XVl] SIGILLARIA 213 



(fig. 200, E) the xyletn consists of both primary and secondary 

 elements (x, x% but in the lamina the latter is poorly if at all 

 represented. In the lamina of the leaves of S. Brardi the 

 primary xylem forms a narrow slightly curved band with two 

 lateral groups of narrower, presumably protoxylem elements; 

 this is surrounded by delicate parenchyma styled by Renault, on 

 very slender evidence, phloem (" liber "). Some dark cells below 

 the xylem are described as sclerous tissue, and surrounding the 

 bundle is a sheath of transfusion tracheae (dotted area in 

 fig. 200, E). It is possible that the elements spoken of with 

 hesitation by Renault as secondary xylem are transfusion 

 tracheae. 



There has probably been some confusion in the minds of 

 authors between sclerous tissue and dark secretory tissue in 

 Sigillarian leaves ; the crescentic band, a, shown in fig. 142, B, 

 which corresponds in position with the sclerous tissue of 

 Renault in S. Brardi leaves, appears to be of the nature of 

 secretory tissue. 



The diagram shown in fig. 142, B, illustrates a type of leaf 

 very like those already described, except that there are two 

 xylem strands, x. The difference between the double strand and 

 the single bundle seen in figs. 142, A, C and 200 E, is com- 

 paratively small, but it is a real distinction. This type of leaf 

 (fig. 142, B) was originally described by Renault' under the 

 generic title Sigillariopsis. The genus was founded on a French 

 petrified specimen consisting of part of a ribbed stem possessing 

 a stele of the Sigillarian type and characterised by separate 

 primary xylem strands, like those of S. Brardi described by 

 Brongniart in 1839. Renault considered the presence of two 

 xylem strands in the leaf a sufficient reason for the institution 

 of a new genus and named the specimen Sigillariopsis Decaisnei. 

 Prof. Bertrand of Lille kindly photographed for me Renault's 

 type-specimen and sent several prints with explanatory notes. 

 The transverse section of the leaves shows very clearly the two 

 xylem strands; each strand consists of a triangular group of 

 primary tracheae with the protoxylem apex pointing towards 

 the lower surface of the lamina. Below each primary strand 

 1 Renault (79) Pis, xri. xiii. p. 270 ; (96) A. p. 245. 



