CUCURBITA 



613 



active and produce strands of phloem adjacent to the primary 

 xylem. (Fig. 3 19.) The lateral sectors of the perixylary cambium 

 remain inactive for some time, but later produce groups of phloem 

 which extend tangentially from the flanks of the meta- and 

 secondary xylem of the bundle. 



As the hypocotyl matures, the radial elongation of the paren- 

 chymatous cells separating the inner phloem from the protoxylem 



-/ — m r 



Fig. 319. Transection of centrad portion of hypocotyledonary bundle showing develop- 

 ment of inner and connective phloem : ca, inner cambium ; con ph, connective phloem ; i ph, 

 inner phloem; /«r, medullary ray ; /»/», phloem; />/, pith ; atj i, primary xylem; x)- 1, second- 

 ary xylem. 



increases the interval between the two, and the inner cambium 

 may cut off some xylem elements centrifugally, forming an obcol- 

 lateral bundle. In addition to the phloem produced by the outer, 

 inner, and perixylary cambiums, a limited amount of intraxylary 

 phloem may be produced in the xylem parenchyma. This has been 

 noted by Holroyd (18) in C. Pepo, and by Rutledge (34) in 

 C. maxima. The situation with respect to the phloem is further 

 complicated by the fact that strands of ectocyclic phloem are 

 originated in the pericycle outside the bundles, and also in the 



