180 PLANT LIFE 
formation of carbohydrates in the plant. 
In other words, the predisposition to infec- 
tion in this instance is probably connected 
with a disturbance of the photosynthetic 
processes. 
The whole matter of immunity is evidently 
very closely related with nutrition. It may 
be the result either of a defective hereditary 
constitution or of some property of the 
environment (e. g. excessive or deficient sup- 
plies of essential food elements) which inter- 
feres with the chemical processes of the 
manufacture, distribution, or utilisation of 
food within the organism. The part played 
by the fungus depends on its physiological 
capability to take advantage of the host 
plant. It must be able to enter the body of 
its victim, and either utilise there whatever 
stores of nutriment are directly available, or 
it must modify the vital processes, and in 
this way secure for itself the nourishment it 
needs. Some‘ of the extremely specialised 
parasites, and especially some of the rusts, are 
limited to particular species, and even sub- 
species, of plants as hosts. This can only be 
interpreted as meaning that they are adapted 
to live on a very special kind of food, and 
perhaps also that they are easily affected in a 
prejudicial manner by substances which occur 
in species nearly related to their own proper 
hosts. But even the specialised parasites 
are capable of further extending their range. 
For example, the Brome grasses, of which 
