270 ABSORPTIOxN OF WATER 



begin to enlarge, principally in the direction of the long axis of the root. The 

 division of meristematic cells and their subsequent elongation results in pro- 

 jecting the growing region and root cap forward through the soil, and accounts 

 for the growth in length of the roots. The region of cell elongation is seldom 

 more than a centimeter in length, and is usually less. This contrasts markedly 

 with the corresponding region of a stem tip which may be as much as ten 

 centimeters in length, or even longer in some species. Only a small part of 

 the root tip — a few millimeters in length at most — is pushed through the soil. 

 Since, in a growing root tip, the elongation of cells ensues as soon as they are 

 formed by cell division, a cell which is one day in the region of cell division 

 is the next day in the region of cell elongation, subsequent divisions in the 

 meristematic tissue having produced additional layers of cells beyond it. 



The anatomy of a representative root as shown in cross section through 

 the root hair zone, is illustrated in Fig. 70. The structure of a young root 

 shows a number of distinctive features. The cortex is relatively thicker than 

 that of stems. This characteristic is especially noticeable in fleshy roots in 

 which the cortex often has a diameter many times that of the stele. The 

 intercellular spaces of the root cortex are also more prominent than those in 

 the stem cortex in which the cells are rather densely packed. An endodermis 

 is almost invariably present in roots; this is generally considered to represent 

 the innermost layer of the cortex. While an endodermis is found in the 

 stems of many species, it is by no means of universal occurrence. Just within 

 the endodermis is present a narrow zone of parenchymatous pericyclic tissue. 

 Usually this is continuous, but in some species, as described below, it may be 

 discontinuous. Lateral roots arise in the pericycle. 



In roots the primary xylem and the primary phloem are present in a 

 radial pattern. The primary xylem as seen in cross section appears as a 

 number (usually 2 to 5, although sometimes as many as 20) of radially 

 situated strands. In many roots the center of the stele is composed of xylem; 

 in some, especially monocots and herbaceous dicots, it is composed of pith. 

 Usually the xylem strands terminate in contact with the pericycle, but in some 

 species they abut directly on the endodermis, breaking up the pericycle, as 

 seen in cross section, into a discontinuous series of arcs. The primary phloem 

 of roots occurs as patches of tissue (as seen in cross section) which alternate 

 with the strands of xylem (Fig. 70). 



The structure of the individual types of cells occurring in the root tissues 

 is essentially similar to the structure of corresponding types of cells occurring 

 in the stem. 



With few exceptions the roots of all perennials and many annuals grow 

 in diameter as they increase in age by means of a cambium layer much as do 



