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 



