6ix THE STRUCTURE OF ECONOMIC PLANTS 



peripheral portion of the secondary xylem. They are small and 

 isodiametric except that those adjacent to the vessels are stretched 

 periclinally. Occasionally, the sectors of secondary xylem may 

 be partially segmented by xylem rays of thin-walled parenchyma 

 which extend about half way to the center of the axis. 



The total amount of secondary phloem produced is not large as 

 compared with the secondary xylem. It occupies four zones out- 

 side the secondary xylem, the groups being separated from each 

 other and bounded externally by pericyclic tissue. The sieve 

 tubes are among the largest known, attaining an average diameter 

 of 0.05 mm., and Yasuda (41) has reported a width of 0.067 ^im. 

 for C. Pepo. The sieve plates have very large pores located in the 

 horizontal or slightly inclined end walls. The companion cells 

 are relatively large, and there may be three or four of them in a 

 linear series adjacent to a single sieve tube. 



Secondary Thickening of the Hypocotyl. — Above the transi- 

 tion region where the bundles are strictly endarch, secondary 

 thickening is initiated in the usual manner, and the fascicular 

 cambium in each of the ten or more bundles produces secondary 

 xylem and phloem elements similar to those described for the root. 

 As the stele enlarges, the ray and pericyclic tissues keep pace so 

 that the bundles are separated from each other by parenchymatous 

 tissue. The outer phloem is limited outwardly by the persistent 

 pericycle, which forms a cap of fibers. The cortex and epidermis 

 persist longer than in the root but ultimately disintegrate and a 

 phellogen of pericyclic origin produces a periderm. The medullary 

 parenchyma also disintegrates so that the hypocotyl is hollow at 

 maturity. 



The inner cambium becomes active soon after the outer one, 

 producing phloem elements centripetally and parenchymatous 

 cells centrifugally. (Fig. 319.) As Holroyd (18) has pointed 

 out, the outer cambium of each bundle may become continuous 

 with the inner cambium around the lateral and anterior faces of 

 each bundle, forming what he terms "perixylary" cambium. From 

 the inner cells of this cambium, strands of inner phloem originate 

 centrad to each bundle. The growth of the inner phloem is some- 

 what variable; and, since the inner cambium also cuts off paren- 

 chymatous cells in large numbers, the phloem strands in old hypo- 

 cotyls are frequently separated from the protoxylem of the bundle 

 by a wide zone of parenchyma. In some cases, this may become 



