114 



PRACTICAL COURSE IN BOTANY 



125. Movement of water. — It has already been shown 

 (71, HI) that a constant interchange of Hquid 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 

 the stem, while the down- 

 ward flow of elabonated 

 sap from the leaves takes 

 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. 



Fig. 130. — The .stump of a larj^e oak that 

 was injured by lightning many years ago. The 

 interior is completely decayed, leaving only 

 a hollow shell of living tissue, from which 

 branches continue to put forth leaves year 

 after year. 



