BETA VULGARIS 169 



chyma zone, and an inner zone containing obliterated phloem. The 

 first two zones have been formed by centripetal growth of the outer 

 ring, while the third zone is the product of centrifugal growth of the 

 older ring." 



Tertiary thickening in the hypocotyl is like that described 

 for the fleshy portion of the root except that the initial secondary 

 cambium is pericyclic in origin rather than arising in the phloem 

 parenchyma. The central portion of the axis consists of a pith 

 which widens out toward the crown stem, and the hypocotyl 

 may become hollow at its upper limit. A union of rings occurs 

 in the upper hypocotyl which is related to the manner of the 

 insertion of the leaf traces in which the bundles of each leaf trace 

 extend inwardly for different distances, the median bundle of 

 each trace reaching the center of the axis while the lateral bundles 

 differentiate centripetally for shorter distances. This vascular 

 arrangement, together with the anastomosing of the annular 

 rings, results in an interconnection between all of the leaves of the 

 rosette and all of the rings of the fleshy axis. 



The Floral Axis. - — Slightly below the growing point of the 

 floral axis, a circular zone of procambial tissue is formed which 

 separates the included pith from the cortex. In this zone, col- 

 lateral vascular bundles are differentiated, the first elements 

 formed being narrow, thin-walled phloem cells which are difficult 

 to distinguish until they are nearly mature. Protoxylem elements 

 of a loose spiral type are next formed and finally, in the inter- 

 vening zone of procambial tissue, a fascicular cambium arises 

 which produces secondary xylem and phloem. The cortex is paren- 

 chymatous, except for ridges of collenchyma near its periphery; 

 and the epidermis is single-layered. 



At this stage, the floral axis is roughly polygonal in outline 

 and there are five large cauline bundles at its angles, with numerous 

 smaller bundles in the intervening sectors of the vascular ring. 

 After the fascicular cambium has functioned for some time in 

 the usual manner, an interfascicular cambium is differentiated 

 which is oriented in a way that is unlike the arrangement found 

 in most stems. One arm of the interfascicular cambium is con- 

 tinuous with the fascicular cambium, while the other may arch 

 around outside the phloem of the adjacent bundle so that a uni- 

 lateral connection with the fascicular cambium, rather than a 

 bilateral one, results. This method of development produces an 



