114 PRACTICAL COURSE IN BOTANY 
125. Movement of water. — It has already been shown 
(71, 111) that a constant interchange of liquid is taking place 
through the stem, between the roots, where it is absorbed from 
the ground, and the leaves, where it is used partly in the man- 
ufacture of food. Just what causes the rise of sap in the stem 
is one of the problems of vegetable physiology that botanists 
have not yet been able to 
solve. There are how- 
ever, certain forces at 
work in the plant, which, 
though they may not ac- 
count for all the phenom- 
ena of the movement, 
undoubtedly influence 
them to a great extent. 
From experiments 58- 
61, we can obtain an 
idea of what some of 
these forces may be. 
126. Direction of the 
current. — These experi- 
ments show that the up- 
ward movement of crude 
sap toward the leaves is 
mainly through the ducts 
— in the woody portion of 
wan, 130, Rue sme of s large oak Wet the stem, while thedown- 
interior is completely decayed, leaving only ward flow of elaborated 
a hollow shell of living tissue, from which 
branches continue to put forth leaves year sap from the leaves takes 
after’ year. place chiefly through the 
soft bast and certain other vessels of the cortical layer. The 
action of the leaves in giving off part of the water absorbed, as 
shown in Exp. 59, probably has also an important influence 
on the course of sap movement. If loss of water takes place 
in any organ through growth or other cause, the osmotic flow 
of the thinner sap from the roots will set in that direction. 
: 
