228 Eoyal Society : — 



tained intermediate in size between the above example and the 

 small stems more usually met with. The vascular laminae increased 

 in thiclcness as they proceeded from withiu 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 parench^Tna, 



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

 The innermost one is a deUcate parenchyma closely investing the 

 hgueous zone, its cells being continuous -nith those of the medullary 

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

 parenchymatous layer of greater thicluiess than the inner one. 

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

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

 third or prosenchymatous layer, which presents very different 

 features according to the aspect in A^'hich it is regarded. In the 

 transverse section it consists of radiatmg bands of parenchyma 

 alternating with narrower and very dark-coloured ones of woody 

 prosench'STna, the latter looking very like the Eoman numerals upon 

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

 are fibrous laminae, which pursue an undulatory 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. Xo vascular bundles enter these areolae ; hence 

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

 Lepidodendra. 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 

 areolae, 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 yoimg stems. 



Vascular bundles of large size ascend vertically through the two 

 inner parench;^niiatous layers of the bark. In some instances 

 each of these bundles exhibits, in the transverse section, an oval 

 outline, with 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 incoi-porated with it. It 

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

 at the exterior of the hgneous zone, corresponds T\ith the spaces 

 intervening between the de^^ached masses of the medullary cvhnder 

 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. 



