XVl] SIGILLAEIA 211 



.shows that they persisted, in some cases at least, for several 

 years as in Araucaria imhricata. The lower surface of the 

 lamina was characterised by a prominent keel (fig. 142, A 

 and C) which dies out towards the apex ; on either side of it 

 are well-defined stomatal grooves (figs. 142, g,g] 143, A ; 200, 

 D, g). The upper face may be characterised by another groove 

 (fig. 142, B) but without stomata. The occurrence of the 

 stomatal grooves, the abundance of transfusion tracheae 

 (fig. 142, t) surrounding the vascular bundle, and the pre- 

 sence of strengthening hypodermal tissue suggest that the 

 leaves of Sigillaria were of a more or less pronounced xerophi- 

 lous type and had a fairly strong and leathery lamina. The 

 mesophyll tissue consists either of short parenchymatous cells 

 or of radially elongated palisade-like elements and has the loose 

 or lacunar arrangement characteristic of the aerating system 

 in recent leaves ; the slight development or absence of palisade- 

 tissue may indicate exposure to diffuse light of no great intensity. 



In most species there is a single vein, but in others the xylem 

 forms a double strand (fig. 142, B). Sections of the lamina near 

 the apical region present a more circular form, owing to the 

 gradual obliteration of the upper groove and lower keel and to 

 the dying out of the stomatal grooves. 



The transverse section of the leaf diagrammatically repre- 

 sented in fig. 142, A, A', shows the two stomatal grooves, g, 

 and a prominent keel ; the single vein consists of a small group 

 of primary tracheae, x, some delicate parenchyma, and a brown 

 patch of imperfectly preserved tissues, a, resembling the secre- 

 tory zone tissue of a Lepidodendron. The whole is surrounded 

 by a sheath of rather wide and short thinner-walled spiral or 

 reticulate tracheids, which may be spoken of as transfusion 

 tracheae, t, and compared with similar elements in the leaves 

 of many recent Conifers. To this tissue Renault applies the 

 epithet "water-bearing" and it is very likely that this may have 

 been its function. The shaded portions of the lamina, in 

 fig. 142, A, represent the distribution of thicker-walled hypo- 

 dermal tissue. The section of a leaf 3 mm. wide shown in 

 fig. 142, C, shows an almost identical structure; the transfusion 

 tracheae are richly developed especially on the sides and lower 



