XV] LEPIDODENDRON 177 



both recent and extinct spores and denote their origin in 

 tetrads. The spore shown in fig. 191, E', illustrates the external 

 features. The apical region of the prothallus of a megaspore 

 of Lepidodendron Veltheimianum described by Mr Gordon^ 

 consists of smaller cells than those occupying the greater part 

 of the spore-cavity, a differentiation which he compares with 

 that of the prothallus of Selaginella. 



There can be little doubt that the petrified shoots described 

 by Williamson^ from the Calciferous Sandstone beds of Burnt- 

 island as Lepidodendron brevifolium are identical with specimens 

 possessing the external features of i. Veltheimianum. In 1872 

 Dawson expressed the opinion that Williamson's species should 

 be referred to L. Veltheimianum, and evidence subsequently 

 obtained confirms this view. The stele of this species is of 

 the medullated type, differing from that of L. fuliginosum and 

 L. Harcourtii in the absence of prominent ridges on the external 

 surface of the primary xylem, and from L. vasculare in the 

 possession of a parenchymatous pith. In younger twigs the 

 cortex consists of fairly homogeneous tissue, but in older 

 branches there is a greater distinction between a delicate 

 middle cortex and a stronger outer cortex. Fig. 186, A, 

 represents a stem in which the vascular cylinder is composed 

 of a primary xylem ring, x, 15 mm. broad, succeeded by 

 a zone of secondary wood 1"2 cm. in breadth. The junction 

 between the primary and secondary xylem is shown on a larger 

 scale in fig. 186, B. The tissues abutting on the secondary 

 xylem have not been preserved; the outer cortex, which 

 consists chiefly of secondary elements, is divided superficially 

 into unequal ridges corresponding to the leaf-cushions which 

 have been more or less obliterated as the result of growth 

 in thickness of the stem. 



9. Lepidodendron Pedroanum (Carruthers). 



In 1869 Mr Carruthers described some specimens of vege- 

 tative stems and isolated sporangia, collected by Mr Plant in 

 Brazil, as Flemingites Pedroanus^. From a more recent account 



1 Bennie and Kidston (88) PL vi. figs. 20, a—s. " Gordon (08}. 



' Williamson (72). * Carruthers (69^). 



