands just beneath the epidermis of the host. The individual spores are very small and can be well studied only with a microscope of high power (× about 400). They are brown two-celled bodies with a thick wall (Fig. 281). Since they are the resting or winter-spores, they are termed _teleutospores_ (“completed spores”). Usually they do not fall, but remain in the sori during winter. The following spring each cell of the teleutospore puts forth a rather stout thread, which does not grow more than several times the length of the spore and terminates in a blunt extremity. This germ tube, _promycelium_, now becomes divided into four cells by cross walls, which are formed from the top downwards. Each cell gives rise to a short, pointed branch which, in the course of a few hours, forms at its summit a single spore called a _sporidium_. This in turn germinates and produces a mycelium. In Fig. 282 a germinating teleutospore is drawn to show the promycelium, _p_, divided into four cells, each producing a short branch with a little _sporidium_, _s_. [Illustration: FIG. 282.—GERMINATING TELEUTOSPORE OF WHEAT RUST. ] 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 neighbourhood 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. [Illustration: FIG. 283.—LEAF OF BARBERRY WITH CLUSTER-CUPS. ] [Illustration: FIG. 284.—SECTION THROUGH A CLUSTER-CUP ON BARBERRY LEAF.] 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.