THALLOPHYTA: FUNGI 



143 



ing throughout the entire body of the host. It does not directly kill the 

 host cells, but lives on their food materials, which it absorbs by means 

 of haustoria. During the late spring and early summer numerous spores 

 are produced. They break through the epidermis of the leaves, groups 

 of them, known as uredinia, appearing on the surface as reddish brown 

 streaks or lines (Fig. 117). These spores are called uredospores. Each 

 consists of a stalked binucleate cell with a rather thick cell wall. They 

 are scattered by the wind, directly infecting other wheat plants, and are 

 chiefly responsible for the rapid spread of the disease, especially during 

 a wet season. Successive crops of uredospores may be produced through- 

 out the summer. 



Fig. 118. Puccinia graminis. Section through a telium on a leaf sheath of wheat, showing 

 teliospores in various stages of development, X 200. 



Later in the season, at harvest time or thereabouts, the same mj^celium 

 that produced the uredospores earlier now gives rise to elongated groups 

 of black spores called teliospores. These groups, known as telia, appear 

 chiefly on the stems and leaf sheaths (Fig. 118). The teliospores are also 

 stalked but are two-celled and heavy-walled. At first each cell has two 

 nuclei, but the members of each pair fuse as the spore matures. The 

 teliospores do not germinate until the next spring, thus carrying the 

 fungus over the winter. Upon germination, one or both cells of the 

 teliospore gives rise to a short filament. This filament is the basidium 

 (Fig. 119.4). It consists of four cells, each of which sends out a short 

 branch, called a sterigma, bearing a small terminal basidiospore. The 

 basidiospores cannot infect wheat plants. They are carried by the wind 

 to leaves of the common barberry {Berberis vulgaris), where they germi- 

 nate and produce an extensive internally parasitic mycelium. It is 

 mainly this species that is susceptible to infection by the basidiospores of 

 wheat rust. Most other barberries are immune. 



The mycelium produced by the basidiospores on the barberry develops 

 spermogonia (pycnidia), small flask-shaped organs appearing on the upper 



