NUTRITION: THE WORK A.ND THE MATERIALS. 9 



much more permeable to water than it is to protoplasm ; 

 and so it happens that, while water readily penetrates the 

 membrane and diffuses itself in the protoplasm, protoplasm 

 does not nearly so readily permeate the membrane as the 

 water. Ingress of water is easy and of constant occurrence, 

 egress of protoplasm is rare and exceptional. 



Pure water or weak saline solutions, such as are gene- 

 rated in the soil under certain circumstances, pass readily 

 through membrane that is, the molecules of the one shift 

 and change places with those of the other while those of 

 gummy or albuminoid substances like protoplasm do not. 

 After a time, if there is no outlet for the water absorbed, 

 or if it is not utilised within the plant in some way, absorp- 

 tion and diffusion cease, the cell becomes saturated with 

 water, and until something happens to disarrange the 

 balance, no more is absorbed. But, even in the case where 

 the cell is saturated with water, it may still take up other 

 liquids, because the diffusive power of those other liquids, 

 in relation to the cell-wall and to the protoplasm, is diffe- 

 rent from that of water, and this absorption may go on in 

 its way till saturation point is reached for each one of 

 them, just as in the case of water. On the other hand, it 

 may happen that the plant may be saturated with other 

 substances, and incapable of taking up more of them, 

 while at the same time pure water may be freely taken up. 



Quantity absorbed. Just so much and no more of each 

 particular substance is absorbed, the exact quantity of each 

 being regulated in all cases by the condition and require- 

 ments of the cells, their membranous walls, and their con- 

 tents. Thus it happens that some particular substances 

 may be found by the chemist to exist in large relative pro- 



