THE PROPERTIES OF SOIL 35 



plant, and thus helps those of the leaves in 

 their characteristic work of decomposing the 

 carbonic acid gas of the air, which enters 

 into the leaf, chiefly through the minute 

 apertures (stomala) so abundant on the sur- 

 face of leaves, and also, though to a less 

 extent, on young shoots. It is from the 

 carbonic acid gas of the air that plants get 

 their carbon, a substance that constitutes 

 about one-half of the dry matter of a crop, 

 all of which has been abstracted from the 

 atmosphere with which the leaves and stems 

 are bathed. The mineral matter may not 

 exceed five per cent, of the dry weight 

 of a crop, and yet it is just as essential 

 to growth as the carbon which is so much 

 more abundant. Mineral matter is all 

 absorbed from solutions in the soil, not enter- 

 ing through any definite apertures in the 

 roots or root hairs, but passing through 

 the bounding membrane of the root hairs, 

 which themselves are lateral outgrowths of 

 epidermal cells. There would be no move- 

 ment of water and plant food from the soil into 

 the roots were it not for the fact that the solu- 

 tions (cell sap) in the root hairs and epi- 

 dermal cells for some plants have no root 

 hairs are stronger than the solutions in the 

 soil. But, given these conditions namely, 



B 2 



