910 MR A. C. SEWARD AND MR A. W. HILL ON THE 



secondary wood are clearly bordered (PL IV. figs. 25, 26 and 29), and the position of 

 the pit-closing membrane is occupied by numerous delicate threads extending from one 

 thickening band to the next ; the appearance of these in surface-view is seen in figs. 

 25 and 26. In tangential longitudinal section the walls of the tracheids have a double 

 contour, and each pit-closing membrane appears to consist of a double series of irregular 

 fine threads, which have probably been formed by the contraction and tearing of the 

 thin membrane which originally connected the scalariform bands. Precisely similar 

 threads occur in Lej) telodendron vasculare (Binney),* Lepidopliloios Wunschianus 

 (Will.),t L. Harcourtii (Witham),| Lepidodendron mundum (Will. ),§ L. brevifolium 

 (Will.), || L. squamosum (Gopp.),1T and Stigmaria jicoides, Brongn. # * 



Count Solms-Laubach, in describing the same structure in a Culm species of Lepido- 

 dendron, speaks of the two sets of delicate threads, which appear as two lines in the 

 drawing of PL IV. fig. 29, as probably corresponding to the " grenzhautchen " of the 

 tracheid wall, the middle lamella of which has disappeared. The same limiting mem- 

 brane is shown in PL IV. fig. 26, and the irregular darker substance in the middle of 

 each thickening band represents the shrunken internal portion of the wall. This 

 histological feature is probably the result of post-mortem changes, and cannot be 

 regarded as of any importance as a specific character. ft 



In contact with the narrower tracheids, already referred to, in the outer portions of 

 the primary xylem, there are numerous short and more delicate elements characterised 

 by fine spiral and reticulate thickening bands ; some of these are shown in fig. 24, 

 PL IV., but it is more convenient to deal with this feature in describing the origin and 

 structure of the leaf-traces. At two points in the ring of primary xylem, indicated by 

 in the diagram, fig. 33, PL IV., there is an indentation occupied by thin -walled 

 parenchyma and a few fairly large isodiametric tracheids. 



iii. Secondary xylem. — The broad band of secondary wood consists of radially 

 disposed scalariform tracheids of smaller diameter than the innermost elements of the 

 primary wood. At some points the smaller internal tracheids which compose the inner 

 ends of the rows of secondary xylem are seen in contact with the elongated narrow 

 elements of the projecting teeth of the corona, but for the most part the rows of 

 secondary tracheids are separated from the primary tracheids by a zone of varying 

 breadth consisting of elements of a different type. These intervening cells |j are short 

 or isodiametric in form, and have spiral or reticulate bands on their walls ; they are, in 

 fact, short and delicate tracheids (PL II. fig. 13 and PL IV. fig. 24). When secondary 



* Hovelacqle (92), p. 42, fig. 6. SOLMS-LAtTBACH (92), pi. ii. fig. 6. 



t Williamson (80), pi. xiv. fig. 4. + Ibid. (93), p. 20. § Ibid. (89), pi. v. fig. 14 A. 



t| E.^., specimen 511 (Williamson Coll., British Museum). 



IT Solms-Lauhach (92), p. 76. ** Ibid., p. 76. 



tt In a specimen of Williamson's Lepidode?idroii brevifolium, some of the tracheids show the pit-closing membrane 

 intact, while in others it has the form of fine threads. 



;} "Filues primitives" of Bertrand. It is probable that these elements described by Bertrand as cells with smooth 

 walls possessed originally reticulate thickening bands. 



