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Prof. Williamson on the Fossil 



[Mar. 7, 



to the same plant, of which the structure has varied at different stages of 

 its growth. The specimens were obtained from some thin fossiliferous 

 deposits discovered by Mr. G. Grieve of Burntisland, in Fifeshire, where 

 they occur imbedded in igneous rocks. The examples vary from the very 

 youngest, half-developed twigs, not more than T ^ of an inch in diameter, 

 to arborescent stems having a circumference of from two to three feet. 

 The youngest twigs are composed of ordinary parenchyma, and the imper- 

 fectly developed leaves which clothe them externally have the same struc- 

 ture. In the interior of the twig there is a single bundle, consisting of 

 a limited number of barred vessels. In the centre of the bundle there can 

 always be detected a small amount of primitive cellular tissue, which is a 

 rudimentary pith. As the twig expanded into a branch, this central 

 pith enlarged by multiplication of its cells, and the vascular bundle in like 

 manner increased in size through a corresponding increase in the number of 

 its vessels. The latter structure thus became converted into the vascular 

 cylinder, so common amongst Lepidodendroid plants, in transverse sections 

 of which the vessels do not appear arranged in radiating series. Simulta- 

 neously with these changes the thick parenchymatous outer layer becomes 

 differentiated. At first but two layers can be distinguished — a thin inner 

 one, in which the cells have square ends, and are disposed in irregular 

 vertical columns, and a thicker outer one consisting of parenchyma, the 

 same as the epidermal layer of the author's preceding memoir. In a short 

 time a third layer was developed between these two. 



When the vascular cylinder had undergone a considerable increase in its 

 size and in the number of its vessels, a new element made its appearance. 

 An exogenous growth of vessels took place in a cambium layer, which in- 

 vested the preexisting vascular cylinder. The author distinguishes the 

 latter as the vascular medullary cylinder, and the former as the ligneous 

 zone. The newly added vessels were arranged in radiating laminae, sepa- 

 rated from each other by small but very distinct medullary rays. At an 

 earlier stage of growth traces of vascular bundles proceeding from the 

 central cylinder to the leaves had been detected, These are now very 

 clearly seen to leave the surface of the medullary vascular cylinder where 

 it and the ligneous zone are in mutual contact ; hence tangential sections 

 of the former exhibit no traces of these bundles, but similar sections of 

 the ligneous zone present them at regular intervals and in quincuncial 

 order. Each bundle passes outwards through the ligneous zone, im- 

 bedded in a cellular mass, which corresponds, alike in its origin and in its 

 direction, with the ordinary medullary rays, differing from them only in its 

 larger dimensions. At this stage of growth the plant is obviously iden- 

 tical with the Biploxylon of Corda, with the Anabathra of Witham, and, so 

 far as this internal axis is concerned, with the Sigillaria elegans of Bron- 

 gniart. The peculiar medullary vascular cylinder existing in all these 

 plants is now shown to be merely the developed vascular bundle of 

 ordinary Lycopods, whilst the exogenous radiating ligneous zone enclosing 



