MANNER IN WHICH KOOTS TAKE UP FOOD. 107 



stances held in solution by the water moving about in 

 the soil may not, under certain circumstances, be ab- 

 sorbed by the roots of the plants. But it is based upon 

 the assumed fact, that the roots receive their food from 

 the thin layer of water which, retained by capillary 

 attraction, is in intimate contact with the earth and 

 with the root surface, and not from more remote layers' 

 of water ; that between the root surface, the layer of 

 water, and the earthy particles, a reciprocal action goes 

 on, which does not take place between the water and 

 the earthy particles alone. It also assumes as proba- 

 ble, that the nutritive substances adhering, in a state 

 of exceedingly minute division, to the outer surface of 

 the earthy particles, are in direct contact with the fluid 

 of the porous absorbent cell-walls, by means of a very 

 thin layer of water ; and that the solution of the solid 

 elements is effected in the pores of the cell-walls, 

 whence they pass immediately into the system of the 

 plant. 



The facts in support of this view, briefly recapitu- 

 lated, are as follow : The roots of all land-plants, and 

 of most marsh-plants, are in direct contact with the 

 earthy particles. These particles of earth have the 

 power of attracting the most important elements of 

 food conveyed to them in watery solution (such as pot- 

 ash, phosphoric acid, silicic acid, ammonia), and of re- 

 taining them, just as charcoal retains colouring matters. 

 In most cases that have been investigated it has been 

 found that the water moving about in the ground ex- 

 tracts from the soil scarcely any appreciable quantities 

 of ammonia, no phosphoric acid, and potash in such 

 trifling quantities, that all these together are quite in- 

 sufficient to afford the requisite supply of these ele- 

 ments to the plants growing in the field. 



solid state through the bladder into the water, is only apparent. Both 

 salts are dissolved in the pores of the membrane where they come in con- 

 tact with the acidulated water, and as the evaporation of the water from 

 the bladder somewhat diminishes the inner pressure as compared to the 

 outer, the stronger outer pressure, assisted by the solvent power of the 

 water, forces the solution inward. 



