CLASSIFICATION OF DISEASES. 421 



enlargement of the flesh of a fruit, the entire or 

 partial absence of seed, are the results of disease, 

 strictly considered ; and yet these conditions are 

 very desirable in fruit culture. Therefore, practically, 

 we will regard as disease whatever is unfavorable to 

 the growth of the plant for the purpose intended. 

 This view is taken by Mr. Berkeley, of England, who 

 defines disease as " a deviation from the normal con- 

 dition of species." He treats the subject practically 

 only as it is unfavorable to these deviations from 

 nature produced by cultivation. 

 ' Species have no natural tendency to disease, be- 

 cause this might destroy them ; yet individuals or 

 varieties have such a tendency, which they mani- 

 fest in Avhatever condition they may be placed, and 

 which they transmit to their offspring. 



Berkeley thus classifies diseases : Internal, or Con- 

 stitutional; External, or Accidental; Doubtful, or 

 Conjectural. Weigmann arranged them thus : Those 

 which affect the organs of Nuirition, of Respiration, 

 and of Reproduction. But we shall adopt a classifi- 

 cation better suited to the object of this chapter, 

 and shall speak of the diseases of different species 

 of fruit according to the part of the plant which 

 they affect. This arrangement will enable the fruit- 

 grower to avail himself readily of the results of our 

 investigation. 



"While far the larger part of the diseases of plants 

 may be rather mechanical and accidental than con- 

 36 



