THE STEM 167 



through a membrane. The solution of sugar which can 

 be coloured to make it more conspicuous will quickly 

 rise in the glass tube and soon reach a considerable height 

 (/). The explanation of this fact is as follows : accord- 

 ing to the laws of diffusion the water and the solution of 

 sugar tend to mix, the one moving towards the inner 

 vessel, the other towards the outside of it. But the 

 particles of water move more quickly than those of 

 sugar ; therefore the water will pass into the sugar 

 with greater rapidity than the sugar into the water ; 

 moreover, water passes through a bladder far more 

 easily than sugar ; therefore, by the joint working of 

 these two causes, the current of water into the inner 

 vessel will be far more rapid than the opposite movement 

 of sugar, and hence the rise of the solution in the tube 

 which at first seemed incomprehensible because it 

 contradicts the laws of hydrostatics. We should get 

 the same effect, though less clearly, if instead of sugar 

 we took albumen, gum, or some other substance gener- 

 ally found in vegetable cells. Therefore here also the 

 phenomenon resolves itself into diffusion, complicated 

 by the presence of the membrane. Phenomena of this 

 kind have been called osmotic. The rate of this diffusion, 

 all other conditions being equal, will also depend on the 

 area of contact between the two liquids, in our case on 

 the size of the opening closed by the bladder. Suppose 

 we grant that our apparatus presents a certain resem- 

 blance to a root-cell, a root-hair, and remember how large 

 is the area of contact between such root-hairs and the 

 water of the soil : we shall then soon realise what 

 the result must be if such an apparatus is multiplied a 

 million times, however microscopic it may be. Every 

 cell greedily absorbs water and squeezes it through its 

 presumably thinner inner wall into vessels which send 

 it up the root into the stem. 



Such is the explanation we can give of the water-raising 

 capacity of the root or root pressure. Alone it is probably 



