THE RELATION BETWEEN ROOTS AND SOIL WATER 275 



thickened walls are suberized and sometimes partly lignified. In a second 

 type (Fig. 73, B and C) a thickened strip is present on the inner surface 

 of the radial and transverse walls, this thickening often being suberized. The 

 width and general configuration of these Casparian strips, as they are called, 

 varies with the species. Regardless of type, most thickened endodermal walls 

 are pitted. In some species having the thick-walled type of endodermis, there 

 are present, opposite the outer end of each area of xylem tissue (as seen in 

 cross section) isolated thin-walled endodermal cells called passage cells. These 



Caspar/an 

 sfr/p 



(1_ endodermis 



X^ceU 



corf/ca/ 

 ce// 



sfe/e «■ 



Fig. 73. {A) thickened tangential walls of endodermis of a maize root, as seen in 

 cross section. (B) Casparian strip in the stem endodermis of Piper macropliylla, as 

 seen in cross section. (C) perspective diagram showing position of Casparian strip in 

 an endodermal cell. 



are supposed to facilitate the movement of water and dissolved salts through 

 the endodermis. 



After passing through the endodermis water moves into the xylem ducts, 

 in most species after traversing a few intervening layers of pericyclic cells. 

 The route followed by water through the rest of the plant has already been 

 described in Chap. XV. 



The Relation between Roots and Soil Water. — From the standpoint of 

 the absorption of water by plants a clear distinction should be made between 

 soils in which capillary movement of water occurs readily and those in which 

 such movement of water is slow or non-existent. In soils of the former 

 type translocation of water may occur toward the young roots whenever they 

 are absorbing water. There are two principal conditions under which capil- 

 lary translocation of water can occur in soils at appreciable rates : ( i ) in 

 any zone of soil which is not more than a few feet above a water table, and 



