SECT. IX. CAUSES OF DESCENT. 



also. In the language of Mr. Knight they are 

 merely a continuation of the external tubes already 

 noticed, which after quitting the base of the foot- 

 stalk he describes as not only penetrating the inner 

 bark, but descending along with it and conducting- 

 the proper juice to the very extremity of the root. 

 In the language of M. Mirbel they are the large or 

 rather simple tubes so abundant in the bark of 

 woody plants, though not altogether confined to it ; 

 and so well adapted by the width of their diameter 

 to afford a passage to the proper j uice. 



SECTION IX. 

 Causes of Descent. 



THE proper juice then, or sap elaborated in the 

 leaf, descends by the returning vessels of the leaf- 

 stalk, and by the longitudinal vessels of the inner 

 bark, the large tubes of Mirbel and external tubes 

 of Knight, down to the extremity of the root. 

 What is the cause of its descent ? It appears that According 

 the descent of the proper juice was regarded by the earlier 

 earlier phytologists as resulting from the agency 

 of gravitation, owing perhaps more to the readiness 

 with which the conjecture suggests itself than to 

 the satisfaction which it gives. But the insufficiency 

 of this cause was clearly pointed out by Du Hamcl, 

 who observed in his experiments with ligatures that 

 the tumor was always formed on the side next to 



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