98 THE NUTRITIVE FUNCTIONS. 



play, that of capillary attraction. This last force together 

 with that of endosmosis, sufficiently explains all the pheno- 

 mena connected with the motion of the sap in plants. 



It is well known that if two fluids of different densities 

 be separated from each other by an animal or vegetable mem- 

 brane, a mutual action will commence, the tendency of 

 which is to produce an equilibrium of density between the 

 fluids ; and that the denser fluid will draw the lighter 

 through the membrane with a force proportional to the 

 difference of density of the two fluids. It is also known 

 that if a number of delicate tubes of different sizes be 

 immersed in water, the water will rise within the tubes 

 above its natural level on the outside of them, in propor- 

 tion to the fineness of their calibre or bore. 



Now in winter vegetable life is passive. The huge 

 oak tree, equally with the acorn that it has cast upon the 

 earth, is torpid and inactive, and can no more put forth 

 branches than the acorn can germinate. By the fall of the 

 leaves the evaporating orifices have been removed. The 

 cicatrices or leaf scars are all healed, and every pore is 

 carefully closed and sealed up against the severity of the 

 weather, sometimes by secretions especially elaborated for 

 this purpose. The fluids in the interior of the plant are at 

 this time in a state of equilibrium. Capillarity cannot 

 act. Fluids do not rise in capillary tubes closed at the 

 top. 



With the return of heat and light to the earth in spring, 

 the fountains of nutrition are again unsealed. The resinous 

 exudation on the buds is melted, the pores are opened, and 

 the store of starch, oil, and other secretions, which always 

 exists in the neighborhood of all growing points, is changed 

 into dextrine and sugar, in consequence of the absorption 

 of the oxygen of the atmosphere. The cells immediately 



