86 



OEGA.NOGRAPHY, 



ference, or in young stems some of them would reach the roots. 

 When we make a vertical section therefore of an endogenous 

 stem, we find these vascular bundles intersecting each other in 

 various ways, as shown in fig. 174. 



Fig. 172. 



173. 



Fig. 174. 



^■grs. l?2and 173. Diagrams showing the course of the flbro-vascnlar 

 bundles of aMonocotyledonous stem, a, b, c, d. Fibro-vascular bundles. 



Fig. 172 exliibits the course of the bundles as formerly supposed. 



Fig. 173, according to Mohl's system, as now jiroved to be correct. 



Fig. 174. Vertical section of the stem of a Palm, showing (fv) 



the vascular bundles intersecting each other as they pass downwards. 



Tlie vascular bundles in their course down the stem generally 

 become more attenuated, which circumstance arises from certain 

 differences which take place in their structiu'e as they descend. 

 Thus when they first originate they consist, as we have seen (see 

 p. 66), of spiral, pitted, and other vessels, mixed with paren- 

 chymatous and woody tissues. In their descent they gradually 

 lose the spiral and other vessels, so that when they terminate at 

 the circumference they consist chiefly of liber-cells bound together 

 by parenchyma. The ri7id or false hark of endogenous stems 

 is thus chiefly formed of the ends of the vascular bundles which 

 originate in the leaves, and hence we see the principal reason 

 why the rind cannot be separated, as in exogenous stems, from 

 the wood beneath. 



It follows from the mode of growth of the vascular bundles, 

 as indicated above, that the term endogenous, commonly ap- 

 plied to such stems, is not altogether correct, as the bundles 

 are only endogenous for a portion of their course, terminating 



