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 

 will take up abun- 

 dantly what its pred- 

 ecessor has rejected. 

 In this sense, plants 

 are said to exercise a 

 selective power in 

 the absorption of nu- 

 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 



FIG. 72. Root absorbing mineral food from 

 rock. The large sycamore, whose base is partly con- 

 cealed by the trumpet creeper on the left of the pic- 

 ture, is growing in very hard, stony soil, and one of 

 its main roots has molded itself so completely to the 

 ledge of rock protruding on the right, that when a 

 portion of it was torn away, as shown where the light 

 streak ends at a, the impress of its fibers was so 

 strongly marked on the rock as to give the latter the 

 appearance of a petrified root. 



