228 



THE SECOND BOOK OF BOTANY. 



FIG. 400. 



Fig. 399. That portion of the bundle which looks 

 toward the centre of the stem answers to wood, and 

 the outer portion answers to the inner bark, while 

 the cellular tissue, through which the bundles are 

 interspersed, answers to the medullary rays and pith 

 of the stems of dicotyledons. In Fig. 399 L is a 

 region of fibres, with thick walls and spiral ducts 

 (T). Then, in the midst of cells and fibres (P), we 

 have barred and dotted ducts (Y); beyond, at L, 

 are thick fibres, like the liber, and, still outside of 

 these, the laticiferous ducts ( V L). 



The vertical section of a stem formed from these 

 bundles presents an appearance like Fig. 400. These 

 woody bundles, scattered irregu- 

 larly through the cellular tissue, 

 remain single and isolated. There 

 is no such thing as a separation 

 of the stem into a woody system, 

 and a region of bark, with cam- 

 bium interposed, as in dicotyle- 

 dons. There are differences, 

 however, between the central 

 and exterior parts of the stem, 

 somewhat analogous to these, 

 which we will endeavor to ex- 

 plain. 



Figs. 401 and 402 will assist 



in understanding these differences. The dark lines 

 represent woody bundles. Each bundle, traced from 

 above downward, starts from a point on the stalk, 

 where a leaf is inserted, descends obliquely toward 

 the centre of the stem (mark this), then, curving out- 

 ward, descends obliquely again toward the circumfer- 



