XV] LEPIDODENDRON 113 



of lepidodendroid plants^ are worthy of notice in connexion 

 with the recent work of Mr Gwynne-Vaughan^ who has shown 

 that in many recent ferns the scalariform bands in the xylem 

 elements are not connected by a thin pit-closing membrane, but 

 are separated from one another by open spaces. In the Lepido- 

 dendron tracheae we seem to have a stage in which the inter- 

 vening membrane is in process of absorption. It is, however, 

 possible that the threads may be the result of contraction and 

 splitting of the membrane during drying or decay. 



The stele of Lepidodendron vasculare, before the addition of 

 any secondary xylem, may be described as a protostele, a term 

 originally proposed by Professor Jeffrey*, in which the central part 

 of the conducting strand of xylem elements has been converted 

 into rows of parenchyma and short tracheids, the latter being 

 better adapted to storage than to conduction. It is probable 

 that this type of stelar anatomy, which distinguishes L. vasculare 

 from other species, represents a comparatively primitive arrange- 

 ment forming a transition between the stele of L. esnosten.se, 

 which consists of a solid rod of tracheids, and the stele of 

 L. Harcourtii (fig. 179, A) and other species in which the xylem 

 forms a cylinder enclosing a large parenchymatous pith. 



Parenchymatous cells occur in contact with the outer edge 

 of the xylem-cylinder some of which are distinguished by an 

 irregular reticulate pitting. The tangential section repre- 

 sented in fig. 148, B, illustrates the appearance of a shoot of 

 L. vasculare in which no secondary xylem is present: the central 

 strand of tissue consists of the parenchyma abutting on the 

 xylem with several leaf-traces Qt) passing upwards in an almost 

 vertical course from the outer edge of the stele. 



The secondary xylem (fig. 148, A, x^) consists of radially 

 arranged scalariform tracheae with associated rows of paren- 

 chymatous cells which form medullary rays (fig. 149, mr). 

 Leaf-traces pass through the medullary rays in the secondary 

 xylem cylinder in a direction at right angles to the primary xylem 

 stele from which they are given off, but at the outer edge of the 



J Solms-Laubach (92) PI. ii. fig. 6 ; Seward and Hill (00) PI. iv. fiR. 26. 

 See p. 910 of the latter paper for other references. 



2 Gwynne-Vanghan (08). ^ Jeffrey (98). See also Tansley (08) p. 37. 



