On the Physiology of Parasitism. 125 
In all cases vigorous penetration took place provided the 
spores were in a condition to germinate. 
The view which seems at present most likely to the writer is 
that the stimulation to penetration is a contact one. It is quite 
feasible of course that chemical factors may influence the nature 
and degree of this “ thigmotropic ” response. It is however prema¬ 
ture to dogmatise on this subject as the amount of reliable data 
available is insufficient for the purpose of generalisation. 
Geneyal Considerations on Plant Immunity and Resistance. 
The general trend of this research has been to show that 
nothing short of a full physiological investigation of fungal 
metabolism on the one hand and of at least certain aspects of 
higher plant metabolism on the other hand must be undertaken 
if any insight into the problems of resistance and immunity is to 
be obtained. The extraordinary diversity of the observational 
data on this subject is sufficient to prove the hopelessness of trying 
to explain these phenomena on the basis of one or two features of 
the fungus on one hand and of the parasite on the other. The 
establishment of parasitism by a particular fungus on a particular 
plant follows upon a long series of physiological actions on the 
part of the fungus and simultaneous actions or consequent reac¬ 
tions on the part of the plant. With highly specialised “ obligate 
parasites ” the reactions between host and parasite are of a very 
recondite nature, and as yet there seems no possibility of subject¬ 
ing them to experimental analysis. The application of the methods 
of the present research to such cases is thus problematical. In 
the case however of less specialised forms such as Botrytis, the 
reactions of host and parasite are comparatively simple in virtue 
of the phenomenon of “ action in advance.” On this account the 
host plant can be treated as a special kind of culture medium. Thus 
the problem entails simply the investigation of the growth of the 
parasite in a culture medium together with that of the enzymic 
process by which the living plant is converted into a dead culture 
medium. Nevertheless though the whole problem can be attacked 
on the lines of known methods, the phenomena presented are of a 
complex character, and as has been already emphasised a complete 
physiological investigation of a large number of processes must be 
carried out if any advancement is to be made. 
A scheme of research adequate to the needs of the problem 
may best be devised by consideration of the successive stages by 
