172 BOTANY. 



neath the epidermis, which is soon ruptured, exposing the 

 spores (Fig. 94, II) in reddish lines or spots upon the leaves 

 and stems. This is the Red-rust stage, so common before 

 -wheat-harvest. These red spores fall easily, and quickly 

 germinate, producing more Red-rust (Fig. 95, 2>), and so 

 rapidly increasing the parasite. 



357. (m) Somewhat later in the season the same para- 

 sitic filaments which have been producing Red-rust spores 

 begin to produce lines or spots of dark-colored, thick- walled, 

 two-celled bodies (teleutospores), the so-called spores of 

 the Black-rust (Fig. 94, III). Being thick-walled, 

 they endure the winter without injury, and when spring 

 comes (IV) they germinate on the rotting straw and pro- 

 duce several minute spores, called sporids (Fig. 95, A and 

 B). This is the fourth and last stage of the rust. The 

 sporids fall upon Barberry-leaves and germinate (Fig. 95, 

 G), giving rise to cluster-cups again. 



These stages are so difEerent in appearance that for a long time tliey 

 were regarded as distinct plants, and received different names. Thus 

 the first stage was classified as a species of ^cidiura, the second as a 

 species of Uredo, and the third as a Puccinia. "We still preserve 

 these names by sometimes calling the spores of the first secidiospores, 

 and of the second uredospores, while the third name is retained as 

 the scientific name of the genus. 



The spoi-ids cannot ordinarily produce rust directly upon wheat, 

 probably because of the toughness of the epidermis; but it has re- 

 cently been discovered that when sporids germinate upon very young 

 leaves of wheat-seedlings, they penetrate the epidermis and then soon 

 give rise to a red-rust stage. In such cases the cluster-cup stage is 

 omitted. This is no doubt the mode of propagation of rust on the 

 spring wheat so largely grown in the Mississippi Valley, and also 

 upon oats and bailey (sown in the spring), which are affected by the 

 same species. 



There are many kinds of rusts, distinguished mainly by their te- 

 leutospores, which are one-celled (Uromyces and Melampsora), two- 

 celled (Puccinia and Gymnosporangium), or many-celled (Phragmldi- 



