52- 



THE STRUCTURE OF ECONOMIC PLANTS 



slightly different situation occurs in Zea and other monocotyledons, 

 in which the entire lateral root is formed from derivatives of the 

 pericycle, except that the endodermis constitutes an exterior layer 

 over the root cap. 



In the organization of the growing point, the histogens are 

 differentiated in the same manner as described for the primary root, 

 and the type of ontogeny is similar to that of the axis from which 

 the lateral root arises, although the number of points of protoxylem 

 differentiated is not necessarily the same. The elongation of the 

 lateral root primordium proceeds with varying degrees of rapidity, 



CO-- 



latrt-- 



FiG. xo. Longisection of the primary root of Pastinaca showing the origin of a lateral 

 root: CO, cortex; e«, endodermis; to r/^, lateral root; mx, metzxylem. 



depending upon the plant and the environmental conditions affect- 

 ing it; but, eventually, the cells of the endodermis, cortex, and 

 epidermis of the main axis are either mechanically pushed aside 

 and crushed, or are resorbed by the adjacent tissues. The point of 

 origin of the lateral root is such that the cells of its stele are dif- 

 ferentiated directly in contact with the vascular elements of the 

 main axis; and, thus, the xylem and phloem of the lateral root may 

 be continuous with the corresponding tissues of the main axis, but 

 this is not true of the cortex and epidermis. (Fig. xo.) 



In cases where secondary thickening of both the lateral and 

 primary axes occurs, the cambium of the primary root and that of 

 the lateral root may be continuous. As a result, successive addi- 

 tions of secondary vascular tissue to the two axes proceed simul- 

 taneously, and continuity of the vascular regions is maintained. 



