302 Ward.- — On Relations between Host and Parasite 
a species which differs so much from the one on which the 
Fungus giving rise to the Uredospores was raised, that the 
incipient mycelium has to accept food-supplies differing con- 
siderably from those on which it was brought up, and to face 
antagonistic influences of other kinds, enzymes and possibly 
anti-toxins, &c. } on the part of the cells it is in contact with, 
it may be dominated entirely, and the infection fails. Or it 
may just manage to struggle along and even to develop 
a few feeble spores, and it may depend on the vigour of these 
latter whether the reinfection of this species can occur, and 
so on. 
ii. Factors in the Host. 
It is evident that some factor or factors on the part of the 
host-plant must be concerned in rendering infection successful, 
or the reverse ; because, as we have seen, even when the 
external conditions are favourable to germination, and even 
when spores have successfully germinated on the leaf, and 
the germ-tubes reached the stomata, infection only follows 
in the case of certain species or varieties, while others are 
‘resistant’ if not ‘immune/ The question thus arises, In 
what does this capacity for ‘ resistance,’ or ‘ immunity/ consist ? 
Several possibilities may be suggested in answer. The 
cell-walls might be supposed to be too thick for the hyphae, 
even when safely ensconced in the inter-cellular passages, to 
penetrate them ; or the stomata may be imagined to be too 
small for the tip of the tube to force its way in ; or the hairs 
on the epidermis may be a hindrance, and so forth. In most 
of the few cases where such questions have arisen, some such 
anatomical hindrances to infection have been suggested, and 
in at least one similar instance — that of wheat-rust, such 
anatomical hindrances to infection have been asserted 1 and 
denied 2 by different observers. 
But it is obvious that no mere direct examination of the 
structure can be expected to supply a sufficient answer to 
such questions, and still less that speculative suggestions 
1 Cobb, Agric. Gaz. N. S. Wales, vol. iii, 1892, p. 1. 
3 Eriksson, Die Getreideroste, 1896, p. 351. 
