CIRCULATION OF SAP IN THE STEM. 247 



the leaves and back again. This, with the support of the 

 leaves, is the mechanical function of the stem. But the 

 stem has a chemical or nutritive function also; for as the 

 sap ascends through it to the leaves it undergoes certain 

 'changes, by which it is fitted for assimilation intfie leaves. 



The water which enters the roots and which contains 

 certain nutritive substances in solution, passes on to the 

 stem and ascends to the leaves where it diffuses itself over 

 their exceedingly large aggregate surface, and then de- 

 scends through the stem to the roots. When the sap, or 

 what remains of it after having deposited its load of ali- 

 ment through the leaves, branches, and stems, reaches the 

 roots again, it is necessarily much changed in character, 

 having been exhausted of its burden or so much of it, as 

 has been utilized by the plant. It then deposits in the 

 cells of the roots what these require for their growth, and 

 it is then perhaps completely used up or has been deprived 

 of its dissolved matter. At any rate then it either mingles 

 with the upward current, or it escapes from the roots as 

 useless matter. 



But what causes this upward and downward motion of 

 the sap? It has been attributed to the action of the ab- 

 sorptive and decomposing agency of membraneous matter 

 referred to as osmose; and this is the most probable cause 

 of it; it has also been explained by the action of capillary 

 attraction; by the pressure of the atmosphere acting upon 

 a vacuum produced in the plant; but whatever may be the 

 nature of the force by which the circulation is produced, 

 its results are the same, and its effects upon the growth of 

 the plant are not changed. 



The outer covering of the stem of a tree which is called 

 the bark, and the leafy envelope of the stem of a grass 

 have much in common in regard to the nutrition of the 

 plant, and what happens in one case is duplicated in anoth- 

 er with but very little variation; excepting that in such 

 plants as wheat, which have but little foliage, the assimilat- 

 ing functions of the leaves are performed by the stems in 

 great part. The sap then which is conveyed upwards from 



