286 



STRUCTURE OF STEMS AND ROOTS 



low power, and make a sketch showing the 

 arrangement of the bundles. The sections, if 

 mounted in a permanent way, as in balsam, 

 may be kept for further study of the bundles. 

 Compare with Fig. 454. 



486. Exogenous Stems. — The fibro-vascular 

 bundles in exogenous (or dicotyledonous) 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 453. Diagram 



bundles 



show the course 

 of fibro-vascular 

 bundles in mono- 

 cotyledons. 



454. Fibro-vascular bundles of Indian corn, much 

 magnified, a, annular vessel; a', annular 

 or spiral vessel; tt', thick-walled vessels; 

 w, tracheids or woody tissue; f, sheath of 

 fibrous tissue surrounding 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. 



and often 

 extending 

 from the central pith to 

 the outer cortex. These 

 usually are prominent in 

 young stems of woody 

 plants and in vines. (Fig. 

 457.) All trees and nearly 

 all other woody plants 

 of the temperate regions, 

 as well as many herba- 

 ceous plants, show this 

 plan of stem. The me- 

 dullary 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. 



487. To study arrange- 

 ment of bundles in ex- 



