7.6z THE STRUCTURE OF ECONOMIC PLANTS 



with the insertion of the bundles of the leaf traces. There may 

 also be a reduction in the number of rings in the narrow tapering 

 portion of the lower root. Chiefly because of differences in 

 cultural conditions, there is considerable variation in the develop- 

 ment of the beet with respect to the number of rings, their width, 

 and the amount of parenchymatous storage tissue, but the funda- 

 mental organization of the fleshy axis is identical in all cases. 



At the time when the primary tissues are completely matured, 

 the root has a single-layered epidermis, a cortical region consist- 

 ing of a few layers of parenchyma limited centripetally by the 



Fig. 12.8. Transection of fleshy portion of the axis showing central zone of primary and 

 secondary vascular tissues and concentric rings of tertiary tissues produced by the secondary 

 cambiums. (After Artschwager, /oar. Agr. Res.^ 



endodermis, and the stele. The stele is bounded by a uniseriate 

 pericycle enclosing the primary xylem and phloem, and these 

 are separated from each other by a zone of interstitial parenchyma 

 which increases in amount during the maturation of the primary 

 vascular tissues. This takes about lo toiz days and is complete 

 when the axis is only i or x mm. in diameter in contrast to a final 

 one of 15 cm. or more which may be attained in the mature sugar 

 beet. 



When the primary tissues are nearly mature, the first lateral 

 roots originate in the pericycle approximately outside the proto- 

 xylem points. They occur in two double longitudinal rows, 

 since the root primordia are initiated slightly to the right and 

 left of radii passing through the two protoxylem points. The 



