PLANT NTTTKITION. 25 



of these root-hairs when they are present. The upper 

 portions of the fibril do not act as absorbent organs, the 

 root hairs do not exist in this part of the root, the struc- 

 ture of which becomes gradually less and less adapted 

 for absorption, so that the actual space in each fibril de- 

 voted to absorption is relatively small in relation to its 

 length. The remainder of the fibril acts as a conduit for 

 the transmission of the absorbed fluids upward from cell 

 to cell by osmosis and imbibition, and as a holdfast. 



The passage of the insoluble matters in the soil into 

 the root is effected by an acid liquid produced by the 

 root-hair or cell in consequence of its contact with the 

 particle of soil, aided by the water in the soil. This acid 

 fluid saturates the cell walls, corrodes, and effects the 

 solution of the surface of the particle of the soil in con- 

 tact with the fibril or root-hair. No passage of acid fluid 

 out of the cell takes place, root excretions having no ex- 

 istence ; but the corrosive, and as it were digestive, action 

 above mentioned, is due solely to the absolute contact of 

 the cell of the root with the particle of the soil. 



The soil, therefore, is not to be looked on as contain- 

 ing so much liquid food ready for instant use ; that may 

 be so as regards water, but for other substances the di- 

 gestive action of the roots is necessary. 



In addition to the absorption of liquids as just detailed, 

 roots have the power of freely absorbing the oxygen gas 

 contained in the soil, and if a supply of oxygen be cut 

 off, the roots die from suffocation. One use, therefore, 

 of the plowing and harrowing operations is to keep the 

 soil open and permeable, and thus allow the access of oxy- 

 gen to the roots. 



On the other hand, roots do not absorb carbonic acid 



gas nor exhale oxygen as the leaves do in the sunlight 



but they do give off carbonic acid gas, which, with the 



aid of water, converts the insoluble carbonates of the soil 



2 



