OF YITAL MOTION. 29 



fluids, and the vapours, passing off into the atmo- 

 sphere, will leave their former spaces to be filled from 

 the same sources; and hence (the condition of the 

 atmosphere being such as to favour evaporation and 

 radiation) the same agent which causes the primary 

 rush of fluids from the earth will provide for the con- 

 tinuance, and convert the rush into a continuous current. 



The downward movement of the sap during the 

 colder months of winter, when the temperature of the 

 air is too low to produce any considerable expansion 

 in the tissues of the aerial system of the plant, and 

 when, for the same reason, the evaporation of fluids 

 will be retarded, would seem also to be of similar sig- 

 nificance ; for at this time we may suppose the under- 

 ground portions of the plant to be relatively warmer 

 and more expanded than the aerial ; and, consequently, 

 on the same principles we may account for the change 

 of current which has taken place. 



Other powers may, and do, operate in these move- 

 ments; but the actual exudation and absorption of 

 sap from the roots, witnessed by Biot, on the super- 

 vention and cessation of a frost, together with the 

 changes in the main current which characterize the 

 circulation of the plant in summer and winter, all 

 point to heat as a grand and important agent. 



In the circulation of the plant it has been already 

 stated that the fluids move indifferently through the 

 cells and vessels, or through the insterstitial spaces, 



