FRUITS FROM THE TROPICS 297 



For that matter, a column of water in even a 

 relatively small tree like the orange would prob- 

 ably exert a deleterious pressure on the cellular 

 structures. 



But in reality the water in the plant is con- 

 tained largely in the cells of the plant tissue, and 

 is passed on by osmosis or exudation from one 

 cell to another. 



It seems probable that the laws of osmosis as 

 developed by the Dutch physicist Vant Hoff, 

 partly in response to questions raised by Profes- 

 sor de Vries, give a clew to the entire subject of 

 the rise of sap in the tree. 



According to Vant HofFs theory, osmosis or 

 the passage of water through a membrane from 

 a weaker to a stronger solution, is due to the 

 pressure of the molecules in the stronger solution 

 which, in virtue of their greater numbers, beat 

 against the cell wall and exert a pressure exactly 

 comparable to the pressure of a gas. The push of 

 the molecules against the cell wall suffices to 

 squeeze water through the wall until there is an 

 equalization of pressure on both sides. 



As the cell sap in the cells of the rootlets 

 of a plant is more concentrated than the watery 

 solutions in the soil about it, osmotic action is 

 established, which results in the cells taking up 

 a certain amount of water. But cells that thus 



