70 EGYPTIAN AGRICULTURE. 



of a more or less permeable membrane. In explanation 

 of this consider the case of a strong solution of a substance 

 separated from a weaker solution or from water by a 

 membrane permeable to water and to the substance. In 

 time the two solutions will come to the same strength, 

 partly by water moving from the weak solution to the 

 strong one, partly by the dissolved substance moving from 

 the strong solution to the weak one. As the former 

 movement is much the greater of the two the strong solu- 

 tion gains in volume at the expense of the weak one ; in 

 other words there is a flow of liquid from the latter to 

 the former. Were the membrane to be impermeable to 

 the dissolved substance then, with a solution on one side 

 arid pure water on the other, uniformity of composition 

 could never be attained, but water would continue to pass 

 into the solution in an endeavour to make the two solutions 

 equal. If the result of this accumulation of liquid on the 

 one side of the membrane is to set up an increased pres- 

 sure there, a time will at last come when this pressure will 

 balance the inward flow of water and no more water will 

 enter, or more correctly, what enters will exactly balance 

 what passes out. As a matter of fact probably no mem- 

 brane is completely impermeable to dissolved substances, 

 but some allow certain substances to pass through them 

 only very slowly. Such is the character of the proto- 

 plasmic layer in the root-hair. Easily permeable to water 

 and to many of the salts dissolved in soil water, it is almost 

 impermeable to many of the substances found dissolved 

 in the cell-sap. Osmosis, the name given to such pheno- 

 mena as have just been described, has therefore to do with 



