CHAP, iv.] The Food of Trees 75 



often totally deficient in it, their wants must be met by the 

 nitrogenous compounds set free on the decomposition of 

 organic matter within the soil ; and, even from this alone, one 

 important function of the invaluable humus or leaf-mould can 

 easily be seen. The warmer a soil, the more thorough its aeration, 

 and the more its supply of moisture is removed from either of 

 the extremes of dryness or wetness, so much the more rapidly 

 and successfully will the nitric acid (so essential for agricultural 

 crops especially) be evolved from decomposing organic remains. 

 But as nitric acid is apt to be easily washed out of porous 

 soils, this partially explains why loose sandy soil is less fertile 

 than loams and other soils of a more retentive nature. 



Water (H 2 O) is also one of the essentials to the support of 

 plant life. It not only forms a very direct source of nutriment, 

 but it is also requisite for the solution of inorganic substances 

 (nutrient salts\ which can only be absorbed by the root-hairs 

 when held in solution. By far the greatest portion of the water 

 that is imbibed is transpired through the foliage. A certain 

 quantity, however, is utilized in the formation of the organic 

 tissue, the requisite hydrogen being exclusively obtained from 

 this source; whilst, during the process of assimilation, the 

 organic substances are formed from water and carbonic acid 

 by the elimination or setting free of oxygen (when CO 2 + H 2 O 

 becomes CH 2 O + O 2 &c. : see note i to page 71). 



The requirements of woodland crops as to water vary greatly 

 according to the species of tree, and for one and the same 

 species according to the nature of the soil, the climate, and 

 the exposure of any given locality. The finer the particles of 

 soil, and the richer the admixture of humus, the more moisture 

 can a soil contain. In situations with humid atmosphere the 

 spiracles of the leaves open much wider than in dry air ; for 

 the leaves have a certain power of accommodating themselves 

 to the temporary conditions of the atmosphere so as to stimu- 

 late transpiration during a humid, and to retard it during a dry, 

 state of the air. They are thus endowed with a certain 



