386 THE STRUCTURE OF ECONOMIC PLANTS 



xylem strand of the lateral root lies in the same plane as the strand 

 of the root axis from which it originates. 



As the plerome increases in size, anticlinal divisions occur in the 

 two outer layers of the pericycle (<«', a"^ to compensate for this 

 growth, and the innermost of these two layers (a"^ begins peri- 

 clinal divisions at a point farthest removed from the tip. These 

 periclinal divisions occur progressively in this layer toward the tip 

 of the primordium; and, subsequently, their innermost derivatives 

 also undergo periclinal divisions. This stage is illustrated at the 

 right in Figure 196, £, where the basal portion of the layer a" has 

 divided once and one of the innermost derivatives is undergoing 

 another division as indicated by the mitotic figures. In this man- 

 ner, the number of cell layers is increased by successive periclinal 

 divisions from the newly formed layers of the periblem. (Fig. 

 196, F.) In each instance, the divisions of the inner layers are 

 continued progressively toward the tip of the root; and, finally, 

 the remaining cells of layer a" are all divided once periclinally and 

 the basic plan of the periblem is established which is similar to that 

 described for the primary root. (Fig. 196, H.) The outermost 

 layer of the periblem divides only anticlinally, producing a single 

 layer which differentiates into the hypodermis. At the tip of the 

 root, the innermost layer of the periblem continues to divide and 

 produces the other six to eight layers of cortical cells, the innermost 

 becoming the endodermis. 



By the time that the periblem has formed about three cell layers 

 around the base of the conical plerome, the outermost one, a' , 

 undergoes periclinal divisions at its tip. (Fig. 196, F.) Of the 

 two layers thus formed, the outer one becomes the first layer of the 

 root cap, while the inner one remains meristematic and by further 

 divisions produces additional layers of the root cap. As in the 

 primary root, the laterally oriented cells of the layer a' continue 

 anticlinal divisions and function as a dermatogen which gives rise 

 to the epidermis. Following the establishment of the histogens, 

 subsequent stages in the ontogeny of the lateral root are similar to 

 those of the primary root. 



Vascular Transition. — Vascular transition takes place in the 

 hypocotyl although the stelar tissue is not completely collateral 

 and endarch until the bases of the cotyledons are reached. At 

 approximately the ground level, the stele of the lower hypocotyl is 

 root-like with a diarch primary xylem strand, laterally placed 



