ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 



87 



line, for as a rule they describe a spiral as they elongate. Their movement seems 

 as though it were for discovering the most favourable parts of the earth for absorp- 

 tion and attachment. In this manner they penetrate into the interspaces in the 

 earth which are filled with air and water. They also have the power of thrusting 

 aside minute particles of earth, especially if the latter consists of loose sand or mud. 

 If they strike perpendicularly a solid immovable bit of earth, they bend aside 

 and grow round it with their surfaces closely adpressed to that of the obstacle until 

 they reach the opposite point on the other side, when they once more resume their 

 original direction (fig. 12 3 ). When they encounter large grains of earth they 



^ 



Fig. 12.— Absorptive Cells on Root of Penstemon. 



'Seedling with the long absorptive cells of its root ("root-hairs") with sand attached. = The same seedling; the sand 

 removed by washing. * Root- tip with absorptive cells; xlO. * Absorptive cells with adherent particles of earth. 'Section 

 through the root-tip ; x60. 



sometimes stop and swell up to the shape of a club. The club divides into two or 

 more arms, which grasp and cling to the granule like the fingers of a hand. Many 

 fragments of earth remain thus in the grasp of finger-like processes, whilst others 

 are held fast in the knots and spirals of corkscrew-shaped root-hairs which are 

 often found tangled together. But the retention of most of the earth-particles 

 which adhere to a plant, including fragments of lime, quartz, mica, felspar, &c, as well 

 as plant-residues, is due to the fact that the outermost layer of the absorptive cells 

 is sticky, it being altered into a swollen gelatinous mass which envelops the 

 particles. When this sticky layer becomes dry it contracts and stiffens, and the 

 granules partially imbedded in it are thereby cemented so tightly to the absorptive 

 cells that even violent shaking will not dislodge them. 



In the case of most seedlings, and in that of grasses, the absorptive cells which 

 proceed from the roots and which are especially numerous in the latter, are generally 

 thickly covered with particles of earth (see fig. 12 4 ). If such a root is pulled out 

 of sandy soil it appears to be completely encased in a regular cylinder of sand (fig. 

 12 ] ). A root of Clusia alba, taken from coarse gravel, had its root-hairs so tightly 



