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 dicotyledoa (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. 



