STEMS, BULBS AND BUDS 45 



In most cases it is not well defined, and its function is not well 

 understood, although it is supposed to serve for the conduction 

 of carbohydrates. 



The fibro-vascular bundles each consist of two parts which are 

 separated by a layer of living cells known as the primary meri- 

 stem or cambium. The outer part of the bundle is known as the 

 phloem and is composed of bast fibers, sieve tubes and some other 

 cells known as accompanying or companion cells. The inner 



FIG. 24. Diagrammatic drawing of section of dicotyledonous stem showing epidermis 

 (a) ; cortex composed of collenchyma (6) ; parenchyma (c) ; medullary ray (d) ; fibro-vascular 

 bundle composed of phloem (e); xylem (/); fibro-vascular cambium (g) interfibro-vas- 

 cular cambium (/i) ; and central axis composed of parenchyma (t). 



part of the bundle is known as the xylem and is composed of wood 

 fibers and tracheary tubes. 



The central axis or pith is made up of parenchyma tissue, 

 which varies greatly in amount in the various plants. It is 

 sometimes very large, constituting the greater part of fleshy 

 stems and serving for the storage of water and reserve food. 



The Structure of the Monocotyledonous Stem. A cross- 

 section of a solid monocotyledonous stem, such as the corn, shows 

 numerous fibro-vascular bundles scattered through a mass of 

 parenchyma tissue, the entire structure surrounded by a hard, 



