STRUCTURE OF STEMS 63 



103. Microscopic structure of an annual stem. The pith, 

 cortex and medullary rays are composed of thin walled cells of 

 nearly equal diameter and belong to the fundamental tissue or 

 parenchyma. The outer layer of cells is the epidermis. In some 

 stems just underneath the epidermis the cells for several layers 

 have walls which are thickened at the angles. This gives addi- 

 tional strength to the stems. 



104. The fibro-vascular bundle. This should be studied in 

 cross and longitudinal sections. In cross section each bundle is 



Fig. 59- 



Xylem portion of bundle. Cambium portion of bundle. Bast portion of bundle. Section 

 of vascular bundle of sunflower stem. 



seen to be divided into two parts, an outer (toward the outside of 

 the stem) and an inner one. The outer portion is the bast portion, 

 while the inner is the woody portion. The bast portion is char- 

 acterized by numerous bast cells, which have whitish thick walls, 

 and form very long fibers. In the wood portion are the vessels 

 which in cross section appear like large angular or circular cells 

 with thick walls. Intermingled with the vessels are the thick 

 walled w r ood fibers and some thin walled parenchyma. Between 

 the bast and woody portions of the bundle there is a group of cells 

 with very thin walls and rich in protoplasm. These cells are 



