87 
injured tissues that it can give rise to fresh crops of conidia, but 
these conidia are not capable of infecting normal barley 
foliage. Putting the matter crudely we may say that their 
temporary sojourn on barley has not sufficiently altered their 
constitution to make them capable of attacking it directly—it 
can still only respond to wheat, its normal host. . 
Such considerations show clearly that the plant itself when 
growing normally has a capacity for resisting the infection of 
certain parasites and they lead one to ask whether by changing: 
the external conditions under which the plant is growing this 
power of resistance can be altered, whether it can be increased 
or diminished? If in any way it can be increased those whose 
mission it 1s to combat fungoid attacks will have a powerful 
weapon placed in their hands. So far though we have little 
evidence one way or the other. Marshall Ward has shown that 
the condition of the plant as determined by starving it of cer- 
tain food materials such as phosphates, nitrates, calcium com- 
pounds and so on, does not alter its lability to infection, but 
Hennings states that plants highly susceptible to the attacks of 
certain parasites become far less so when transplanted into 
richer soils. On the other hand it js generally believed that 
wheat, for instance, grown on rich soil is more susceptible to 
yellow rust than when grown on poor soils. A probable ex- 
planation of the latter phenomenon is that the fungus has 
greater stores of foodstuffs to draw upon in the former case and 
can consequently produce far more spores and so become more 
obvious. 
The whole subject is rendered more complex by the fact that 
it does not follow that because a plant, say for example wheat, 
can resist the attacks of yellow rust it is immune to the mildew 
as well. 
When one seeks for an explanation of the fact that of two 
very similar plants one may be highly susceptible to the attacks 
of a Particular fungus and the other immune, the answers given 
are conflicting. Speaking of wheat and its rusts a recent text= 
200k considers immunity to be due to such external characteris- 
tics as toughness of the epidermis (though infection takes place 
through the stomata) and the narrowness and erectness of the 
leaf-blade. It also states that early sown and early ripening 
Varieties escape rust best. At first sight there is a superficial 
probability that these factors are of importance but a more 
detailed examination shows that they fail to carry the burden 
Pitedaet them: — "The one extensive histological examination 
made for this purpose by Marshall Ward has shown that the 
Bou paty or otherwise of infection occurring among the species 
‘ romus When inoculated with uredospores of the biologic 
tm of Puccinia dispersa could not be correlated with such 
