=403. Uredospores of the red-rust form.=—If we make a similar preparation from the pustules of the red-rust form we see that instead of two-celled gonidia they are one-celled. The walls are thinner and not so dark in color, and they are covered with minute spines. They have also short stalks, but these fall away very easily. These one-celled gonidia of the red-rust form are called “uredospores.” The uredospores and teleutospores are sometimes found in the same pustule. It was once supposed that these two kinds of gonidia belonged to different plants, but now it is known that the one-celled form, the uredospores, is a form developed earlier in the season than the teleutospores. =404. Cluster-cup form on the barberry.=—On the barberry is found still another form of the wheat rust, the “_cluster-cup_” stage. The pustules on the under side of the barberry leaf are cup-shaped, the cups being partly sunk in the tissue of the leaf, while the rim is more or less curved backward against the leaf, and split at several places. These cups occur in clusters on the affected spots of the barberry leaf as shown in fig. 215. Within the cups numbers of one-celled gonidia (orange in color, called æcidiospores) are borne in chains from short branches of the mycelium, which fill the base of the cup. In fact the wall of the cup (peridium) is formed of similar rows of cells, which, instead of separating into gonidia, remain united to form a wall. These cups are usually borne on the under side of the leaf. =405. Spermagonia.=—Upon the upper side of the leaves in the same spot occur small, orange-colored pustules which are flask-shaped. They bear inside, minute, rod-like bodies on the ends of slender threads, which ooze out on the surface of the leaf. These flask-shaped pustules are called _spermagonia_, and the minute bodies within them _spermatia_, since they were once supposed to be the male element of the fungus. Their function is not known. They appear in the spots at an earlier time than the cluster-cups. [Illustration: Fig. 217. Section of an æcidium (cluster-cup) from barberry leaf. (After Marshall-Ward.)] =406. How the cluster-cup stage was found to be a part of the wheat rust.=—The cluster-cup stage of the wheat rust was once supposed also to be a different plant, and the genus was called _æcidium_. The occurrence of wheat rust in great abundance on the leeward side of affected barberry bushes in England suggested to the farmers that wheat rust was caused by barberry rust. It was later found that the æcidiospores of the barberry, when sown on wheat, germinate and the thread of mycelium enters the tissues of the wheat, forming mycelium between the cells. This mycelium then bears the uredospores, and later the teleutospores.