36 Mi: ROBERT KIDSTON ON 



BRONGNIART tlien proceeds to consider the structure of Lepidodendron Harcourtii* 

 the only Lepidodendron whose internal organisation was then known. 



In Lepidodendron Harcourtii the primary vascular bundle consists of a closed ring 

 of scalariform tissue, sharply defined on its inner side, where the larger-sized vessels are 

 placed. On its outer surface the vessels become much smaller, and form a curious 

 crenulate line caused by bay-like hollows with dividing projections. The elements 

 composing the bundle are disposed without any definite order, and there are no 

 medullary rays. The foliar bundles spring from the outer smaller vessels. 



From the structure exhibited by this species, Brongniart came to the conclusion that 

 Lepidodendron belonged to the vascular cryptogams, as the arrangement of the vessels 

 was such as one constantly observes in Lycopods. In contra-distinction to this, he 

 believed that the disposition of the ligneous tissue (secondary xylem) of Sigillaria 

 elegans, composed of radiating series of vessels, was a character foreign to all crypto- 

 gams and characteristic of Dicotyledons, and for these and other reasons, which 

 Brongniart fully gives, he believed the affinities of Sigillaria elegans were with the 

 gymnosperms. To show, however, that his opinions were not free from doubt, he 

 says : — " But in the meantime it may be difficult to establish this in a positive manner, 

 because there are numerous differences between this plant and the gymnosperms which 

 we know," and of these differences he gives a summary which need not be repeated 

 here, but concludes with the following suggestive sentence : — "All these circumstances 

 lead to the conclusion that the Sigillaria and Stigmaria constitute a special and 

 extinct family probably belonging to the great division of the Dicotyledonous gymno- 

 sperms, but of whose fruit and leaves we are still ignorant," and he further asks whether 

 Stigmaria may not be the root of Sigillaria. 



Tin' great point on which Brongniart founded his conclusions that Lepidodendron 

 was Lycopodiaceous, and Sigillaria gymnospcrmous, was the absence of a secondary 

 xylem in the former and its presence in the latter, accompanied with medullary rays 

 and a radial arrangement of the vascular elements. 



It is a curious circumstance that Lepidodendron Harcourtii, the species examined 

 by BRONGNIART while he was instituting his comparison between the internal structure of 

 Sigillaria and Lepidodendron, is one of the very few Lepidodcndra which has not yet 

 \ ielded any specimens showing the development of the secondary xylem. In regard to 

 the absence of secondary xylem in Lepidodendron Harcourtii,^ Professor Williamson 

 remarks :■ -"No specimen of it has yet been found showing a trace of secondary xylem. 



* Wi'iHA.M, "On tlie lepidodendron Harcourtii," Trans. Nat. Hist. Soc. Nor thumb., Durham, and Newcastle, vol. 

 ii., read Blarcfa 1832. Ibid., "Internal Structure of Fossil Vegetables found in the Carboniferous and Oolitic De- 

 posits of Great Britain," pp. 51 and 75, pi. xii. figs. 1-7, and pi. xiii., Edin., 1833. Lindley and Hutton, Lepidoden- 

 dron Harcourtii, Fostil Flora, vol. ii. p. 45, pis. xcviii. and xcix., 1833. 



t WILLIAMSON, " On the Light Thrown upon the Question of the Growth and Development of the Carboniferous 



Arborescenl Lycopods by a Study of the Details of their Organisation," Mem. and Proc. Manchester Lit. and Phil. Soc, 



4th ser. vol. ix., session 1894-5, p. 47, 1895. See also Brongt., Hist. d. ve'ije't. foss., vol. ii. p. 37, pis. xx. and xxi., 1837 ; 



1 liiiTKAND, " Remarqaes sni Le Lepidodendron Harcourtii de Witham," Travauz el Mffmoircs des Faculte's de Lille, vol. ii. 



No, <;. plfl. i.-x. Lille, 1801. 



