384 CLASS BASIDIOMYCETEAE 



infection, thus isolating the parasite from the host tissues and causing the 

 early death of the fungus before it does much damage to the host. The 

 death of the affected parts of the host plant is gradual in most cases. In 

 some cases, e.g., the rusts of small grains, the splitting of the epidermis 

 by the numerous elongated sori of the rust seems to increase the water 

 loss of the host plant to a very detrimental degree. (Fig. 126.) 



The promycelium of the Rusts is normally four-celled, each cell 

 having but one nucleus. It emerges through a thin spot in the tehospore 

 wall, the germ pore. In shape the promycelium is variable depending 

 upon the species. It may be long and slender and nearly straight, each 

 cell with a long sterigma, or short and thick and curved, constricted more 

 or less at the septa. On such a curved promycelium the sporidia always 

 arise on the convex side, often on rather short sterigmata. In a number of 

 genera the promycelium does not entirely emerge from the teliospore. 

 Thus in Zaghouania the swelling of the teliospore bursts its thick wall 

 allowing the emergence of a thin-walled, four-celled promycelium whose 

 basal portion still remains enclosed within the old cell wall. In Coleo- 

 sporium and Gallowaya the teliospore divides by cross walls into four cells 

 without emerging from the cell wall, thus producing a promycelium that 

 is entirely internal. From each cell a long slender sterigma grows up 

 through the gelatinous stratum that covers the tops of the layer of later- 

 ally adhering teliospores. Weir (1912) reported that occasionally in this 

 genus the four cells produced by the division of the teliospore are cru- 

 ciately arranged, resembling the condition in Tremella. In Chrysopsora 

 each teliospore of the two forming the stalked compound teliospore 

 divides transversely as in Coleosporium. In Gopla7ia dioscoreae (B. & Br.) 

 Cummins (1935b) the teliospores arise from the base of a gelatinous 

 matrix and push up into it but not through to the exterior. Each telio- 

 spore, as in Coleosporium, divides by transverse septa into four cells from 

 each of which a slender sterigma pushes out to the surface where the 

 sporidia are formed. This is very similar to the case in the gelatinous 

 species of Auriculariales. 



The uninucleate sporidia may be long, ellipsoid, pointed or rounded 

 at one or both ends, or may approach a spherical shape. The cell contents 

 are usually somewhat yellow. The sporidium is shot off with more or less 

 violence from the tip of the sterigma. Usually the whole contents of the 

 promycelial cell pass into the single sporidium arising from that cell but 

 occasionally a case is met with where the cell nucleus divides and one 

 nucleus and part of the cytoplasm pass into the sporidium, so that a 

 second sporidium may be produced by the same cell, as occurs more 

 frequently in the Ustilaginales. A sporidium that fails to fall upon a 

 suitable host is capable, under proper conditions of moisture, of pro- 

 ducing a secondary sporidium at the tip of the sterigma and this in turn 



