72 



FOSSIL PLANTS. 



that have lost their epidermal layer, or had it converted into coal ; this layer, strengthened 

 by the bast-tissue of its inner surface, having remained as a hollow cylinder, when all the 

 more internal structures had been destroyed or removed. 



" From this type the author proceeds upwards, through a series of examples in which 

 the vessels of the medulla become separated from its central cellular portions and retreat 

 towards its periphery, forming an outer cylinder of medullary vessels, arranged without 

 order, and enclose a defined cellular axis ; at the same time the encircling ligneous 

 zone of radiating vessels becomes yet more developed, both in the number of its vessels 

 and in the diameter of the cyhnder relatively to that of the entire stem. As these changes 

 are produced, the medullary rays separating the laminse of the woody wedges become 

 more definite, some of them assuming a more composite structure, and the entire 

 organization gradually assuming a more exogenous type. At the same time the cortical 

 portions retain all the essential features of the Lepidodendroid plants. 



" We are thus brought by the evidence of internal organisation to the conclusion 

 that the plants which Brongniart has divided into two distinct groups, one of which he 

 has placed amongst the Vascular Cryptogams, and the other amongst the Gymnospermous 

 Exogens, constitute one great natural family 



" Siigmaria is shown to have been much misunderstood, so far as the details of 

 its structure are concerned, especially of late years. Li his memoir on Sigillaria elegans, 

 published in 1839, M. Brongniart gave a description of it, which, though Hmited to a 

 small portion of its structure, was as far as it went a remarkably correct one. The plant, 

 now well known to be a root of Sigillaria, possessed a cellular pith without any trace of 

 a distinct outer zone of medullary vessels, such as is universal amongst the Lepidodendra. 

 The pith is immediately surrounded by a thick and well-developed ligneous cyhnder, 

 which contains two distinct sets of primary and secondary medullary rays. The primary 

 ones are of large size, and are arranged in regular quincuncial order ; they are composed of 

 thick masses of mural cellular tissue. A tangential section of each ray exhibits a lenticular 

 outline, the long axis of which corresponds with that of the stem; these rays pass 

 directly outwards from pith to bark, and separate the larger woody wedges, which consti- 

 tute so distinct a feature in all transverse sections of this zone, and each of which consists 

 of aggregated laminse of barred vessels, disposed in a very regular radiating series. The 

 smaller rays consist of vertical piles of cells arranged in single rows, and often consisting 

 of but one, tw^o, or three cells in each vertical series ; these latter are very numerous, 

 and intervene between all the numerous radiating laminse of vessels that constitute the 

 larger wedges of woody tissue. The vessels going to the rootlets are not given off from 

 the pith, as Goeppert supposed, but from the sides of the woody wedges bounding the 

 upper part of the several large lenticular medullary rays ; those of the lower portion of the 

 ray taking no part in the constitution of the vascular bundles. The vessels of the region 

 in question descend vertically, and parallel to each other, until they come in contact with 

 the medullary ray, when they are suddenly deflected, in large numbers, in an outward 



