LEPIDODENDRON 143 



the stele, are accompanied by a broad arc or zone of 

 secondary wood and phloem, the only case of the kind 

 hitherto found in a Lepidodendroid, as distinguished 

 from a Sigillarian, stem. The specimen is fully 

 described in the Transactions of the Royal Society of 

 Edinburgh, by Prof. A. C. Seward and Mr. A. W. Hill. 1 



Another form, which in its primary condition some- 

 what resembles Lepidodendron Harcourtii,. is the Burnt- 

 island species named by Williamson L. brevifolium. 

 This, like L. Wunschianum, comes from the Calciferous 

 Sandstones, at the base of the Carboniferous formation. 

 Twigs and branches of all sizes have been found in 

 abundance, and the first beginnings of secondary 

 growth have thus been traced. The species differs 

 strikingly from L. Wunschianum, in the dimensions of the 

 branches with secondary tissues. In L. brevifolium, even 

 comparatively small twigs have a zone of radially seriated 

 wood of cambial origin, surrounding a primary cylinder, 

 sometimes only about 3 mm. in diameter. In the larger 

 branches, the secondary zone of wood attained a thick- 

 ness enormously greater than that of the primary ring. 

 An example of this stem, at a fairly advanced stage, is 

 shown, in transverse section, in Fig. 58. 



For the more detailed study of the structure and de- 

 velopment of the stem in a Lepidodendron with secondary 

 growth, we will choose, however, another example, 

 namely L. selaginoides,' which, from the abundance ol 

 specimens and their extraordinarily good preservation, 

 has proved exceptionally favourable for investigation. 



In this form, which is abundant in the Lower Coal- 

 measures, the specimens showing structure have been 



1 I.e. vol. xxxix. Part iv. 1900. 



