202 THE ROYAL SOCIETY OF CANADA 



intact and appear entirely normal. At no stage of the disease is 

 there an extensive killing of the host tissue. As remarked by Marshall 

 Ward, "A uredine, when flourishing in a leaf, does not act as a de- 

 vastating parasite, but as one which slowly taxes its host, and even 

 stimulates the cells for some time to greater activity." 



Infection of a Resistant Host 



On Kanred, a wheat variety which, on the basis laid down in 

 Part I of this paper (see Table II), is described as immune to Form 

 XVII, the spores of this form germinate quite normally. The long 

 germ tubes follow the surface of the epidermis, dipping into depres- 

 sions, in the same manner as was observed in Marquis. On reaching 

 a stoma the tip of the germ tube swells to form an appressorium, and 

 practically all of the protoplasm flows into it, leaving the germ tube 

 almost empty (Plate \T, 3). Often the appressoria formed by two 

 or three spores may be found crowded together at a single stoma 

 (Plate VI, 4). 



In spite of this, it appears that in many cases the germ tube fails 

 to get right through the stoma. It forms an appressorium and there 

 stops (Plate VI, \5). Out of many hundreds of sections examined it 

 was possible in 50 or more to observe satisfactorily the relation of the 

 appressoria to the stomata. The formation of sub-stomatal vesicles 

 was observed in only about one-third of these cases. Since, however, 

 the technical difficulty associated with the detection of these vesicles 

 is much greater than in the case of the appressoria, it is possible that 

 a larger proportion of the latter may have made good their penetra- 

 tion. Further, it is not to be supposed that all the appressoria make 

 good their entrance even into a susceptible wheat. In the course of 

 her work the author has frequently observed sections of the sus- 

 ceptible Marquis variety, in which appressoria had apparently failed 

 to get through the stomata. 



As previously noted. Miss Allen (1) was of the opinion that only 

 a few appressoria of the rust form with which she worked, a form to 

 which Kanred was highly resistant, succeeded in penetrating the 

 stomata of this variety, and suggested that this may have been due 

 to the narrow stomatal openings. If this observation be correct, it 

 would seem that the very heavy infection of Kanred by such forms as 

 III, XI, XII, XV, XVIII and XXXII, reported in the early part of 

 this paper, "COuld only be explained on the assumption that these 

 forms have smaller germ tubes. The present writer measured the 

 average diameter of the germ tubes produced by spores of Form 



