HEALTH AND DISEASE. 93 



not seen by the ordinary observer, but those who 

 bud, graft and multiply by cuttings on a large 

 scale know that such bud-variations are important, 

 quite apart from more extensive " sports " which 

 occasionally occur. 



On the other hand, we have reason to believe 

 that many species have died out gradually as 

 the environment altered. These plants died 

 because they did not vary suiificiently, or did not 

 vary in the right directions; they became diseased 

 with respect to the then prevailing conditions of 

 normal physiology or health. 



Disease, therefore, may be said to be variation 

 of functions in directions, or to extents, which 

 threaten the life of the plant, the normal in all 

 cases being the state of the plant characteristic 

 of the species. 



Even now, however, we have not obtained a 

 complete definition, because, since all plants die 

 sooner or later, we have not excluded the natural 

 demise of the individual or its parts, and no one 

 would call the autumnal fall of leaves, or the 

 withering of an annual after flowering, death from 

 disease. Clearly then the idea of disease implies 

 danger of premature death, and probably this is 

 as near as we shall get to a satisfactory definition. 

 Since this matter is of primary importance for our 

 present theme, I will add the following instances 

 for consideration. 



A plant in perfect health and in the fullest 

 exercise of all its functions, has its roots in a soil 

 which is suitably warmed and aerated, contains 



