PHYSIOLOGY OF VEGTABLES. llf 



In this respect the course of the sap in vegetables dif- 

 fers essentially from the circulation in animals. 



5. Growth in genera!. 



The growth of the trunks and branches, as it re- 

 pects thickness, takes place by means of successive 

 layers, which are annually added, one to the bark, and 

 another to the wood, in a manner already described. 



The elongation of trunks and branches of trees and 

 shrubs takes place by annual shoots, which are made at 

 their extremities, No part already formed is carried 

 upward or enlarged in proportion in all its parts, as is 

 the case with animals. 



The Sword-leaved plants, such as Iris and some 

 species of Lily, elongate by additions to their base, 

 while the parts perfectly formed are carried upward ; 

 hence they appear of a darker green at their extremi- 

 ties than near the ground. Most of the grasses elon- 

 gate by additions to the bases of their several joints. 



Observation. I have determined these points by placing" stakes 

 beside the growing- plants and marking- them. 



That part of a plant which is the shooting part, 

 whether it be the base or extremity, is usually whitish 

 anil tender. 



6. Duration of the Vascular Systems. 



All perennial plants have annually a new set of radi- 

 cles to their roots, a new growth of leaves to their 

 branches, whether the former fines have fallen or not, 

 a new layer of bark, and a new one of wood added to 

 their bodies. Dr. Smith says, <* the vascular system 

 of all plants is strictly annual. This of course is ad- 

 mitted in annual plants, the existence of whose stems, 

 and often of the whole individual, is limited to one 

 season ; but it is no less true with regard to trees. 

 The layer of alburnum, on the one hand, is added to 



