Host and Parasite in certain Diseases of Plants. 397 



Fig. 3. The same oaks as those of fig. 2, photographed from the same spot on. 

 July 20th, 1888. The cumulative injury .to the leaves in successive years 

 results in the death of the trees. 



of the co-operating factors are to be brought into the foreground, 

 though, until this is decided, it may be a hopeless task to consider 

 prophylactic measures. As examples of the complex interactions 

 that may be met with in the first group of diseases as arranged 

 above, we might consider the following : — 



A soil is said to be unsuitable as regards aspect, or elevation, or 

 steepness, but it will be evident that the degree of unsuitableness 

 may vary with the depth and structure of the soil, and with the lati- 

 tude, the proximity of mountain ranges or the sea, and other factors 

 which influence the climate ; instances of disease, in the broad sense 

 of the w T ord, are frequent enough where two neighbouring crops or 

 growths of the same species of plant suffer in very different degrees, 

 •owing to slightly different combinations of such factors of the 

 environment ; and the difficulty of referring the disease more espe- 

 cially to any one cause only increases with experience. Or, take 

 the structure, &c, of the soil. It may vary in chemical composition, 

 in capacity for retaining water, in physical texture, and so forth ; 

 and the enormous differences to be met with are best known to those 

 who have to cultivate large estates or continuously observe large 

 tracts of country. But it is matter of general experience that the 

 ohemical composition of a soil is one of its least important features 



