[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.=
Mycelium between cells of leaf in affected spots. Spermagonia (sing. spermagonium), small flask-shaped bodies sunk in upper side of leaf; contain “spermatia.” Æcidia (sing. æcidium), cup-shaped bodies in under side of leaf. Wall or peridium, made up of outer layer of fungus threads which are divided into short cells but remain united. At maturity bursts through epidermis of leaf; margin of cup curves outward and downward toward surface of leaf. Central threads of the bundle are closely packed, but free. Threads divide into short angular cells which separate and become æcidiospores, with orange-colored content. Æcidiospores carried by the wind to wheat, oats, grasses, etc. Here they germinate, mycelium enters at stomate, and forms mycelium between cells of the host.
_Uredo stage (red-rust) on wheat, oats, grasses, etc._
Mycelium between cells of host. Bears uredospores (1-celled) in masses under epidermis, which is later ruptured and uredospores set free. Uredospores carried by wind to other individual hosts, and new crops of uredospores formed.
_Teleutospore stage (black rust), also on wheat, etc._
Mycelium between cells of host. Bears teleutospores (2-celled) in masses (sori) under epidermis, which is later ruptured. Teleutospores rest during winter. In spring each cell germinates and produces a promycelium, a short thread, divided into four cells. Promycelium bears four sterigmata and four gonidia (or sporidia), which in favorable conditions pass back to the barberry, germinate, the tube enters between cells into the intercellular spaces of the host to produce the cluster-cup again, and thus the life cycle is completed.