RAPHANUS SATIVUS 



2-93 



(Fig. 147, /4.) The cortex is comprised of a few layers of large 

 parenchymatous cells, and the epidermal cells are also thin-walled, 

 many of them becoming root hairs. 



The development of the primary root corresponds to type three 

 as described by Janczewski (8). The terminal meristem of the root 

 is differentiated into three histogens, the calyptrogen-dermatogen, 

 the periblem, and the ple- 

 rome, each consisting of a 

 single layer of cells. (Fig. 

 146.) In the ontogeny of 

 the axis, the root cap and 

 epidermis arise from the 

 calyptrogen-dermatogen 

 layer, the cortical cells 

 from the periblem, and 

 the stelar tissue from the 

 plerome. 



The root cap consists 

 of several layers of cells 

 which form a series of 

 overlapping cones sur- 

 rounding the meristematic 

 region of the root tip. 

 As the calyptrogen cuts 

 off successive cell layers 

 to form the root cap, the 

 distal or marginal cells of 

 the conical histogen cease 

 periclinal division and, 

 by continued anticlinal 

 divisions, form the single 

 layer of epidermal cells. 

 This method of differen- 

 tiation results in an epidermis with a stair-step arrangement in 

 which each step of the epidermis abuts one layer of the root cap 

 cells, rather than an epidermis with a continuous smooth surface. 

 The steps become progressively shorter toward the apex of the 

 root and finally end in one which is a single cell in height, touching 

 the last-formed layer of the root cap. In consequence, the number 

 of steps corresponds to the number of layers of the calyptra, except 



,M^e^ 



Fig. 145. 



Srages in development of seedling, variety 

 Red Globe. 



