CH. V, 4] ABSORPTION BY ROOTS (231 



salts dissolved in the soil water with which they are absorbed 

 into the plant. Though other mineral matters are also 

 absorbed, only those which contain these elements are 

 invariably essential ; and if we add the three elements, car- 

 bon, hydrogen, and oxygen, we have a list of ten elements, 

 indispensable to the life of the higher plants. 



Not all of the mineral salts dissolved in the soil water are 

 absorbed equally by plants, or in the same proportions by 

 different plants; but in how far this seeming "selective 

 power" of roots is merely incidental to their physical and 

 chemical constitution, and in how far it acts adaptively to 

 the needs of the plant, is still uncertain. Probably, as in 

 most such phenomena, something of both is involved. 



Such is the method of the primary function of roots, that 

 of absorption. The second function, anchorage of the plant 

 in the ground, is chiefly mechanical and comparatively simple. 

 Against the lateral strains upon stems from the action of 

 winds, a suitable resistance is provided in the radiating 

 disposition of the roots, with their tough cord, or cable, 

 type of construction. There is good reason to suppose that 

 roots subjected to the greatest strains may become thicker 

 and tougher in adaptive self -adjustment thereto, in the 

 very same way that our own muscles grow stronger through 

 exercise. 



In addition to the two functions which roots perform as 

 their peculiar contribution to the economy of the plant as a 

 whole, they have also certain others essential to their own 

 individual well-being, notably respiration and growth. 

 Respiration in roots has precisely the same method and 

 meaning as in other parts of the plant (page 165). Roots, 

 accordingly, require air, and this need has a dominating in- 

 fluence upon many features- of their habits and structure. 

 In plants which live in bogs, marshes, swamps, and other 

 places of standing water, the air is usually transferred to the 

 roots from the leaves along the intercellular air system, 

 which in such cases is specially developed. By ordinary 



