2.30 THE STRUCTURE OF ECONOMIC PLANTS 



into two, one of which is added to the root-cap, the other to the 

 dermatogen as a permanent member of it. . . . The cells of the der- 

 matogen and root-cap, which owe their origin to the division just 

 described, divide further by walls perpendicular to the surface; from 

 each therefore is produced a section of a layer consisting of several or 

 many cells." 



The cells of the root cap, which later become oriented in a 

 lateral position because of the elongation of the root axis, may 

 divide anticlinally. In this manner, they compensate for the 

 growth in length of the root axis, and each series of root cap 

 cells keeps pace with the elongation of the root axis until the 

 layer of which they are a part becomes the outermost one and dis- 

 integrates. The cells of the calyptra which form the terminal 

 vertical rows of the cap do not usually undergo further division, 

 except for one or two cells which are proximal to the calyptrogen; 

 but they enlarge in all three dimensions until they are two or three 

 times the size of the calyptrogen initials. As a result of the anti- 

 clinal divisions of the lateral rows of the calyptrogen and the 

 concurrent enlargement of the cells in the vertical rows, the root 

 cap consists of a very regular series of cell layers which overlie 

 one another. (Fig. no, B.) 



The cortex is derived from the two-layered periblem which forms 

 a terminal cap over the small group of plerome cells at the root 

 apex. The outer layer of the periblem divides only in anticlinal 

 planes, in this manner giving rise to the outermost cortical layer 

 which becomes the hypodermis. The initials of the inner layer 

 divide in all planes to produce the four- to seven-layered paren- 

 chymatous portion of the cortex and the endodermis. 



The pericycle is the first tissue of the stele to differentiate and is 

 derived from the outermost layer of the plerome. The pericyclic 

 cells can be identified early in ontogeny by their dense contents and 

 greater axial dimension as compared with adjacent cortical cells. 

 Following the differentiation of the pericycle, the primary phloem 

 and xylem develop simultaneously. The protophloem is comprised 

 of elongated parenchymatous cells with relatively small diameters. 

 The primary phloem ducts are formed by the breakdown of a 

 linear row of cells adjacent to the pericycle, as many as three or 

 four occurring in each phloem group. They are completely dif- 

 ferentiated at a level approximately 1.5 cm. above the root tip, 

 and the number may increase until there are seven or more in each 



