52 NUTRITION. 



Many excellent observers, however, deny the 

 general system of intercellular passages, or of 

 consequence the passing of the sap by these 

 means; the question must therefore be consi- 

 dered as undecided. 



41. Heat and light exercise great influence 

 on the ascent of the sap. A plant exposed to 

 the light takes up a sensibly larger quantity of 

 water than one kept in darkness. The leaves 

 exhaling moisture in great abundance (to the 

 amount of about two thirds of the water taken 

 up) and consequently requiring and receiving a 

 proportionate supply, tend largely to promote 

 the direct ascent of the sap, and a terminal 

 bunch, such as is always left by mulberry grow- 

 ers when the leaves are picked, determines the 

 rise of the sap to the top of the tree, whereas if 

 the summit be left bare, the juices will scarcely 

 be active enough to reach it, and in addition to 

 this vertical action, the cellular envelope which 

 surrounds the branches, and which communi- 

 cates with all the woody and cortical layers by 

 the medullary rays, draws the sap, by the ac- 

 tion of the living cellules, in a transverse direc- 

 tion. In Endogenous plants, in which there 

 are no medullary prolongations, the sap is ne- 

 cessarily drawn to the summit by the leaves, 



