56 THE WORK OF THE ROOTS [chap. 



clover, which contains no silica but much lime, or the 

 ash of mangolds, which, while containing a good deal of 

 lime, is specially rich in potash and soda. These specific 

 differences would seem to show that the roots of a plant 

 possess a power of selection, so that they can in the one 

 case take in silica and in the other reject it ; or, again, 

 that they can take in potash in preference to soda, or 

 both rather than the lime which is so much more 

 abundant in most soils. It is a mistake, however, to 

 attribute this selective power to the roots ; it really resides 

 in the active growing cells of the plant ; the root hairs 

 (and they are the active absorptive organs of the plant, 

 both for water and the nutrients coming in with the 

 water) allow the passage of whatever dissolved substances 

 are presented to them until the sap within possesses 

 the same concentration as the water outside the plant. 

 When this stage has been reached, no more can enter, 

 until the living cells of the plant by withdrawing some 

 of the material for constructive purposes lower the 

 concentration of the sap in that particular substance. 

 Potash, for example, accumulates in a plant rather than 

 soda, even though there may be more soda in the soil, 

 because the plant's cells keep utilising the potash and 

 taking it out of the sap solution; whereas a small 

 quantity of soda maintains the plant sap as saturated 

 as the soil water outside, because so little of it is 

 required by the plant's cells. But though the active 

 agency resides in the growing cells, and not in the 

 roots, the result is the same — the plant as a whole 

 does exert a selective action on the elements of nutrition 

 presented to it, so that all plants of the same kind have 

 a certain characteristic ash composition, in which 

 differences of soil, season, manuring, etc., do not 

 cause wide variations. Furthermore, plants of different 

 species possess sharply differing characteristics in the 



