492. THE STRUCTURE OF ECONOMIC PLANTS 



stl 



COs 



well-defined endodermis, and its outer surface is bounded by the 

 epidermis which produces root hairs. 



In the ontogeny of the root, the meristem is differentiated into 

 three histogens. The plerome and periblem, which give rise to 

 the stele and cortex respectively, are clearly defined; and a layer 



of cells, the calyptrogen-dermat- 

 ogen, overlying the periblem 

 produces the epidermis and the 

 root cap. (Fig. 2.53.) The root 

 cap consists of several layers of 

 cells, and those located at the tip 

 may divide again periclinally. 

 This differential activity results 

 in the formation of several ver- 

 tical rows of cells in the central 

 portion of the cap so that it is 

 thicker at its apex than at its 

 margin. Frequently, irregulari- 

 ties in the orientation of the 

 lateral cells result from these 

 divisions and subsequent cellular 

 enlargement. 



The epidermis is produced by 

 the final periclinal divisions of 

 the most laterally placed cells of 

 the calyptrogen-dermatogen layer. 

 Each final division results in two 

 daughter cells, the inner one be- 

 coming an epidermal initial while 

 the outer forms a portion of the 

 In this manner, suc- 

 cessive cells become a part of the 

 epidermis as the root increases in 

 length; and, because of the manner in which this is accomplished, 

 the outer surface is not smooth near the apex of the root as is the 

 case when the epidermis is formed by anticlinal divisions of a 

 distinct dermatogen. Instead, it forms a series of steps each of 

 which is continuous with a layer of the root cap; and there are as 

 many steps to the epidermis as there are layers in the root cap, except 

 where subsequent periclinal divisions of the latter have occurred. 



Fig. 2.53. Median longisection of primary 

 root : cal, calyptrogen-dermatogen ; co, cor- 

 tex ; en, endodermis ; ep, epidermis ; pbm, foOt Cap 



periblem ; pel, pericycle ; 

 r c, root cap ; stl, stele. 



pi, plerome ; 



