LUTHER BURBANS: 



exerted by the weight of the column of water, car- 

 ried, let us say, to the top of a redwood tree. For 

 that matter, a column of water in even a relatively 

 small tree like the orange would probably exert 

 a deleterious pressure on the cellular structures. 



But in reality the water in the plant is contained 

 largely in the cells of the plant tissiie, 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 HofT, 

 partly in response to questions raised by Professor 

 deVries, give a clew to the entire subject of the rise 

 of sap in the tree. 



According to Vant HofTs theory, osmosis or the 

 passage of water through a membrane from a 

 weaker to a stronger solution, is due to the pres- 

 sure 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 sfdes. 



As the protoplasm in the cells of the rooUets of 

 a plant is more concentrated than the watery so- 

 lutions in the soil about it, osmotic action is 

 established, which results in the cells taking up a 



[284] 



