THE FOSSIL FLORA OF THE YORKSHIRE COAL FIELD. 37 



It appears probable that in this respect the type resembles L. Wunsckianum, viz., that 

 the exogenous zone, or secondary xylem, only made its appearance at an advanced age of 

 growth, and that we have as yet obtained no specimen sufficiently advanced to have 

 entered upon that stage." 



Since Brongniart wrote his memoir, little has been added to our knowledge of the 

 internal organisation of Sigillaria, the only contribution being by MM. Renault and 

 Grand 'Eury, " Recherches sur les vegetaux Silicifies d'Autun : Etude du Sigillaria 

 spinulosa." * This memoir contains some important anatomical discoveries. The 

 primary vascular bundles form an inner circle, but do not touch each other laterally ; 

 they are, however, in contact with the secondary xylem or " ligneous cylinder," which 

 is slightly swollen at the point where they come in contact with it. The form of the 

 primary bundles is exactly similar to the corresponding portion in Sigillaria elegans. 

 In transverse section the bundle is in the form of a crescent, of which the convexity 

 formed by the larger vessels faces inwards, while the slightly concave part composed of 

 smaller vessels faces the slight prominences of the secondary xylem. The large vessels 

 are scalariform or reticulated, but as they approach the secondary xylem they become 

 smaller, and assume the form of spiral vessels or false tracheids. These smaller vessels 

 give rise to the foliar bundles. 



The secondary xylem consists of a cylinder of many regular radiating series of 

 medium-sized vessels separated by medullary rays, but without any division into com- 

 ponent bundles as in Sigillaria elegans. The vessels which compose this secondary 

 xylem are elongate and barred on all surfaces (scalariform), as in Sigillaria elegans. 

 These vessels are disposed in long radiating lines and separated by the slender medullary 

 rays or by the foliar bundles. 



The Medullary rays, which are composed of one, or at most of two, rows of laterally 

 placed smooth-walled muriform cells, have a considerable vertical height. 



The Bark, according to Renault, is composed of three layers : an inner and delicate 

 cellular tissue which is usually destroyed ; a middle, of coarse Dictyoxyloid structure, 

 which he names the suberous layer. This suberous layer is formed of bands of elongated 

 cells with smooth resistant walls, which anastomise longitudinally and transversely, 

 and thus form a strong mesh-like structure, whose meshes are filled with more delicate 

 prismatic cellular tissue. The suberous layer appears to be formed of several zones of 

 this Dictyoxyloid tissue, which, under certain conditions of preservation, separate from 

 each other. In comparison to the size of the vascular system, this middle zone of the 

 bark bulks largely in the size of the stem. 



The third or outermost layer of the bark — the Epiderm — is formed of polyhedric 

 cells, which become elongate as they approach the cellular rays which fill the meshes of 

 the suberous layer, and especially so where they join the mesh-bands. The epiderm 

 forms a continuous layer extending over all the outer surface of the plant, and obscures 



* M4m. Presentes par divers savants a Vacad. d. Sciences, vol. xxii. Paris, 1875. Plates. See also note on p. 62 

 of this communication. 



