STRUCTURE OF STEMS 57 



stem they are arranged in the form of a ring about midway 

 between the center of the stem and the outside. They are 

 arranged in a concentric ring with spaces between them. The 

 central part of the stem is the pith or medulla. The portion 

 outside of the rings of bundles is the cortex. The radiating 

 strands of tissue lying between the bundles and connecting the 

 pith with the cortex are the medullary rays. 



97. 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. - 



98. The fibre-vascular bundle. This should be studied in 

 cross and longitudinal sections. In cross section each bundle is 

 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 wood 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 

 usually quite regularly arranged in rows and are rectangular in 

 form. This is the cambium portion of the bundle. The cells 

 of the cambium grow and divide, thus increasing in number. The 

 older ones on the bast side of .the bundle cease to grow and change 

 into bast cells, others into sieve tubes, etc. The older ones on 

 the wood side of the bundle cease to grow and change into vessels, 

 wood fibers, etc. It will be noticed that the wood and bast at 

 no point meet around the cambium but that the cambium itself 

 extends across the medullary ray and is connected with the cam- 

 bium in the adjacent bundles. In fact, the cambium forms a 



