22.8 Boyal Societij :— 



tained iniei'iiiodiate in size between the above example and the 

 small stems more usually met \\A\h. The vascular laminae increased 

 in thickness as they proceeded from within outwards, and then 

 subdivided, in the ordinary exogenous manner, through the inter- 

 calation of new medullary rays. These rays are remarkable for the 

 great vertical range of each one, as well as for the large number of 

 cells which enter into their composition. In tangential sections 

 they appear as elongated lenticular masses of parenchpna. 



The Bark. — This organ is separable into three, if not four layers. 

 The innermost one is a dehcate parenchyma closely investing the 

 ligneous zone, its cells being continuous wdth those of the medullary 

 rays. At its outer surface this tissue passes gradually into another 

 parenchvmatous layer of greater tliicliness than the uiner one. 

 Both of them have patches of dark-coloured cells scattered through 

 their tissues. But the most remarJvable part of the bark is the 

 third or prosenchymatous layer, which presents very different 

 features according to the aspect in which it is regarded. In the 

 transverse section it consists of radiating bands of parenchyma 

 alternatiug with narrower and very dark-eoloui'ed ones of woody 

 prosenchyma, the latter looking very like the Eoman numerals upon 

 the face of a clock. Tangential sections show that the black bands 

 are fibrous lamiufe, which pursue an undidatory course as they 

 ascend through the stem, and which, as they alternately approach 

 and recede from one another, divide this part of the bark into a 

 series of lenticular or rhomboidal areas, occupied by various forms 

 of parench}Tna. No vascular bundles enter these areolae ; hence 

 they are something altogether different from the leaf-scars of the 

 Lepidodendm. Externally to this prosenchymatous layer some 

 specimens exhibit detached traces of a very thin external layer of 

 parench^nna, apparently derived from the cells of the rhomboidal 

 areohe, which have extended beyond the fibrous laminae and spread 

 themselves over the surface of the bark as a continuous layer ; 

 but this condition appears to be confined to very young stems. 



Avascular bundles of large size ascend vertically through the two 

 inner parenchymatous layers of the bark. In some instances 

 each of these b;indles exhibits, in the transverse section, an oval 

 outline, -oith faint traces of a vertical division into two parts. But 

 ordinarily the two halves of the bundle have separated, forming two 

 distinct bundles, which are some distance apart. They exhibit little 

 or no tendency to diverge from the hgneous cylinder as they ascend, 

 and in some instances actually become incorporated with it. It 

 is remarkable that the position of each of these double bundles, 

 at the exterior of the hgneous zone, corresponds with the spaces 

 intervening bet\Aeen the detached masses of the medullary cylinder 

 within it, as if the former were designed to act as buttresses 

 strengthening these weaker points in the vascular axis. It not 

 unfrequently happens that exogenous additions are made to such 

 of these bundles as are encompassed by the innermost layer of 

 the bark, in the shape of a few radiating laminae of vessels developed 

 on their outer or peripheral surfaces. 



