126 On the Physiology of Parasitism. 
which parasitism on a particular plant becomes established. 
When fungal spores come in contact with the plant surface, the 
first desideratum is a sufficiency of water to enable germination 
to take place. An investigation of hygrometric conditions is thus 
necessary as determining the presence of this water, and also as 
preventing at a later stage the spreading of the attack by the drying 
up of the part affected. Incases where the initial spore germination 
is dependent on dew formation an investigation of the speed of 
germination is of great importance. Granted a sufficiency of 
moisture for germination, the capacity of the particular spores to 
germinate in the drop falls for consideration. This problem as 
has been shown entails a two fold investigation, viz., the capacity 
of the spores to germinate in water in virtue of energy stored 
within themselves, and the passive exosmosis of food materials 
into the drop from the underlying plant tissue. The next essential 
step in the process is that the fungus with the food material avail¬ 
able to it should be able to penetrate the cuticle (or, in certain cases, 
should be able to advance as far as the neighbouring stomata and 
so into the interior of the plant). The treatment of this problem 
entails the examination of the intrinsic penetrative power of the 
fungus on the one hand, and of the mechanical resistance of the cuticle 
on the other, together with an estimation of possible tropic factors 
concerned. The difficulty in this regard is one simply of methods 
of experiment. Once penetration of the cuticle has taken place, the 
problem becomes simply an enzymological one. if parasitism of 
the type shown by Botrytis cinerea is to be established, the 
fungus must be able to kill the adjacent host cells, and continue 
to live in and progress through the dead remains. The problem 
is therefore one of the action of the fungal principle upon the 
particular living cells, and of the capacity of the fungus to live 
and excrete its active principle in the juice of the plant. 
In the case of the so-called wound parasites (among which 
most bacteria would appear to fall) the problem presented is much 
simpler, as the problems which arise antecedent to the penetration 
of the cuticle do not come into consideration. Such parasites are 
thus to be investigated on the lines of the latter part of the scheme 
sketched above. 
The investigation of a number of parasite and host types, 
according to the scheme advocated in the present paper, would 
prepare the way for a treatment on physiological lines of the 
phenomena of resistance and immunity. 
