142 LYCOPODIALES [CH. 



only by the more frequent preservation of the middle cortex, a 

 fact due to a difference in minute structure, but chiefly by 

 the peculiar structure of the secondary tissue added to the 

 stele ; this is in part composed of radial series of parenchy- 

 matous cells and of a varying amount of tracheal tissue the 

 elements of which are narrower than in other species and are 

 characterised also by their sinuous vertical course. As is pointed 

 out in the sequel, the anatomical features of L. fuliginosum, as 

 at present understood, are not confined to one type of Lepido- 

 dendron stem. Specimens have been described with leaf- 

 cushions of the form characteristic of L. aculeatuvi, L. obovatum 

 and Lepidophloios combined with the anatomical features of 

 Williamson's species : it is possible that the two species 

 L. obovatum and L. aculeatum are not really distinct \ but it is 

 certain that shoots with both the Lepidodendron and Lepi- 

 dophloios cushions may have the same type of anatomical 

 structure. 



A more detailed knowledge of the structural features of 

 Lepidodendron shoots may enable us to define anatomical 

 species with more exactness than is possible at present. There 

 can, however, be little doubt that well-marked anatomical 

 features may be associated with more than one specific form of 

 shoot as defined by the form of the leaf-cushions. 



Solms-Laubach proposed the name Lepidodendron William- 

 soni for the anatomical type L. fuliginosum of Williamson, but 

 the latter name has been generally adopted. 



In the following account special attention is directed to the 

 nature and origin of the secondary stelar tissue and to the 

 secretory zone, as difference of opinion exists as to the interpre- 

 tation of these features. Among the best examples of shoots 

 of Lepidodendron fuliginosum without secondary tissue or in 

 which it is feebly developed are those originally described by 

 Binney. The stele includes a large parenchymatous pith, the 

 cells of which frequently show signs of recent division, a feature 

 observed also in the pith of the large stem of L. Wiinschianum, 

 represented in figs. 181, 182. The primary xylem cylinder has 

 an irregularly crenulate outer edge like that of L. Wiinschianum 

 ' Tliej- are regarded as identical by Fischer (04). 



