242 
MORPHOLOGY 
thus formed; while tracheae (true vessels) are developed by a fusion of 
cells end to end, so that a continuous tube of considerable length may be 
formed. A system of tracheae always ends in tracheids, which are 
therefore at least the end cells of any vascular system. 
FIG. 541. Transverse section of vascular cylinder of a young dicotyledon (Ricinus): 
the regions, beginning outside, are epidermis (single layer of cells); cortex (a zone of 
several layers), including an almost continuous band of fibrous cells (heavy walls); a zone 
of several layers (the outer ones being phloem, the inner cambium) ; the zone of xylem 
strands (separated by pith rays, the innermost vessels in each strand being protoxylem, 
the outer and larger ones metaxylem) ; and the pith. 
The characteristic element of the phloem is the sieve vessel (fig. 545), 
so named because in the wall there occur definite areas full of perforations 
known as sieve plates (fig. 546). These vessels also arise by cell fusion, 
as do the tracheae. 
The vascular system. The vascular system of dicotyledons and of 
monocotyledons is so different that the two groups must be considered 
separately. 
