92 



ORGANOGRAPHY. 



Fig. 180. Fig. 181, 



bundles may be traced to the leaves, from which organs they are 

 at first directed towards the interior of 

 the stem {fig. 181, a, h, c, d), along 

 which they descend generally for some 

 distance, and then gradually curve out- 

 wards again and terminate at the circum- 

 ference in the rind or false bark, or in 

 young stems some of them 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. 

 182. 



The vascular bundles in their course 

 down the stem generally become more 

 attenuated, which circumstance arises 

 from certain differences which take place 

 in their structure as they descend. Thus 

 when they first originate they consist, as 

 we have seen (see p. 72), of spiral, an- 

 nular, and other vessels, mixed with 

 parenchymatous and liber-cells. In their 

 descent thcv gradually lose the spiral 



/=^(i7s. 180andl81. Diagrams , .1 ' 1 .., ^1 ..t, * 



showing the course of the ^"^1 Other vesscls, SO that when they ter- 

 fibro-vascuiar bundles of minate at the circumfercuce they consist 



aMonocotyledonous stem. 1 • n c ^■\ n i, 1 j. .^1 i 



fl, 6, c, d. Fibro-vascuiar chicfly of libcr-cells bouud togetlier by 



bundles Fig. 180 ex 



liibits the course of the 

 bundles as formerly sup- 

 posed F/ff.lSl, accord- 

 ing to Mohl's system, as 

 now proved to be correct. 



Fig. 18: 



Fig. 182. Vertical section 

 of the stem of a Pulm, 

 showing ('./V) the vascular 

 bundles intersecting each 

 other as they pass down- 

 wards. 



parenchyma. The rind or false bark 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 latter 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 endoge- 

 nous for a portion of their course, as 

 tliey terminate ultimately at the circum- 

 ference. This term endogenous has been 

 therefore altogether discarded of late 

 years by many botanists, who use instead, 

 one which is derived from the structure 

 of the embryo of such plants which we 

 have seen ])ossesses but one cotyledon, 

 and hence their stems are called Mono- 



