1 4 BOTANY. 



The medullary rays, or plates, or the silver trrain of carpenters, keep up 

 a communication between the bark and tlic ])ith, these being generally 

 separated by vascular layers. They consist of cellular tissue, which has 

 been gradually compressed, so as to give a niuriform appearance to the cells. 

 The space they occupy, at first large, is diminished more and more with 

 increasing age. A transverse section of a woody stem presents the 

 appearance of narrow lines running from the centre to the circumference. 

 A longitudinal section shows that these i-ays are not laminas continuous 

 from one end to the other, but are broken up b}^ the intervention of woody 

 fibres. 



We have thus described the normal character of the exogenous stem. 

 There are, however, certain anomalous appearances in certain plants, which 

 are not readily reducible to rule. In place of the concentric arrangement 

 of the vascular layers, there are sometimes only a few rows of wedge-shaped 

 Imndles, and additions made by the interposition of new bundles, just as in 

 the young herbaceous normal stem ; sometimes these vascular bundles are 

 arranged in zones. Again, in some cases the separating layers are cellular, 

 not fibrous ; sometimes the woody layers are arranged in a very irregular 

 manner. In some Bignonias the layers are divided into four wedge-shaped 

 portions, probably by an introversion of the liber. In Paullinia a central 

 Avoody mass is sometimes surrounded by others likewise cylindrical. In some 

 Malpighiacese, the outer surface, instead of being cylindrical, exhibits very 

 irregular lobes and indentations. 



^ The stems of endogenous plants present many features different from 

 those which we have found to exist in exogens, and especially in that there is 

 no absolute or visible distinction into pith, medullary rays, wood, and bark. 

 There is an intermixture of bundles of fibro-vascular tissue among a mass of 

 cellular tissue, the whole overlaid by a zone of denser cellular and woody 

 tissue, inseparable from the stem. In the young plant the centre of the stem 

 is occupied entirely by cells, around which the vessels are grouped, increasing 

 in number towards the circumference. The central cells are sometimes 

 ruptured and absorbed, leaving a cavity ; more generally, however, they are 

 persistent, becoming gradually encroached upon by the increasing vascular 

 system. The external layer of the endogenous plant occupying the place 

 of bark, and known as fqlse hark, is a dense layer of cellular tissue, into 

 which the lower ends of the vascular fibres dip, losing their vascularity as 

 soon as they reach it. 



The opinion originally entertained that the new layers of vascular fibres 

 were developed inside the old ones, and pushed these out towards the cortical 

 envelope, appears not to be strictly correct, as, although at first they are 

 thus internal, yet, subsequently, they curve outwards to run into the 

 exterior, as already mentioned. After all, the true distinction between 

 exogenous and endogenous stems consists in this : in the former, the woody 

 or vascular layers increase indefinitely at their periphery; in the latter, 

 they are arrested in lateral growth at a definite epoch. When it is one 

 terminal bud alone of an endogen that developes, the stem may be truly 

 cylindrical ; when several develope, however, the stem will be conical. A 

 14 



