52 



ENDOGENOUS OR MONOCOTYLEDONOUS STEM. 



found not to continue in a parallel direction, but to arch outwards, so 

 as ultimately to reach the circumference. Hence, the newly-formed 

 fibres really become external at the base, although internal above. 

 On making a vertical section of an endogenous stem, as of a Palm, 

 there is observed an interlacing of fibres, similar to what is represented 



in fig. 114, 2, where the four vas- 

 cular bundles, abed, are first 

 directed towards the centre, and 

 <* r^ '' d then curve outwards towards the 



circumference, so that those last 

 formed ultimately become external. 

 The term Endogenous, will, there- 

 fore, only apply strictly to the 

 fibres at the early part of their 

 course. On this account, the terms 



a 



\ 





\ 



r/U 



(' Endogenous and Exogenous have 



been recently discarded by many 

 writers, the terms Monocotyledon- 

 ous and Dicotyledonous being sub- 

 stituted. The true distinction 

 between Exogenous and Endogen- 

 ous stems consists in this, that in 

 the former, the woody or vascular 



\ / \1 I I y bundles increase indefinitely at 



their periphery, while in the latter, 

 they are arrested in their transverse 

 growth at a definite epoch. 



95. The composition of the vas- 

 cular bundles, in different parts of 

 then* course, varies. Thus, at the 

 11 upper part, where they proceed 

 from the leaves towards the centre, 

 they contain spiral vessels, porous 

 vessels with some cellular tissue, a 

 few laticiferous vessels, and woody 

 fibres resembling those of liber (fig. 

 112). As the bundles descend, the 

 spiral vessels disappear, then the 

 porous vessels, and when they have 

 reached the periphery, and have become incorporated with it, nothing 

 but fibrous tissue, or pleurenchyma, remains, forming by its division a 

 complicated anastomosis, or net-work. Thus, at the commencement 



