56 PRACTICAL COURSE IN BOTANY 
by overfeeding. This action of osmosis in withdrawing 
the contents from a cell is termed plasmolysis, and you can 
easily understand how very important a knowledge of the 
principles governing it is to the farmer in determining the 
application of fertilizers to his crops. 
Dead cells, although powerless to carry on the life processes 
of a plant, have nevertheless important uses in serving the 
purposes of mechanical support and also to some extent in 
assisting in the work of absorption, though their function 
here is a purely mechanical one. 
60. Selective absorption. — Different plants through 
their roots absorb different substances from the soil water, or 
the same substance 
in varying degrees. 
Hence, one kind of 
crop. will exhaust 
the soil of certain 
minerals while leav- 
ing other kinds in- 
tact, or very little 
diminished; and vice 
versa, another kind 
' ye : j é will take up abun- 
Fic. 72, =e of a tree enveloping a rock. dantly what its pred- 
The large sycamore, whose base is partly concealed a 
by the trumpet creeper on the left of the picture, ecessor has rejected. 
is growing in very hard, stony soil, and one of ] 
its main roots has molded itself so completely to the In this BEMIS plants 
ledge of rock protruding on the right, that when a are said to exercise a 
portion of it was torn away, as shown where the light F oy 
.streak ends at a, the impress of its fibers was so selective power in 
strongly marked on the rock as to give the latter the the absorption of nu- 
appearance of a petrified root. F 
trients. The expres- 
sion must not be understood, however, as implying any kind 
of volitional discrimination. It is merely a short and con- 
venient way of saying that the cells of different plants possess 
different degrees of permeability to certain substances, some 
being more permeable to one thing, some to another. But 
beyond this rejection of untransmissible substances there is no 
