PRINCIPAL GROUPS OF PLANTS. 69 



These spores consist of one or more cells surrounded by a thick 

 black wall, and they produce the " black rust " seen on foliage 

 at the end of the season. 



Wheat Rust. — The most important member of the Rust Fungi 

 is Puccinia, of which there are a large number of species that are 

 destructive to economic plants, as wheat, plum, cherry, red cur- 

 rant, etc. The one whose life history has been best studied is the 

 wheat rust {Puccinia graminis), which requires two dilYerent 

 plants to complete its life history, namely, wheat and barberry. 

 The Teleutospores, or '* winter spores" (Fig. 41, //), as they 

 are called, because of their carrying the life of the plant over the 

 winter season, consist of two cells. These spores exist on the 

 leaves and stems of wheat over winter, and in the spring they ger- 

 minate (Fig. 41, A). From each cell a mycelium (promycelium 

 or basidium) consisting of two to four cells arises (Fig. 41, A, p), 

 and from the tip of each branch of the promycelium a spore 

 known as a sporidium develops (Fig. 41, A, s). The Sporidia 

 are scattered by the wind, and when they fall on the barberry 

 leaves (Fig. 41, B) they germinate, producing a dense mass or 

 mycelium which penetrates into the tissues of the host. 



Sooner or later, just within the under surface of the leaf, 

 there is formed a more or less spherical, dense mass, which grows 

 outward, breaking through the surface, forming a cup-like re- 

 ceptacle known as an secidium (Fig. 41, C). The JEcidis., or 

 cluster cups, are orange or yellow and are filled with perpendicular 

 rows or chains of spores which arise from the basidium-like 

 mycelium below. The spores, which have received the name 

 Aicidiosporcs, are somewhat spherical or polyhedral, and contain 

 a reddish-yellow oil. They are scattered by the wind and, falling 

 upon the wheat plant (Fig. 41, E) , germinate immediately, form- 

 ing a dense mycelium. At first it produces what is known as a 

 " Summer spore," or Uredospore (Fig. 41, G), giving rise to the 

 reddish-brown spots and stripes on the leaves and stalks of the 

 wheat plant. The Uredospores are i -celled, and are carried by 

 the wind to other wheat plants, thus rapidly spreading the disease. 



The Uredospores arise in much the same way as the Teleuto- 

 spores (Fig. 41, //), which form brown patches later in the sea- 

 son, and which have been already considered. The Teleutospores 



