CHAPTER XX. 



SYMPTOMS OF DISEASE {Continued). 



Spotted leaves The colours of spots White, yellow, 

 brown, and black spots on leaves Farti-coloured 

 spots The browning, etc., of leaves. 



Discoloured spots or patches on the herbaceous 

 parts of plants, especially leaves, furnish the 

 prominent symptoms in a large class of diseases, 

 due to many different causes, and although we 

 cannot maintain this group of symptoms sharply 

 apart from the last, as seen from the considerations 

 on albinism, it is often well marked and of great 

 diagnostic value. By far the greater number of 

 spot-diseases are due to fungi, but this is by no 

 means always the case. The most generally use- 

 ful method of subdividing the classes, though 

 artificial like all such classifications, will be accord- 

 ing to the colour of the spots or flecks, which, 

 moreover, are usually found on the leaves. It is 

 necessary to note, however, that various conditions 

 may modify the colour of spots on leaves. Many 



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