The Ascent of Sap 123 



Anybody who has bought one of the toy bal- 

 loons made of colored bladder and filled with gas 

 has had a chance to learn that gases pass through 

 membrane. After a day or two, the balloon can 

 no longer fly up to the ceiling. There is not a split 

 nor a prick in it — yet it has lost all its bounce. 

 The gas has passed out and air has come in 

 through the thin bladder of which the balloon is 

 made. This happened because the air 

 on the outside of the bladder was much 

 denser than the gas on the inside. 



In the same way, and in obedience to 

 the same natural law, a denser and a 

 lighter liquid will mingle slowly through 

 the delicate wall of a plant cell. 



So the earth-water in the root-hair 

 oozes slowly through the thin wall which 

 separates it from the sap in the next cell. 



Soon this second cell becomes rrom Farmer's 'BuUetia 

 ,, ... No. 173. 



swollen to many times its orig- 



. , . / . ,, Fig. 32- Wood of 



inal size, and its wall gets a spruce showmg its 



' _ ® cellular structure. 



stretched till It can stretch no 

 more. Then in the same slow way water is drawn 

 through this wall into the third cell — and so It 

 mounts on up to the top of the tree. 



We may say that the living cells pass the water 

 on from one to another all the way up to the 

 highest twig. This is the " vital movement " of 

 the sap, and it has been called " the greatest 

 wonder of the spring." 



