RUST-RESISTANT VARIETIES OF WHEAT 45 



(Plate IX, 1, 2, and 4). The hyphal tips may branch very profusely 

 but they become vaculoated very early in most cases and appear un- 

 thrifty. It requires 8 or 9 days for the stage of infection to be 

 reached on Khapli that is reached in 3 or 4 days in susceptible varieties. 

 Short, thick, or rounded hyphal segments are quite common, those 

 at the end of a branch often containing as many as 5 or 6 nuclei, some 

 of which appear to be disintegrating. 



The tips of hyphae naturally die when a group of host cells among 

 which they are growing is killed. However, they may disintegrate 

 without having first killed host cells (Plate IX, 1, 2, 3, and 4). There 

 may be many variations. The hyphae may not send out haustoria and 

 die in consequence, or, even if they do send them into the cells, death 

 may occur. Branches of hyphae which have sent haustpria into host 

 cells frequently become vacuolated and gradually die, or the pro- 

 toplasmic contents may change to granular, deep-staining masses. 

 The whole appearance suggests fungous hyphae growing in an un- 

 favorable nutrient solution. 



In about 11 or 12 days a distinct tendency toward the formation 

 of hyphal wefts can be observed. These vary greatly both in size 

 and position. They may be mere aggregations directly under the 

 epiderm or deeper down in the tissues; or they may begin to wedge 

 the epiderm cells apart after the manner of young pustules. Often 

 small, unruptured pustules are formed in which there is a fairly large 

 number of abortive spores. The pustules may rupture the epiderm 

 but they are always extremely small,. and as a rule the spores are 

 small. The average size of a large number of spores measured was 

 29.69 x 20.68 /*, whereas the average size of the wheat rust spores 

 which were used in making the inoculations was 35.38x21.39 yw. 



After about 20 days practically all the host cells and a large 

 number of the hyphae in an infected region are dead. Haustoria may 

 be present in the host cells in fairly large numbers, but most of them 

 are dead, their protoplasmic contents having broken up into granular, 

 deep-staining masses. The nuclei of the host cells are often dis- 

 integrating also. The infection by this time, and usually earlier, has 

 completely run its course. Comparatively very few spores have been 

 produced, and, under natural conditions, secondary infections would 

 probably not occur to any extent. 



THE COURSE OF INFECTION IN OTHER RESISTANT FORMS 



Substantially the same sequence of events occurs in other resistant 

 forms, such as Arnautka 288, Kubanka 2094, emmer 1522, einkorn 2433 

 (sometimes), and in such cases as the infection of rye by barley rust. 

 The differences seem to be in degree rather than in kind. In the cases 

 of emmer and einkorn the killing of host cells is rarely found but 



