28 OF VITAL MOTION. 



are determined by the very same agents which Bring 

 about the special movements of the cell and latex 

 vessel. 



1. Of extra-organic force as the agent in this 

 movement. 



A statement of M. Biot, quoted by Dr. Lindley, 

 affords a true clue to the discovery of the secret 

 action of these forces. According to this observer, 

 there is an abundant exudation of sap from the 

 spongioles in frosty weather, which continues until a 

 thaw supervenes, when it gives place to an opposite 

 process of absorption, of which the vigour and activity 

 is directly related to the degree of warmth and heat. 

 In other words, there are changes which seem to show 

 that the tissues of the plant contract when exposed to 

 the influence of cold, and expand when heated, just as 

 any inorganic substance ; the waters being expelled 

 from or absorbed into the porous textures, according 

 to circumstances. 



The powerful upward current of the sap, during 

 the warm weather, would also seem to point to the 

 same order of changes, and to meet with its explana- 

 tion in them ; for at this time the trunk and branches, 

 with their several appendages, will be expanded by 

 the heat, and vacua produced, into which fluids may 

 be supposed to rush. The agent, also, which produces 

 the expansion will vaporize a large quantity of these 



