3(58 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 



different vegetable principles necessary for the economy 

 of the plant are removed from it, and a certain quantity 

 goes down to the roots, partly to aid in their growth, 

 and partly to throw new quantities of ascending sap 

 into the tree. In this descent the elaborated sap moves 

 through a system of vessels which anastomose with one 

 another in the same manner as the capillary vessels of 

 animals. 



There are, therefore, two points in this circulation 

 which require attentive consideration the spongiole 

 and the leaf. The spongioles are nothing but the young 

 succulent extremities of the roots, which have been re- 

 cently formed from portions of the descending sap, and 

 that sap is itself a species of mucilaginous solution. Pre- 

 cisely, therefore, as water will pass through the tissue of 

 a bladder the interior of which is filled with gum-water, 

 so will moisture from the ground flow through the spon- 

 giole. There is no difficulty in thus accounting for the 

 rise of the ascending sap on the principles of capillary 

 attraction, arid indeed this is the explanation generally 

 received by vegetable physiologists. 



Guided by the principle above laid down, I offered 

 the following as an explanation of the action of the leaf. 

 The ascending sap, which we may assume to be a weak, 

 watery solution, rises to the upper face of the leaf. It 

 there obtains carbonic acid from the air ; of this the sun- 

 light effects the decomposition, with the production of 

 gum, the result being a change from water to a muci- 

 laginous solution. In the tissue of the leaf we have, 

 therefore, two liquids engaged water and a mucilagi- 

 nous solution. On the principle above indicated, the 

 water will drive the mucilaginous solution before it, and 



O 



force it back along its proper vessels into the stem. 



What, then, is the reason that the light of the sun 

 controls the rapidity with which the ascending current 



