98 ANATOMY OF STEMS, ROOTS, AND LEAVES 



and partly by the increase in number of bundles. The hard 

 outer covering of the corn and similar plants is composed of 

 these nbro-vascular bundles. 



The dicotyledonous stem has bundles made up of the same 

 kind of cells which are arranged very differently. The outer 

 part of the bundle contains the bast ; the inner, the woody cells ; 

 and between the two groups lies the cambium which persists 

 throughout the entire life of the plant. The cells of the cam- 



Fia. 72. Fibrous tissue; bast and wood cells. 



bium form a continuous layer of living, growing cells. These 

 cells divide repeatedly cutting off inner layers which go to form 

 layers of wood and outer layers which eventually help to form 

 the outer parts of the stem. All the other types of cells are 

 derived from these cambium cells. Among the bast cells are 

 the peculiar, sieve cells (Fig. 73) or tubes, so called because of 

 the peculiarly perforated cross walls. On the opposite side of 

 the cambium are a number of cells and tubes which have peculiar 

 thickenings on the inside of the walls. If the cells are short, 

 more or less tapering at the ends and with numerous pits in the 

 walls, they are known as traclieids, but if they are elongated into 

 tubes in which the wall thickenings take the form of spirals, 

 ^ings, pits, etc., they are known as tracheary tissue. (Fig. 71.) 

 This term is also used to include the tracheids. This tracheary 

 tissue is surrounded by long fibrous or wood cells. (Fig. 72.) 

 The outer part of the bundle containing the sieve and bast cells 

 is called the phloem region ; the inner part containing the trach- 

 eary and wood fibre is called the xylem region. 



