DISEASES OF PLANTS GENERALLY. 119 



here naturally present itself, as to what constitutes a dis- 

 ease, and the botanist would reply that, strictly speaking, 

 every departure from the normal healthy condition of a 

 plant would constitute a disease. In agriculture, however, 

 we should take exceptions to this rigid adherence to prin- 

 ciples, as one of the objects of cultivation is to depart from 

 normal conditions, and to induce increased development 

 of various parts of the plant, which must have some influ- 

 ence upon its natural condition and habits of growth. We 

 should rather look upon disease as existing only when the 

 plants cultivated, whatever they may be, do not possess the 

 power of carrying on their processes so as to complete satis- 

 factorily that state of development which constitutes the 

 difference between a vigorous and an unhealthy crop. Mr. 

 Berkeley, in an excellent article on the Diseases of Plants, 1 

 tells us that "there are two main causes of diseases ; the one 

 arising from the derangement of the conditions necessary 

 to healthy growth, the other from injuries inflicted directly 

 by other organized beings, whether belonging or not to 

 the same great division with themselves. The first class 

 will comprise what may be considered as internal disease, 

 so far as it is independent of the presence of other organ- 

 ized beings, as well as all injuries arising from the greater 

 or less intensity of those outward agents and elements 

 which are conducive in their proper proportion to healthy 

 development; the latter, injuries derived directly from 

 the depredations of beasts, birds, or insects, or from the 

 parasitic development, whether of phenogams or crypto- 

 gams, as also the diseased structures due to the presence 

 of insects, whether in a more or less perfect state of de- 

 velopment. It is quite obvious that the more usual distri- 

 bution of the diseases of vegetables into internal and 

 external is not tenable without giving some latitude to the 

 terms ; for far the greater portion of diseases called inter- 



1 Cydo. of Agri., p. 650. 



