[newton] wheat stem RUST 203 



XVIII, a form attacking Kanred heavily, and XVII, a form to which 

 it is very resistant, and could find no appreciable difference between 

 the two. Embedded material of Kanred, infected with Form XVIII, 

 to which it is very susceptible, is now on hand, and with this it is 

 hoped to determine the approximate proportion of cases in which 

 the appressoria make good their entry in these circumstances 



It should be added here that preliminary experiments with 

 Mindum (the susceptible variety used by Miss Allen) brought to view 

 cases in which the growing germ tubes passed directly over stomata 

 without forming appressoria (Plate VI, 1 and 2). This, together with 

 the tendency already noted for the fungus to develop appressorium- 

 like bodies in places other than over a stomata (Plate IV, 7), appears 

 to support the view that chemotropic attraction is not a factor in 

 rust infection. 



As noted above, in at least a considerable proportion of cases, 

 the germ tube may develop in a resistant host the usual sub-stomatal 

 swelling or vesicle. The latter sometimes fails to send out infection 

 threads. It merely remains beneath the stomatal slit and becomes 

 vacuolated (Plate VI, 6). However, the number of such cases 

 observed was not sufiicient to justify any assumption that this 

 condition is more characteristic of resistant than of susceptible 

 varieties. In most cases one or more hyphae are sent out. These 

 hyphae may grow until they meet with a cell, where, at the point of 

 contact, they form a swelling (Plate VI, 7 and 8), and apparently 

 cease growth. In no case were they found to send haustoria into 

 the host cells. The length of time that these hyphae remain capable 

 of growth varies. In some three-day preparations the hyphae were 

 already dead and shrivelled; in no leaves six days after inoculation 

 could hyphae be found which had not obviously reached the end of 

 their capacity for growth. 



From the beginning of growth in the host, it is easily discernible 

 that the vigour of the hyphae is not nearly as great as in the case 

 of those growing in the susceptible Marquis wheat. The nuclei of 

 the hyphae become smaller and appear to degenerate, and the whole 

 contents become highly granular and stain deeply. Abnormal 

 symptoms are prompt to appear also in the host cells. Those in the 

 vicinity of the fungus take on a shrunken appearance, and the nucleus 

 and chloro plastids show definite signs of disintegration (Plate VI, 

 7 and 8). The contest between host and parasite is short and decisive, 

 only a very few host cells being killed. The hyphae seldom develop 

 sufficiently to give any external evidence that the germ tube has even 

 entered. 



