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. 
