40 MR ROBERT KIDSTON ON 



Professor Williamson concludes that the addition to the primary vascular bundle 

 ( which i.s not separated into distinct lateral bundles) must have arisen through a 

 conversion into vessels of the cells of the central or medullary parenchyma by a centri- 

 petal process of development.* 



Outside of this primary bundle we have the exogenously developed centrifugal 

 xylem, composed of small scalariform tissue, radially disposed, between which run the 

 medullary rays. In the specimen figured by Williamson^ of which the above is a short 

 description, the primary xylem forms a circular band, 4 5 mm. thick, while the secondary 

 xylem is 20 mm. thick. 



The vortex consists of several layers ; the innermost is composed of small-celled 

 parenchyma. Outside of this is a broad zone of very uniform parenchyma, composed of 

 small cells passing externally into another zone of uniform tissue, but composed of cells 

 with thicker walls and of larger size. This is enclosed in the usual zone of prosen- 

 ehyma, which in the arborescent stems attained to a considerable thickness from 

 increase by age..| 



The leaf bundles in Lepidodendron Wunschianum offer no peculiarity. They 

 spring from the outer surface of the primary bundle, and pass out in an upward course 

 to the leaves. 



It is therefore clearly proved that Lepidodendron possessed an exogenous growth 

 quite similar to Sigillaria, and especially like that described in Sigillaria spinulosa, 

 where the secondary xylem is not separable into component bundles, as in the Sigillaria 

 rlegans described by Brongniart. 



A further difference has been presumed to exist between the primary bundles of 

 Lepidodendron and Sigillaria, as illustrated by Brongniart, Renault, and Grand 

 'Eury, iu that the primary bundle in Lepidodendron forms a continuous closed ring, 

 while in Sigillaria it is formed of a number of separate bundles arranged in a circle; but 

 in the figure of Sigillaria spinulosa, given by Sohns-Laubach,§ taken from a specimen 

 presented to the late Professor Williamson by M. Renault, at one part of the primary 

 bundle circle, these bundles are shown to be united, and form part of a ring quite 

 similar to that seen in Lepidodendron Harcourtii or Lepidodendron Wunschianum. 

 As illustrating this point more fully, I figure here the central portion of a transverse 

 section of another specimen of Sigillaria spinulosa, — for which I am also much indebted 

 to Mons. Renault, — that shows still more clearly that the isolated primary bundles have 

 evidently been separated from what appears to have been originally a closed vascular ring. 



* "On the Organisation of the Fossil Plants of the Coal Measures," part x. p. 497, Phil. Tram., part ii., 1880. 



t I. o,. r,t., Mem. \., pi. xiv. tig. 6. 



J Williamson, "General Morphological and Histological Index to the Author's Collective Memoirs on the Fossil 

 Plant- of tli'- ' '"al Measures," part ii. p. 16, Mem. and Proc. M'ter. Liter, and Phil. Soc, session 1892-93, 1893. Note. 

 — If the Halonial branch referred here to Lepidodendron Wunschianum belongs to this species, then the plant is a 

 Lepidophloioi. This does not, however, in the slightest alter the position of the question under discussion as to the 

 occurrence of an exogenous zone in Lepidodendron, as it is present in Lepidodendron selayinoides and other species. 



jj Foiisil Botany, English edit., p. 253, fig. 29, 1891. 



