PART II. — VEGETABLE HISTOLOGY. 



having entered the central cylinder, their course lies directly 

 downward, keeping at about the same distance from the centre 

 of the stem. A cross-section of the stem, therefore, shows a 

 circular arrangement of the bundles as in the Fern type, but the 

 bundles are of the collateral and not of the concentric variety. 

 They are usually shaped 

 like a wedge, and radiate 

 from a central portion of 

 the fundamental tissue 

 called the medulla or pith, 

 toward which each pre- 

 sents its thinner edge. 

 Separating the bundles 

 laterally, and connecting 

 the pith with the primary 

 cortex are plates of funda- 

 mental tissue called med- 

 ullary rays. The inner or 

 thinner portion of each 

 wedge-shaped bundle is 

 composed of xylem, and 

 the outer or broader por- 

 tion of phloem tissues, 

 and these are ordinarily 

 separated from each other 

 by a tangential layer of 

 meristem tissue called the 

 fascicular cambium. This 

 is usually continued from 

 one bundle to the next 

 across the intervening 

 medullary ray, and is here 

 called the interfascicular 

 cambium. The cambium 

 thus forms, in most cases, a narrow zone, separating the phloem 

 and exterior tissues, which constitute the bark, from the xylem 

 and interior tissues, which constitute the woody region of the 

 stem. The facts will be understood by reference to Figs. 450 

 and 451 and their accompanying descriptions. 



Fig. 449. — Part of transverse section of stem of 

 Dracaena (rriflexa?). showing its mode of increasing 

 in thickness, e, epidermis; c, corky layer; j>, corti- 

 cal parenchyma; f, a fibro-vascular bundle passing 

 outward to a leaf; m, meristem layer, in which fibro- 

 vascular bundles are forming; g, a partly formed 

 bundle, the upper or outer part of which still con- 

 sists of thin-walled cells; s, secondary fundamental 

 tissue, with radially arranged cells; t, primary 

 fundamental tissue. Magnified about 40 diameters 

 After Sachs. 



