328 Professor Marshall Ward, On the Question, etc. 



obtained between different individuals of the same variety or 

 species, and that the more the variety or species which yields the 

 pollen differs from the variety or species to which the pollinated 

 stigma belongs, the less likely is the pollen to "take" 1 . 



But if for "pollen" we read " uredospore," for "pollen-tube," 

 "germ-tube," for "pollination," " infection," and for "stigma" we 

 read "leaf," we have an exactly similar order of events in the capacity 

 for infection of the plants I am discussing, and it would be quite 

 in accordance with scientific accuracy to say that the best results 

 in infection are obtained by dusting the leaves with spores from 

 different individuals of the same varieties or species, and that the 

 more the variety or species which yields the spores differs from 

 the variety or species to which the infected leaf belongs, the less 

 likely is the infection to " take." 



The parallelism of the two cases up to this point is clear, and 

 some important results seem to me to follow. It is conceded by 

 all who have had to do with hybridisation that the obstacles to 

 crossing are not merely differences in observable structure in the 

 flowers concerned — though such may occur in particular cases — 

 but reside in obscure inter-relations between the cells of the 

 stigma, ovules, &c. of the one plant, and the pollen and the pollen- 

 grains, &c. of the other. And that is exactly what occurs in the 

 case of the Bromes and their Rust. 



But, such being the case, we may expect that just as cultural 

 variations affect the possibilities of crossing, so too they will affect 

 the possibilities of infection. Just as variation in the properties 

 of stigma and pollen are brought about by changes of environ- 

 ment, so, too, variations in the properties of the leaf-cells and of 

 the uredospores, and in consequent predisposition or immunity, 

 will be brought about by such changes. 



1 See also Bailey, Plant Breeding, 1896. 



