70 Meiideliaii Inheritance and Yellow Rust in Wheat 



suffered least from drought, and their growth was almost normal; if wc 

 limit our attention for the moment to these we note the following facts: 

 (1) they all remained perfectly free from rust up to about July 8th 

 (see Table XI); (2) cultures 66/9 and 11/21 were entirely rust-free until 

 maturity; and (3) the other eight cultures each contained some plants 

 that were attacked to various extents. In most cases these attacks were 

 of a shght character only, though three plants out of a total of 364 had 

 actually bad attacks. Careful observation showed that the parasite did 

 not thrive on the plants in tliese cultures, except in the rare cases just 

 noted. The pustules, though often numerous, were abnormally small, 

 and a large proportion of them failed to burst the epidermis, or only 

 did so as the foliage was beginning to shrivel up. Owing to the difficulty 

 in making due allowance tor tiie milder nature of the attack, as distinct 

 from the relative number of pustules borne, it is probable that the sus- 

 ceptibihty of some of these plants has been overstated by the grades 

 accorded to them. Moreover, a comparison of these cultures with the 

 homozygous susceptible ones growing alongside showed that the rekUive 

 difference in the extent of rust attack was as great as had existed between 

 their respective F., parents in the previous season. 



Cultures 26/22, 28/24, 3/1, 15/5, and 24/3 are grouped by themselves 

 in Table III because they were much affected by drought, and their 

 growth was far from normal. Although linally rusted to a greater extent 

 than the first ten cultures, hke them they remained uninfected till as 

 late as July 8th, and the evidence strongly suggests that they were also 

 derived from "genetically immune" F.,'s. 



It was obvious that the extent to which the power of resistance was 

 disturbed varied largely from culture to culture, and also among plants 

 in the same culture. Further, the greatest "disturbance" occurred in 

 those cultures that were most affected by drought, and possibly this 

 may also partly explain why the attack varied so widely on ilift'erent 

 individuals. Although it is impossible to state the exact extent of this 

 ■' disturbance, '" from the evidence obtained it seems safe to conclude 

 that it was sufficient to allow "genetically iniinuiie" plants to become 

 either slightly or in some cases moderately attacked. This must be borne 

 in mind when considering the remaining Fj results. 



Biffen(3, p. 123) found in his experiments that the immune F., jilants 

 bred true to that character; and in the present case, when all the experi- 

 mental evidence was taken into consideration, the only conclusion one 

 could arrive at was that these fifteen cultures were "immune" in the 

 genetic sense, although in uiauy instances the plant's power to resist 



