14 PLANT LIFE ON THE FARM. 



much more permeable to one of the two fluids than to 

 the other. Thus, in the case of a living cell, the mem- 

 brane or wall is 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 per- 

 meate 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 gener- 

 ated 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 proto- 

 plasm do not. After a time, if there is no outlet for the 

 water absorbed, or if it is not utilized within the plant in 

 some way, absorption 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 different from that of water, and this 

 absorption must 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 

 requirements of the cells, their membranous walls, and 

 their contents. Thus it happens that some particular 

 substances may be found by the chemist to exist in large 



