io6 



CORDAITEAE. 



the middle of the substance of the leaf; in Cordaites duplicinervis, Grand' 

 Eury, the bundles approach nearer first to the upper and then to the under 

 side, being in the former case smaller, in the latter larger on the transverse 

 section. Each bundle together with the adjacent parenchyma, which is 

 often obliterated, is usually surrounded by a firm sheath formed as a rule 

 of a single cell-layer. The bast-portion has in most cases disappeared, and 

 the position of the gap thus produced enables us to determine the under 

 side of the leaf with tolerable certainty. The woody portion of the bundle, 



on the other hand, is 

 marked by a peculiarity, 

 which in our modern 

 vegetation is found only 

 in the leaves of Cycadeae 

 and in those of Isoetes 

 and Ophioglossum. Its 

 j initial group (Fig. 7, c) 

 lies on the bast-side of 

 the bundle, either close 



i 



to the gap or bounded 

 on the side of the gap 

 in several species, ac- 

 cording to Renault, by 

 a group of one or more 



FIG. 7. Transverse section of the leaf of Cordaites angulostriatus, Grand' t-/~vre r\f f rcj/--ti^i/-1/=>e rl-iiVVi 

 Eury : a inner strand of the wood-bundle (bois centripete); b outer strand of 



the same (bois centrifuge); / initial strand lying between the two portions ; V\i\rf i ii-wo/-1 f t-i r cwrn-eo 



c bast; d parenchymatous sheath; h, A' and i fibrous tissue forming the "SVC d. CUFVCU ir<Ult>Vt,rie 



mechanical system in the leaf; / spongy mesophyll. After Schenk in rt/4-I^n fTtirr >i h\ TM 



Zittel's Handbook. SCCtlOn (fig. J,O). in 



the latter case we should 



have exactly the structure of the leaf-stalk of Cycadeae. Inside of the initial 

 group is a stronger and outside of it a weaker mass of wood, and the 

 development of both appears from the accounts given of it to advance, as 

 in Cycadeae, in both directions from the initial group. Both here, and in 

 Cycadeae and elsewhere also, Renault terms the inner portion of the primary 

 woody bundle the centripetal xylem (bois centripete), the outer the centri- 

 fugal xylem (bois centrifuge) ; he speaks of the whole bundle as the 

 diploxylous bundle (faisceau diploxyle), to distinguish it from the normal 

 bundle, the monoxylous bundle (faisceau monoxyle). In treating of the 

 Sigillarieae we shall have further to consider how far this terminology is 

 justifiable, and above all how far it answers its purpose. If, as according 

 to Renault appears to be the case in other leaves of Cordaitae, the outer 

 portion of the primary wood (the centrifugal xylem) is wanting, we have the 

 structure which we find in the vascular bundle of Isoetes. Schenk l in his 



1 Zittel (1), f. 174. 



