STRUCTURE OF THE STEM 



35 



a continuous layer of smooth, tough and impervious cuticula. 

 This layer of cells is the epidermis. It is sufficiently elastic 

 to allow considerable expansion with the growth of the stem, 

 but it may finally be ruptured and scale off, leaving the under 

 parts exposed. 



74. The spaces between the spokes of the parenchyma wheel 

 are occupied by a system of fibers and vessels known as the 

 vascular bundles. The fibers are usually of two distinct 

 types, one, known as bast, is found nearer the surface of the 

 stem, while the other is the chief element of the wood and lies 



f 



B:- 



B 



D 



Fig. II. — Diagrams representing the structural elements of the vascular 

 bundles. A, A fibre of wood, or bast; B, one end of a tracheid, showing spiral 

 markings; C, part of a trachea, or true vessel, with pitted markings and the 

 remnant of the dividing wall which originally separated two of the cells which 

 helped form the vessel; D, part of a sieve tube with the perforated cross wall 

 (sieve). 



nearer the pith. The bast fibers consist of greatly elongated 

 cells with extremely thick walls. The fibers of the wood are 

 similar but the walls are not so thick. The vessels are of several 

 kinds; first, tracheides, consisting of single elongated cells 

 whose walls are unbroken, but variously thickened in Umited 

 areas, forming rings, spirals, annular pits, etc.; second, the sieve 

 vessels, formed by rows of elongated cells placed end to end 

 with the dividing walls perforated by pores forming a sieve; 



