260 



STKUCTUKE OF STEMS AND EOOTS 



arrangement of the bundles. The sections, if 

 mounted in a permanent way, as in balsam, 

 may be kept for further study of the bun- 

 dles. Compare with Fig. 401. 



422. EXOGENOUS STEMS. The fibro- vas- 

 cular bundles in exogenous (or dicotyledon- 

 ous) stems are arranged in a circle around 

 the center, which is usually filled with pith. 

 Outside the ring of bundles is a cortex of 

 fundamental tissue. Around this is either a 

 layer of cork or an epidermis. Layers of 

 parenchyma cells, called medullary rays, are 

 found between the bundles and often extend- 4 ow the course 

 ing from the central pith to the outer cor- 

 tex. These cotytedoM? 



usually are prominent in 

 young stems of woody 

 plants and in vines. Fig. 

 404. All trees and 

 nearly all other woody 

 plants of the temperate 

 regions, as well as many 

 herbaceous plants, show 

 this plan of stem. The 

 medullary rays are very 

 prominent in oak wood. 

 These rays are lignified 

 in the xylem part of the 

 bundle and non-lignified 

 in the phloem part. To 



401. Fibro-vascular bundles of Indian corn, much Study arrangement OJ 



bundles in exogens: 



pare thin c 



of the stems of meni- 



spermuin 



magnified. A, annular vessel; A', annular 

 or spiral vessel; TT'. thick-walled vessels; 

 w, tracheids or woody tissue; F, sheath of 

 fibrous tissue siirrounding the bundle; FT, 

 fundamental tissue or pith; s, sieve tissue; 

 P, sieve plate; c, companion cell; I, inter- 

 cellular space, formed by tearing down of 

 adjacent cells; w', wood parenchyma. 



