12 Practical Plant Biology. 



coloured solution below to the colourless water above. As the 

 cylinders are observed day by day this zone of gradation will be 

 seen to rise higher and higher, until finally the solutes are equably 

 distributed throughout the liquid. Under ordinary conditions, 

 no doubt, currents hasten this uniform distribution but even when 

 these are eliminated, diffusion is adequate, given time, to ac- 

 complish it alone. 



Evidently if we could interpose a partition between the solution 

 and the water above, and if this partition obstructed the passage 

 of the solute while it allowed water to permeate it, the diffusing 

 particles, as they spread through the water would be checked by 

 the partition. As they dash forward they will strike the mem- 

 brane and will be reflected from it into the surging mass of their 

 fellows below: rebounding from these they are again hurled 

 against the membrane. The sum of the myriad impacts of this 

 intense bombardment will act as a pressure pushing back the 

 partition. 



Partitions of this nature, which obstruct the passage of dis- 

 solved substances while they allow the solvent to pass freely, are 

 well known. They are found in nature and they may be artificially 

 made. Ordinary animal and vegetable parchments act more or 

 less completely in this way, and by their means the pressure pro- 

 duced by a diffusing solute may be easily seen. 



One of the simplest ways of observing this is as follows : the 

 open ends of a piece of parchment tube, such as is used by 

 chemists for dialysis, are each bound on to a tightly fitting rubber 

 stopper. One of the stoppers is perforated and supplied with a 

 glass or ebonite stop-cock. Thus fitted, the tube is filled with a 

 strong solution (20 per cent.) of sugar. The stop-cock is closed 

 and the whole is submerged in clean water. After a short time 

 the parchment tube will be found distended and bulged owing to 

 the diffusion of the sugar particles pushing out the parchment. 

 Soon the pressure begins to stretch the parchment and if we open 

 the stop-cock the elastic recovery of the parchment will cause a 

 jet of the solution to issue from the cock, giving a visible demon- 

 stration of the pressure. 



A dried raisin put into water is noticed to swell and ultimately 

 become distended owing to the same phenomenon. Water passes 

 through its skin, and the sugar and other soluble substances 

 within go into solution and tend to diffuse or spread in the water. 

 In this, however, they are opposed by the obstruction offered by 

 the skin to their passage, and the pressure developed by the 

 diffusion of the dissolved particles pushes the flaccid skin out- 



