XL] GINKGO 3 



-Cycad. The large thin leaves with long and slender petioles are 

 scattered on long shoots or crowded on slow-growing branches 

 covered with leaf-scars. These short shoots are occasionally 

 branched^ and, as Tupper^ states, they may branch within the 

 wood of the axis out of which they grow, a feature exhibited by 

 the Triassic Conifer Woodworthia. The deciduous leaves are 

 usually more or less deeply bilobed (fig. 630, A, D, F) but those on 

 short shoots are often smaller, and the margin may be entire or 

 uneven (fig. 630, C). On young and vigorous shoots or on seed- 

 lings the lamina is often deeply divided into several cuneate 

 segments (fig. 630, E). In exceptional cases the lamina may 

 reach a breadth of 20 cm. (fig. 630, A) though as a rule it seldom 

 exceeds half that size. The leaf -scars show two small cicatrices. 

 The considerable range in size and form of the fohage-leaves is 

 an important consideration in connexion with the determination 

 of fossil specimens. Two vascular bundles pass up the petiole: 

 at the summit each divides and the two outer branches follow 

 the outer edge of the lamina, giving off a succession of forked 

 veins. Objection is taken by Prof. Johnson^ to the statement that 

 there are two marginal veins on the lower edge of the lamina ; 

 he regards the 'marginal' vein as the product of the successive 

 fusions of the forked veins of the lamina as they pass towards the 

 leaf-base. Whatever interpretation is adopted, the presence of 

 two broadly diverging marginal veins is a noteworthy feature, 

 and the correct explanation is probably that each is derived from 

 one of the two strands in the petiole and gives off a succession of 

 dichotomously branched veins as it passes along the margin of 

 the leaf-blade. The presence of short secretory tracts at intervals 

 between the veins is a characteristic feature sometimes recognis- 

 able in fossil examples. Throughout the greater part of their 

 course in the lamina the veins are accompanied by a small number 

 of reticulate transfusion-tracheids (fig. 631, G, t) : these increase 

 in amount near the distal end of each vein and the water-con- 

 ducting elements may be eventually replaced by a group of short, 

 pitted, tracheids*. A group of large cells with brown contents 

 occurs above and below each collateral endarch bundle. The 



1 Seward and Gowan (00) B. PI. ix. fig. 42. ^ Xupper (11) p. 376. 



•• Johnson (14) p. 171. ' Sprecher (07) pp. 68—71; Bertrand, C. E! (74). 



1—2 



