[Chap. XLIV THE FUNGI 561 



over the winter on the stubble and straw. After winter dormancy they 

 germinate, and a short chib-shaped hypha (basiditim) develops from 

 each of the two cells. The basidium is four-celled, and from each of 

 its cells a third kind of spore ( basidiospore ) is produced. 



During the remainder of the life cycle of this rust certain remarkable 

 phenomena occur. The basidiospores are blown about by the wind. 

 Hvphae from these spores do not infect wheat plants, but thev do infect 

 the common l^arberry plant and a few of its relatives.^ The hyphae pene- 

 trate the leaves of barberr>'. In a short time there appear on the upper 

 surfaces of the leaves pustules of flask-shaped masses of hvphae, sperma- 

 gonia, which produce a fourth type of reproductive body, spennatiiim. 

 The spermatia may unite with receptive hyphae of the internal mycelium 

 and result in binucleate hyphae. Since the different spermatia in a leaf 

 mav have originated from various races of the rust, such a union makes 

 possible hybrids between different races. Such hybrids still further com- 

 plicate measures of control. 



Further growth of these binucleate hvphae within the barberry leaf 

 results in a mycelium and pustules of cluster cups (aecia, sing, aecium) 

 on the lower surface of the barberry leaf. Within these cups, a fifth re- 

 productive body, the cluster cup spore, or aeciospore, is formed. Aecio- 

 spores in turn continue the life cycle onl\' when the\^ lodge and germinate 

 on young wheat plants and a limited number of other grasses. The 

 mycelium from them grows within the tissues of the wheat plant, and 

 first the red spores, and later the black spores, develop — thus completing 

 the life cycle. 



Here, then, is a parasitic rust that lives successively on two totallv 

 unrelated host plants during a complete life cycle. It has several kinds 

 of spores, which are morphologicallv and physiologically different from 

 each other. Curious as it may seem, the first definite proof that diseases 

 may be caused by parasites was obtained when Anton de Barv discov- 

 ered many of the facts of the life history of wheat rust in 1865. 



It is interesting to note the possible variations in the life cycle of the 

 stem rust of wheat, as given above. Red spores cannot survive the winters 

 of the North Central States. They may, however, be blown many miles 

 southward and there cause infection of growing wheat or other grasses 

 in the autumn. The red spores produced in the South cannot survive the 



* Berberis vulgaris is very susceptible to this rust. The green and red \'arieties of the 

 cultixated Japanese barberry, Berberis thntibergii, are immune to it. Certain other species, 

 such as the American barberry (B. canadensis) and the species of MaJionia. are susceptible. 



