J(JHNS0N — G-inkgophyllum Jciltorkense. 171 



is roughly that of the inverted acuminate apex of a foliage-leaf. In neither 

 case is the forked venation so clearly observable and described as could be 

 wished. 



Seward (7) describes a portion of a leaf from the Permo-Oarboniferous rocks 

 of Kashmir under the name of Psi/cjinophyllimi Rollandi, which is highly 

 suggestive of a form intermediate between GiitkgophyUum liiUorhense and 

 a more recent Ginkgo. Its venation is unfortunately not fully preserved. 



Avchaeopteris archaetypus^chiaalh. possesses Psygmophylluni-like pinnules, 

 the largest being 6 cm. long and of the same width. 



Nathorst states that the lateral veins in his Psygmopliyllam appear to arise 

 by repeated dichotomy or forking of the veins at the base of tlie leaf and 

 not, as he saj's is the case in Ginkgo biloba, from two marginal bundles. 

 This distinction as applied to Ginkgo, if found reliable, would constitute an 

 important feature of distinction. It is one ascribed to Ginkgo by Renault, 

 and has been made use of in the identification of fossil leaves. It is, I think, 

 not maintainable. It is an error of description rather than of observation, 

 due to the way of considering the venation. The leaf-scar of Ginkgo, 

 horizontally oval, shows clearly the two soars or cicatrices indicative of the 

 broken ends of the double leaf-trace. The two bundles running through 

 the long leaf-stalk receive all the vascular bundles of their own halves of the 

 actually or potentially bilobed fan-shaped leaf-blade. The leaf-blade is 

 supplied throughout with dichotomously divided veins. The "marginal" 

 vein is really the vein formed by the fusions, at intervals along the lateral 

 edge of the lamina, of the forked veins, as they traverse the lamina from its 

 expanded apex towards the leaf-base. At the point of constriction of the 

 lamina to form the petiole, the vein on either side having received the fusions 

 of the forking- veins of its half of the lamina, passes with its fellow as two 

 main veins through the petiole, each being independent of the other, and 

 belonging to its own half of the leaf, as the double leaf-trace, into the stem. 

 There is dichotomous venation throughout the lamina of Ginkgo ; and a 

 " marginal " vein, from which forking veins are given off on its inner side 

 towards the body of the lamina, does not exist. 



The true origin of the so-called marginal vein, by the fusion of the 



forking veins in the body of the lamina is well brought out in the figure of 



the leaf-venation of G. biloba in the "Morphology of Gymnosperms," by 



Coulter and Chamberlain (8), (fig. 212, p. 187). Unfortunately this figure 



errs in one important respect. The veins of the lamina are shown fusing 



together to form a single vein in the petiole, though the " double leaf-trace " 



is recognized in the text as a Ginkgo character. 



The Ginkgo type of leaf is unique among seed-bearing plants of to-day, 



2o2 



