NUTRITION. THE MACHINERY. 21 



The soil, therefore, is not to be looked on as containing 

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

 as regards water, but for other substances the digestive 

 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 

 ploughing and harrowing operations is to keep the soil open 

 and permeable, and thus allow of the access of oxygen 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 into 

 soluble bicarbonates, and exercises a similar power of 

 solution in the case of phosphates. 



Leaves and Leaf Action. The two great factors in the 

 feeding of the plant are the roots and the leaves. The soil 

 supplies to the roots, as we have seen, water in large quan- 

 tities, gases, earthy and saline substances ; but the air is 

 an equally important source of nourishment, or even more 

 so, since there are rootless plants, and plants which receive 

 no part of their food directly from the soil, while no plant 

 can exist without air, and no plant that is of direct import- 

 ance to the cultivator can live without light. We insert 

 the word "direct" because there is a whole group of plants 

 which can thrive in the absence of light, but these form no 

 part of the ordinary crops of a farm. Indirectly, however, 

 as has been pointed out, in considering the agency of bac- 

 teria as ferments in the soil, these organisms to whose 



