CHAPTER XXX 

 ROOTS: PROCESSES AND SOIL RELATIONS 



If we include both subterranean and aerial roots in our purview there 

 are few processes in leaves and stems that do not also occur in roots. 

 Cell division, enlargement, and differentiation; respiration; transpiration; 

 sugar, fat, and protein syntheses; digestion; assimilation; absorption and 

 movement of water and mineral salts; and the translocation and accumu- 

 lation of substances are just as characteristic of some kinds of roots as 

 they are of some kinds of leaves and stems. Not all of these processes 

 occur in every type of leaf, stem, or root. 



Photosynthesis in roots is limited to those roots which become green 

 when exposed to light, and is characteristic especially of the epiphytes, 

 such as tropical orchids, bromelias, and ferns. However, the roots of 

 many land plants exposed by current and wave action along lake shores 

 and stream banks may become green and add to the sugar supply of 

 the root. Transpiration occurs from the roots of epiphytes, and from 

 any other root surfaces exposed to air, either above or below the soil 

 surface. 



The soil environment. Before discussing the other processes occurring 

 in roots we should consider a few of the conditions in the soil as an 

 environment in which roots grow. First of all the soil is a mass of larger 

 and smaller particles of minerals derived from underlying rocks, or car- 

 ried in from adjoining regions by water, ice, and wind. Through the cen- 

 turies this "parent material" of the soil is modified by weathering which 

 includes rainfall and drought, freezing and thawing, solution and pre- 

 cipitation, leaching and chemical reorganization. 



Meanwhile plant stems and roots have thickly penetrated all the sur- 

 face layers every year, and the lower layers at least every few years. All 

 or parts of these organs have died each year, and their organic com- 

 pounds have been incorporated in the soil. These compounds become 

 the food supply of an enormous population of bacteria, fungi, and minute 

 animals that alter the organic residues and, according to conditions, in- 



315 



