EACH FIBRO-VASCULAR BUNDLE 97 



value as food depends on the size of the cells, the thinness of 

 the cell-walls, the amount and digestible character of the food 

 (carbohydrates, hydrocarbons, proteids ; Chapter IX) which 

 they contain, and the absence of poisonous or injurious 

 compounds. 



Each fibro-vascular bundle is made up of a variety of cells. 

 In the young bundle of the monocotyledonous stem (Fig. 70) 

 is a small group of cells known as the cambium. They have the 



mmm 



Fig. 71. — Tracheary tissue. 



power of rapid growth and division and are primarily respon- 

 sible for the increase in diameter of the monocotyledon stem. 

 After a comparatively short time they lose the power of division 

 and this checks the increase in diameter of the stem. On one 

 side of the bundle will be a mixture of large and small cells 

 constituting the woody part. Some of these cells have peculiar 

 thickenings of the cell-walls forming rings, spirals, pits, etc. 

 They are known as tracheids and tracheary cells. (Fig. 71.) 

 On the opposite side of the bundle is a group of small, thick- 

 vralled cells constituting the fibre or bast. (Fig. 72.) There 

 are other types of cells which will be described later. A mono- 

 cotyledonous stem increases in diameter for a limited time only, 

 partly by the increase in the size of the fibro-vascular bundles 

 7 



