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 
