STEMS AND THEIR FUNCTIONS 



155 



dermal cells are protected by a thin layer of cutin (a waxy substance 

 related to suberin and, like the latter, impervious to water). Beneath 

 the epidermis lies the cortex, extending inward as far as the stele. The 

 outer layers of the cortex are usually composed of elongated cells with 

 thickened corners, forming a type of mechanical or supporting tissue 

 called collenchyma. Within this zone the cortex is made up of paren- 

 chyma, sometimes with the innermost cell layer differentiated into an 

 endodermis composed of mechanical tissue. 



Inside the cortex is the central cylinder, or stele. The stele of the stem 

 differs from that of the root in having a central pith of large or small 



Fig. 11.3. A stereogram to show the relations of the parts of the dicot stem illustrated in 

 Fig. 11.2. (Modified from Brown, The Plant Kingdom, courtesy Mrs. Mary A. Brown.) 



diameter, enclosed in a cylinder of vascular tissue, 1 the outer surface of 

 which is bounded by the pericycle. The inner portion of the vascular 

 cylinder is composed of xylem; the outer part, of phloem, with a thin 

 cambium layer between. 



The structure and growth of older stems. Lengthening of the stem 

 and its branches is entirely the result of the growth of the twigs. As 

 soon as a twig is fully developed, it ceases to elongate and henceforth 

 increases only in diameter. Its radial growth is accomplished primarily 

 by the activities of the cambium lying between the xylem and phloem. 

 The cells of the cambium layer are mostly tall and slender, and the layer 

 itself is normally only one cell thick. Except for the occasional radial 

 divisions that enable the cambium cylinder to enlarge with the growth 

 of the stem, the cambium cells always divide parallel to the surface 



1 The vascular cylinder may be complete or may be interrupted by pith rays extend- 

 ing from the central pith to the cortex ; the former condition is that described here. 



