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DIVISION III. 



PATHOLOGY. 



1. Under this head we shall only enumerate the au- 

 tumnal or hibernal colouration of the leaf, its fall, and 

 the occurrence of ergot and of galls though the pro- 

 priety of ranking one of these phenomena under this 

 title, may, perhaps, be questioned. 



2. Hibernal colouration, etc., etc. Leaves may be di- 

 vided into two great classes ; those that change their 

 colour and fall off in the autumn, or a little later, and 

 those that change their colour in the autumn, but pre- 

 serve their leaves through the winter. 



3. In some plants a discolouration of the leaf takes 

 place some time before it falls, but in others the fall oc- 

 curs whilst the leaf is yet green, and sometimes the dis- 

 coloured leaf remains for a considerable time upon the 

 stem. 



4. Mohl has stated that the change of colour has no 

 connection with the death of the leaf in which it oc- 

 curs ; however it may be dependent upon its altered 

 functions, and that the death going along with the dis- 

 colouration is only accidental. 



5. The discolouration of the leaf, however, appears 

 to me more nearly related to the decay of vegetative 

 vigour than is by some apparently allowed ; and this 



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