200 THE ROYAL SOCIETY OF CANADA 



Normal Infection of a Susceptible Host 



The development of the fungus on a susceptible (tolerant) host 

 is considered normal infection. The infection of Marquis wheat 

 (susceptible) by Form XVII will be described here, and the abnormal 

 condition found in Kanred wheat (immune) left for consideration in 

 the succeeding section. 



The germination of the urediniospore on the epidermis usually 

 takes place within the first twenty-four hours. The tips of the 

 numerous germ tubes can be seen preparing to enter the stomata 

 during the second day, and by the third day infection is well estab- 

 lished. 



When the spore germinates, two germ tubes frequently appear, 

 but one develops more quickly than the other, and the growth of the 

 weaker one is soon arrested. The surviving germ tube grows rapidly, 

 following the epidermis quite closely for long distances, often for the 

 length of ten to twelve epidermal cells before entering a stoma. 

 When the tip reaches a stoma instead of entering directly it swells 

 up and forms an appressorium. Here practically the entire proto- 

 plasmic contents of the germ tube are concentrated (Plate IV, 5 and 

 6). Bolley (6) has depicted the germ tube passing straight through 

 the stoma to the mesophyll cells below. Further, he says that the 

 germ tube from these urediniospores "may bore its way through the 

 skin of a wheat plant and thus start another point of infection." 

 Neither of these phenomena has been observed by the writer. An 

 appressorium has been formed in all cases observed, and infection 

 was always brought about by way of a stoma. 



The germ tube is not always uniform in thickness. Swellings 

 often appear in places, usually depressions in the leaf surface, that are 

 not directly above a stoma (Plate IV, 7). These swellings have the 

 appearance of young appressoria, as the protoplasm aggregates here 

 more densely than in the other parts of the tubes. In a few it was 

 observed that a swelling appeared above a stoma, as in the formation 

 of an ordinary appressorium, but the tube did not enter the leaf at 

 this point but continued to grow in length, entering by another stoma. 



From the appressorium a thin process passes through the stomatal 

 slit to the substomatal space (Plate IV, 5). As soon as the neck has 

 passed through the aperture it enlarges to form the sub-stomatal 

 vesicle (Plate IV, 5, 8 and 9). Into this vesicle the whole contents 

 of the spore are poured, and the entry of the fungus is completed. 

 The germ tube and appressorium soon wither and are lost to sight. 



