IPAYSIOJLOGY OF VEGETABLES. Ill 



extremity of each annual shoot of the plant. They 

 have throughout, spiral coats see plate 3, fig. 18 &19. 

 The cortical vessels or vessels of the inner bark are 

 very distinct, botli in structure and function, from the 

 sap+vessels ; they are much smaller, and the spiral 

 coats which are conspicuous in the sap-vessels have 

 never been found in them. Through the medium uf the 

 bark of the leaf-stalk these vessels of the inner bark 

 communicate with the cells of the leaf, the same with 

 which the sap-vessels communicate. This is probably 

 the only communication which the two sets of vessels 

 have with each other. 



2. Ascent of the Sap. 



In the following account of the course of the sap, &c. 

 the theory of Mr. Knight is adopted. 



In the extreme vessels of the root which immediate- 

 ly absorb the nourishment, an important function is per- 

 formed. The matter presented to the root is water 

 holding various materials in solution : by an unknown 

 process the extreme vessels of the root, as they absorb 

 this fluid matter of the soil, convert it into sap of the 

 peculiar quality necessary to nourish the plant which 

 absorbs it. 



The sap thus formed differs widely in its properties in 

 different species. Sugar, in a greater or less degree, 

 usually abounds in it ; various salts, acids, gums, oils, 

 and resins, are occasionally found in it. 



The sap is compared to the blood of animals. 



Thus prepared by the root, it undergoes no change 

 of consequence, nor performs any important func- 

 tion until it enters the leaves and fructification. The 

 vesse^ of the sap-wood, in which it passes, seem 

 destined merely to convey it to the leaves and flowers, 

 elevated in their several convenient situations. 



Part of the sap is conveyed into the flowers and 

 fruit, where various fine secretions, such as the volatile 



