=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. [Illustration: 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.)] [Illustration: Fig. 219. _A_, section through sorus of black rust of wheat, showing teleutospores. _B_, mycelium bearing both teleutospores and uredospores. (After de Bary.)] [Illustration: Fig. 220. Germinating uredospore of wheat rust. (After Marshall-Ward.)] [Illustration: Fig. 221. Germ tube entering the leaf through a stoma.] =407a. Teleutospores the last stage of the fungus in the season.=—The teleutospores are developed late in the season, or late in the development of the host plant (in this case the wheat is the host). They then rest during the winter. In the spring under favorable conditions each cell of the teleutospore germinates, producing a short mycelium called a _promycelium_, as shown in figs. 222, 223. This promycelium is usually divided into four cells. From each cell a short, pointed process is formed called a “_sterigma_.” Through this the protoplasm moves and forms a small gonidium on the end, sometimes called a _sporidium_. [Illustration: Fig. 222. Teleutospore germinating, forming promycelium.] [Illustration: Fig. 223. Promycelium of germinating teleutospore, forming sporidia.] [Illustration: Fig. 224. Germinating sporidia entering leaf of barberry by mycelium. Figs. 222-224.—Puccinia graminis (wheat rust). (After Marshall-Ward.)] =408. How the fungus gets from the wheat back to the barberry.=—If these sporidia from the teleutospores are carried by the wind so that they lodge on the leaves of the barberry, they germinate and produce the cluster-cup again. The plant has thus a very complex life history. Because of the presence of several different forms in the life cyle, it is called a _polymorphic_ fungus. The presence of the barberry does not seem necessary in all cases for the development of the fungus from one year to another. =409. Synopsis of life history of wheat rust.= _Cluster-cup stage on leaf of barberry._