SPERMATOPHYTES 



243 



Dicotyledons. — The vascular system of dicotyledons is by no means 

 uniform, nor should it be expected in so large a group, but its general 

 features can be indicated. 



In the mature stem the vascular system consists of a hollow cylinder 

 composed of vascular bundles and inclosing the pith (a siphonostele) 

 (figs. 541, 549). Traversing the vascular cylinder from the pith to the 

 cortex, and hence separating the bundles, are the pith rays. The 

 bundles are collateral endarch, and also open; that is, there is a cambium 

 between the xylem and phloem strands which forms secondary xylem 



Figs. 542-546. — Vascular elements of an angiosperm: 542, spiral vessels (of proto- 

 xylem); 543, spiral and annular vessels; 544, dotted duct (characteristic of metaxylem 

 and secondary xylem); 545, sieve vessel (of phloem) with companion cell; 546, sieve 

 plate, with section of companion cell. — 542, 543, after Bonnier and Sablon; 544, after 

 DeBary; 545, 546, after Strasburger. 



and phloem. The secondary wood (xylem) differs from that of the 

 gymnosperms in containing true vessels {tracheae) instead of tracheids, 

 and most characteristic among these vessels are the clotted ducts (fig. 

 544). The phloem also differs from that of the gymnosperms in that 

 the sieve vessels have companion cells (figs. 545, 546). No trace of 

 mesarch structure is seen, even in the cotyledons, which seems to indi- 

 cate that the angiosperms are further removed from the ferns than are 

 the gymnosperms. 



The only primitive suggestion that remains in the vascular system 

 of the stem is the presence of leaf gaps in the vascular cylinder, connected 



