46o THE STRUCTURE OF ECONOMIC PLANTS 



loci results in their alignment in four longitudinal rows which tend 

 to be paired, although the torsion of the root may make the orien- 

 tation appear to be spiral. 



The ontogeny of the lateral roots is similar to that of the primary 

 root, and its histogens follow the same developmental sequence 

 as in the main axis. As the growing point is formed by tangential 



and radial divisions of the per- 

 —pd icyclic cells, the endodermis is 

 forced outward; and the emer- 

 gence of the lateral root through 

 the cortical and epidermal tissue 

 is accomplished by a rupturing 

 and resorption of the cells. 



Secondary Thickening of 

 THE Primary Root. — The first 

 cambial activity occurs in the 

 interstitial parenchyma immedi- 

 ately centrad to the phloem. 

 There is a progressive lateral 

 activation toward the primary 

 xylem poles which may involve 

 the pericyclic cells over those 

 points. In most instances, the 

 cells produced by the active 

 tissue outside the protoxylem 

 points are parenchymatous so 

 that two well-defined rays are 

 formed. 



The secondary xylem vessels 

 are laid down in radial rows, one 



Fig. 135. Transection of sector of old 

 root showing secondary thickening : ca, cam- 

 bium ; mx, metaxylem ; pc/, pericyclic paren- 



chyma; pd, periderm; ph, phloem; px, pro- ^q j^q qj. ^J^j-^g ^^^^ ^^ width, 

 toxylem; .vj x, secondary xylem. ^ ^ _, 



(Fig. ^35.) This approximate 

 radial alignment may be maintained, except that there is also some 

 radial and tangential separation of the vessels owing to the activity 

 of the adjacent parenchymatous cells. The vessels are large with 

 loosely reticulate or reticulate-pitted walls. The end walls of the 

 vessel segments are commonly transverse and the segments are 

 two to five times as long as broad. The cells of the secondary 

 xylem parenchyma are thin-walled, isodiametric in transection, 

 and approximately the length of the vessel segments. 



