96 



LETTERS ON TREES. 



fluid.* But be its nature what it may, the old roots 

 and stems may be no farther concerned in it, than as 

 being the channels through which the nourishing fluid 

 passes upwards from the soil. No facts yet known to 

 physiologists demonstrate that they are any otherwise 

 concerned in it, while those just adduced sufficiently 

 account for it independently of them. 



6. But in the course of the season, there is a descend- 

 ing as well as an upward or ascending movement of the 

 sap. And this other must equally be regarded as a 

 vital movement, and equally due to vital agency. 

 May not that movement, at least, argue vitality and 

 vital ppvfer in the old stems and roots? I apprehend 

 not ; and for this reason, — that while the ascending 

 current is plainly referable to the processes going on 

 in the buds and leaves, to the evolution of which as 

 well as its own elaboration it is subservient, the 

 descending current now in question is connected with 

 the formation of the woody layer all over the exterior 

 of the tree, and as plainly referable to the processes 

 by which that structure is evolved. This layer is dis- 

 tinct from the layers of previous years ; it is a new 

 formation, of tlie same year's growth with the young 



^ To Dr Alison, unquestionably, the merit is due of being the first, 

 in this country at least, clearly to establish both the reality and the 

 importance of this general principle in physiology ; and likewise to 

 shew how large a part it plays in the morbid as well as in the healthy 

 processes of the living body — in the acts of nutrition and secretion, 

 as well as in the movements of fluids. 



