596 THE STRUCTURE OF ECONOMIC PLANTS 



According to Whiting, lateral root formation is first indicated 

 by radial divisions of cells in the two or three inner layers of 

 cortical parenchyma at points centripetal to the protoxylem. 

 (Fig. 311, A.^ The radial divisions of the regularly arranged 

 cells of the cortex occur while the innermost layers continue to 

 divide tangentially. At this time, the pericyclic cells begin to 

 divide; and those just centrad to the active cortical region also 

 divide tangentially, with the result that the exact identity of 

 the two adjacent layers is soon lost. Thus, divisions in all three 

 planes account for the formation of the lateral root primordium 

 from cortical and pericyclic tissue. 



As additional layers of the cortex are involved, the lateral extent 

 of the root primordium is increased until its base reaches to the 

 phloem on either side of the protoxylem point. (Fig. 311, B, C) 

 The cortical cells remain meristematic and continue division, but 

 the outline of the mother cell can frequently be traced, even after 

 several divisions have ensued. The regular layered arrangement 

 of the cells in that part of the cortex which is involved in lateral 

 root formation tends to be distorted by the increased activity of 

 the tissues of the pericycle, which by successive divisions produces 

 the numerous small, radially elongated elements of the stele. 

 (Fig. 311, D.) 



As the primordium elongates, the cortical cap maintains its 

 position over the stele by divisions of the cells at the base of the 

 primordium and adjacent to the phloem of the parent root. Finally, 

 when all but about three layers of the cortical tissue of the primary 

 root are involved in the root primordium, a meristematic zone 

 similar to that described for the primary root develops. (Fig. 311, 

 D, £.) This is formed by tangential divisions in the second, third, 

 or fourth outermost layers of the cortically derived tissues of the 

 root primordium. As a result of the activity of the apical meristem 

 of the lateral root, its tip is torn away from the cortical tissues 

 of the primary root; and it ultimately penetrates the remaining 

 tissues, emerging into the soil. 



The Vascular Transition. — Investigations of the transition 

 by Rutledge (34) and Whiting (40) are in close agreement with 

 the early interpretations of Gerard (ix) and Dangeard (6), but 

 differ from that of Lamounette (2.5). An essential feature of the 

 transition is that, in relation to the number that occurs in the 

 primary root, there is a doubling of the xylem and phloem strands. 



