214 LYCOPODIALES [CH. 



of centripetal xylem is an arc composed of a few small tracheae 

 which Renault and Bertrand describe as secondary xylem ; it 

 is, however, not clear from the photomicrographs that these 

 are of secondary origin, their position and appearance reminding 

 one of the primary centrifugal xylem of a cycadean foliar 

 bundle. Below this centrifugal xylem is another arc of im- 

 perfectly preserved elements described by Eenault as a 

 protective sheath and by Bertrand as glandular tissue ; the 

 latter term is probably the more correct as the tissue may well 

 correspond to the secretory-zone tissue of Lepidodendron stems. 

 Fairly large groups of transfusion tracheids occur on the flanks 

 of the xylem. Prof. Bertrand points out that one of his sections, 

 cut nearer the apex of a leaf than that figured by Renault with 

 a single xylem strand, contains a double strand and thus shows 

 the latter's description to be an incorrect interpretation of the 

 imperfectly preserved tissues. 



The Sigillariopsis type of leaf was recognised by Scott* 

 in English material on which he founded the species Sigil- 

 lariopsis sulcata. In a section which he has recently figured^ a 

 lacuna below the two xylem strands is described as " representing 

 secretory tissue " ; a band of transfusion tracheae almost encircles 

 the pair of bundles. 



In a note published in 1907, Kidston' demonstrated the 

 association of Sigillariopsis leaves with an undoubted Sigil- 

 larian stem of the Rhytidolepis type and expressed his 

 conviction that Renault's genus is identical with Sigillaria. 

 The correctness of Kidston's conclusion has been proved by 

 Arber and Thomas^ who found that the leaf-traces of Sigillaria 

 scutellata bifurcate during their course through the outer region 

 of the cortex and enter the leaf as two distinct strands of 

 primary xylem. In the section from Dr Kidston's collection 

 shown in fig. 142, B, the lamina, 4 mm. wide, consists mainly of 

 thin-walled assimilating tissue composed of radially elongated 

 cells abutting at the periphery on hypodermal mechanical tissue, 

 except at the edges of the stomatal grooves which are bounded 

 by the small-celled epidermis. A broad sheath of thicker-walled 



1 Scott, D. H. (042). 2 Scott (08) p. 230, fig. 95. 



» Kidston (07=). * Arber and Thomas (08). 



