BETA VULGARIS 



155 



r c 



and Artsch wager (i). Their results are here summarized and 

 quoted in part. The ontogeny follows Janczewski's (10) third 

 type, in which the plerome and periblem give rise to the stele 

 and cortex respectively, while the surrounding calyptrogen- 

 dermatogen layer initiates 

 the epidermis and the root 

 cap. According to Lyle 



(II), 



"this calyptrogen-dermato- 

 gen layer of cells, overlying 

 the periblem, divides peri- 

 clinally adding a layer of 

 cells to the root cap and 

 renewing the initial layer 

 itself. The marginal cells 

 of this new calyptrogen- 

 dermatogen layer farthest 

 from the root apex divide 

 only anticlinally forming 

 epidermal cells. This distal 

 portion of the dual layer is 

 thus a dermatogen while 

 the remaining apical section 

 is the calyptrogen. This 

 method of formation of root 

 cap and epidermis accounts 

 for the stair-step arrange- 

 ment of the latter. (Fig. 

 1x3.) Cells of the layers 

 of the root cap also divide 

 anticlinally thus compen- 

 sating for the increase in 

 length of the underlying 

 zones of the root axis. Seg- 

 ments of a few rows of the 

 root cap may also divide peri- 

 clinally forming additional 

 short layers in the root cap. 



"The cortex is derived from the periblem, which consists of a single 

 layer of cells directly inside the calyptrogen-dermatogen. These cells 

 divide both periclinally and anticlinally resulting in three to seven 

 rows of typical cortical cells and an inner layer of axially elongated 

 cells, the endodermis. 



' The stele is differentiated from a group of plerome cells that divide 

 in all planes. The outer layer forms the pericycle which is composed 



-- cal-dgn 



Fig. 12.3. Longisection of root tip showing 

 histogens : cal-dgn, calyptrogen-dermatogen ; co, 

 cortex ; en, endodermis ; ep, epidermis ; pbm, peri- 

 blem ; pel, pericycle ; pi, plerome ; r c, root cap ; 

 stl, stele. (After Lyle.) 



