^^ DISEASE IN PLANTS. 



escape the conviction that a diseased plant, so 

 long as it lives, is also varying in response to the 

 environment. The principal difference between 

 the two cases is, that whereas the normal healthy- 

 plant varies more or less regularly and rhyth- 

 mically about a mean, the diseased one is 

 tending to vary too suddenly or too far in 

 some particular directions from the mean ; the 

 healthy plant may, for our present purposes, be 

 roughly likened to a properly balanced top 

 spinning regularly and well, whereas the diseased 

 one is lurching here, or wobbling there, to the 

 great danger of its stability. For we must 

 recognise at the outset that disease is but varia- 

 tion in directions dangerous to the life of the 

 plant. Health consists in variation also, but 

 not in such dangerous grooves. That the passage 

 from health to disease is gradual and ill-defined in 

 many cases will readily be seen. In fact we 

 cannot completely define disease. Mere abnor- 

 mality of form, colour, size, etc., is not necessarily 

 a sign of disease, in the usual sense of the word, 

 otherwise the striking variations of our cultivated 

 plants would suggest gloomy thoughts indeed, 

 whereas we have reason to believe that many 

 cultivated varieties are more healthy in the 

 sense of resisting dangerous exigencies of the 

 environment than the stocks they came from. 

 Strictly speaking, no two buds on a fruit-tree are 

 alike, and the shoots they produce vary in position, 

 exposure, number, and vigour of leaves, and so 

 forth. The minute variations here referred to are 



