268 ABSORPTION OF WATER 



largely in crop plants. Generally a plant which is closely surrounded by 

 other plants will have a more restricted lateral spread to its root system than 

 one growing at some distance from its neighbors. The lateral spread of the 

 roots of a crop plant such as wheat, growing in a dense stand, is always less 

 than that of an isolated plant of the same species. Isolated trees or those 

 growing in open stands often have root systems which extend far beyond the 

 spread of the crown. In artificial tree plantations the density of the stand 

 affects the lateral spread of the roots just as it does in crop plants. The 

 same principle also operates in natural forests but is there complicated by the 

 greater diversity of species and of age groups than is usually present in an 

 artificial planting of trees. 



The Absorbing Region of Roots. — Most of the absorption of water and 

 mineral salts occurs in the terminal portions of roots. Measured from the 

 apex, the length of the absorbing region of a root varies greatly with the species 

 and with the conditions under which the root has developed. In general ab- 

 sorption of water and mineral salts can probably take place through any por- 

 tion of a root which has not become encased in cork cells. As shown in the 

 later discussion, however, there are good reasons for believing that, in at least 

 many species, most absorption of water occurs in the apical portions of roots, 

 and especially in the root hair zone. Because of the extensive branching of 

 roots there are often millions of root tips on the root system of a mature plant. 

 From a physiological point of view the number of root tips borne by a root 

 system is probably the most important index of its effectiveness as an absorbing 

 organ. 



The external morphology of a root tip can best be observed in roots which 

 have developed in moist air, peat moss, or sawdust. Upon close examination 

 of a root tip four distinct but intergrading regions can be discerned with no 

 stronger magnification than that afforded by a hand lens, or in many species, 

 even with the naked eye. At the very tip of the root is an extremely short 

 region, white in color, which is known as the root cap. Just above the root 

 cap and partly covered by it is the ?neristematic region, the zone of maximum 

 cell division, which is seldom more than a millimeter in length, and which is 

 usually distinguished by a yellowish color. Next in order above the meris- 

 tematic region is a whiter, smooth region, usually several millimeters in length. 

 This is the region of cell enlargement in which most increase in length occurs. 

 Above this region lies the root hair zone which bears the slender hair-like out- 

 growths of the epidermal cells known as the root hairs. The root hair zone 

 varies in length depending upon the species and the conditions to which the 

 root is subjected during its development. 



