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.
407. Uredospores can produce successive crops of uredospores.—The uredospores are carried by the wind to other wheat or grass plants, germinate, form mycelium in the tissues, and later the pustules with a second crop of uredospores. Several successive crops of uredospores may be developed in one season, so this is the form in which the fungus is greatly multiplied and widely distributed.
Fig. 218.
Section through leaf of barberry at point affected with the cluster-cup
stage of the wheat rust; spermagonia above, æcidia below.
(After Marshall-Ward.)
Fig. 219.
A, section through sorus of black rust of wheat, showing teleutospores.
B, mycelium bearing both teleutospores and uredospores.
(After de Bary.)
