UREDINALES 309 



Stakman, E. C, and J. J. Christiansen, "Heterothallism in Ustilago zeae,'' 



Phytopathology, i7: 827-834, 1927. 

 ZuNDEL, G. L., "Monographic studies on the Ustilaginales attacking Andro- 



pogon," MycoL, 22: 125-158, 1930. 



Uredinales 



The Uredinales, or rust fungi, are a group of approximately 

 100 genera containing 5000 to 6000 species of obligate parasites. 

 They attack ferns and seed plants, producing pustules of rusty 

 appearance. Parasitism and specialization are highly developed 

 among rusts, and thus far all attempts to grow them on artificial 

 media have failed. All so-called cultures of rusts consist of inocu- 

 lated host plants grown under artificial or controlled conditions. 



Developmental cycle of Fiiccinia gi-aviinis. Because the de- 

 velopmental cycle of rusts is so complicated and the terminology 

 so confusing, an account of the cereal rust, Fucc'mia graminis, is 

 given at this point. Toward the end of the growing season elon- 

 gated black streaks may be noted, especially on the cereal 

 (wheat) stems. These streaks are telia (black-rust pustules), 

 compact aggregations of dark, stalked, 2-celled tehospores that 

 are formed within the stem tissues. The teliospores reach the 

 surface by rupture of the overlying tissues. They remain dor- 

 mant in situ over the winter, and in the spring each cell is ca- 

 pable of germination. From a thin spot in the tehospore wall 

 (germ pore), a short, curved hypha (basidium) emerges. This 

 basidium normally becomes 4-celled, each cell having 1 nucleus. 

 Each basidial cell gives rise to an ellipsoid spore (sporidium, 

 basidiospore) that is attached by a short sterigma. Usually the 

 entire content of a promycelial cell passes into the single spo- 

 ridium. Sporidia are forcibly dislodged, then caught up by air 

 currents, and may settle on young leaves of barberry. Here each 

 forms a slender germ tube, which penetrates the cuticle and cell 

 wall between the epidermal cells. The actual pore of entry is 

 very small, smaller than the hypha on either side. Then the hypha 

 elongates, branches, and forms an extensive intercellular, uninu- 

 cleate (monocarvotic) mycelium. The appearance of yello\\ish 

 leaf spots constitutes external evidence of the loci of infected 

 tissues. At this stage small, dark, globular bodies (pycnia), 

 formed within the tissues, appear at the upper leaf surface. 

 Numerous tiny, spherical, uninucleate spores (pycniospores) are 



