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 dotted 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 
