SECT. IX. CAUSES OF DESCENT. JQ5 



in breadth, owing to the retarded descent of the 

 proper juice from want of motion. But this re- 

 mark though founded on the general aspect of plants 

 affecting the habitats in question is not without its 

 exceptions. For the Pine though inhabiting the 

 most lofty mountains is still a lofty tree ; and the 

 Oak though inhabiting the recesses of the grove may 

 still, even in respect of diameter, be the monarch 

 of the wood. In aged subjects the descent of the 

 proper juice, or at least the augmentation of the 

 plant is sometimes promoted by means of paring 

 off the lifeless part of the bark, owing, as Mr. 

 Knight thinks to the increased pliancy or flexibility 

 which is thus communicated to the stem ; and the 

 excision of any decayed part is always of benefit to 

 the plant. But the operations in question will com- 

 municate but little flexibility to a stiff and aged 

 trunk where the mass of wood has become already 

 firm and indurated ; so that the good effect pro- 

 duced is perhaps more properly ascribed to the 

 facility which is thus given merely to the access 

 of air. 



Structure of the vessels. Mr. Knight is of Structure 

 opinion that the vessels of the bark are better fitted ves sels. 

 by their conformation to convey the proper juice 

 towards the root than in any other direction, which 

 opinion he founds upon the following experiment : 

 Four strong shoots of a Vine were selected and 

 converted into layers; and were in the end of the 

 autumn disengaged from their parent stock, at the 



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