THE THALLOPHYTA 297 



sent basidia, which instead of being one-celled are composed of 

 three or four cells, each producing a spore. 



1. Ustilaginales or Smuts. — ^Here are found a number of 

 destructive parasites which attack floral organs, especially among 

 members of the Grass family. The mycelium spreads through 

 the body of the host plant and at flowering time gathers in dense 

 masses, particularly in the ovaries and surrounding tissues, which 

 become swollen and distorted, and form the so-called smut. 

 The mycelium here becomes transformed directly into a mass of 

 black, thick-walled spores which survive the winter. On germin- 

 ating the next spring, each spore produces a short filament of 

 three or four cells, the promycelium. This promycelium is 

 thought to represent a basidium, for each cell bears one or more 

 spores or sporidia, which are capable of infecting new plants. The 

 smuts of corn, oats, and wheat are particularly destructive. 



2. Uredinales or Rusts. — All members of this order are para- 

 sites, often becoming very destructive, and have the most compli- 

 cated life histories of any of the fungi. 



The common wheat rust, Puccinia graminis (Fig. 174), is the 

 best known member of the group. The mycelium of this species, 

 in the tissues of the wheat plant, breaks through the surface 

 and produces clusters of one-celled, reddish, rough-walled spores, 

 the summer spores or uredospores, which give a rust-like appear- 

 ance to the stems and leaves. Each uredospore may germinate 

 directly on another wheat plant and produce a new mycelium. 

 Toward the end of the season, clusters of another type of spore 

 are produced at the surface of the plant. These are black, two- 

 celled and heavy-walled, and are known as winter spores or 

 teleutospores. They five over the winter and in the following 

 spring each cell germinates into a slender, four-celled, saprophytic 

 promycelium somewhat as in the smuts. Each of the cells here 

 produces but a single sporidium, however, so that the structure 

 displays a much closer resemblance to a typical basidium. The 

 remarkable fact has been demonstrated that these sporidia 

 (or basidiospores) do not infect wheat, but will attack only 

 plants of the barberry. The spores germinate on the barberry 

 leaves, penetrate the tissues, and produce there a vigorous mycel- 

 ium. On the upper surface of the leaf soon appear small flask- 

 shaped structures, the spermagonia, which produce enormous 

 numbers of very minute cells or spermatia. As its name implies, 

 this organ has been thought to represent the male sexual appa- 



