LUTHER BURBANK 
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 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 Professor 
deVries, give a clew to the entire subject of the rise 
of sap in the tree. 
According to Vant Hoff’s 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 sides. 
As the protoplasm in the cells of the rootlets 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] 
