CIRCULATION. 99 



surrounding the buds thus become filled with a sap more 

 dense and mucilaginous than that contained in the other 

 parts of the plant. The equilibrium of the fluids is thus 

 disturbed, and a strong endosmotic and capillary ascent of 

 the thin watery juices towards the buds is gradually 

 induced throughdut the entire organism of the plant, tend- 

 ing to its restoration. At length when the buds are 

 developed into branches, and the young leaves are spread 

 abroad in the atmosphere, the ascent of the sap becomes 

 powerfully accelerated by the evaporation which takes 

 place at their surface. 



Notwithstanding that the sap is spread by endosmosis 

 in all directions through the cells of plants, yet it is evi- 

 dent that the current will be the strongest where capillary 

 influences most abound ; consequently, it will move espe- 

 cially in their fibro-vascular framework, which we have 

 seen terminates in the leaves in a system of capillaries 

 which anastomose with each other, in the same manner as 

 the capillary vessels in which the arterial and venous sys- 

 tems of animals terminate. 



The motion of the nutrient fluid in the animal and 

 vegetable capillaries is mainly to be attributed to local 

 influences. It is capable of being proved by experiment, 

 that if two liquids communicate with each other through 

 a capillary tube, for the walls of which both have an 

 affinity, the liquid which has the greatest affinity for the 

 tube will be absorbed into its cavity and drive the other 

 before it. It is obvious that the same effect will take 

 place, if instead of one we suppose any number of tubes to 

 communicate in this manner. If now we suppose that the 

 liquid absorbed into the tube undergoes, whilst there, such 

 a change in its constituents as to have its affinity for it 

 diminished, it is plain that it may be driven out by a fresh 



