THE RUSTS 



261 



the leaves and stems of their hosts. The most complicated life 

 histories in the fungi are found in this group, for many spe- 

 cies require two different hosts to complete their life cycle and 

 form a number of different reproductive spores during their 

 development. These peculiarities are best illustrated by the 

 rust of wheat. 



The wheat rust. The wheat rust (Puccinia graminis) appears 

 on wheat, oats, and other grains and grasses, first as red or 

 yellow streaks or spots upon the leaves and 

 stems (Fig. 229, A). The host is greatly weak- 

 ened and consequently matures only a small 

 yield of grain. Towards the end of the season 

 black streaks (Fig. 229, B) are formed in ad- 

 dition to the red-rust spots, and these indicate 

 the development of resting cells, or winter 

 spores, which are peculiar two-celled struc- 

 tures in Puccinia (Fig. 230). The winter 

 spores, called teleutospores, germinate in the 

 spring, and each cell gives rise to a short 

 filament, the promycelium, usually consisting 

 of four cells (Fig. 231), from which are gen- 

 erally developed four spring spores, or sporidia. 

 The winter spores, promycelium, and spring 

 spores probably correspond to the same stages 

 in the life history of a smut. 



Wherever the barberry is common, as in 



Europe and New England, the spring spores *io. 231. Promyce- 



7 1mm of the wheat 



(sporidia) infect these plants and produce on rust (Puccinia 



their leaves peculiar fructifications called graminis) 

 cluster cups, or cecidia (Fig. 232, A, B), in After Tuiasne 

 which are developed chains of cluster-cup spores, or cccidiospores 

 (Fig. 232, C). There is considerable evidence to prove that 

 the cluster cups represent the remains of what was once a 

 sexual phase in the life history of the rust, but which is now 

 much modified, and indeed entirely suppressed in some forms. 



