252 



importance from the point of view of the geological history of 

 the genus. The broad, wedge-shaped or fan-shaped lamina, with 

 its numerous spreading and dichotomously branched veins, at 

 once suggests a comparison with such recent ferns as Adiantum 

 reniforme, L., Trichomanes reniforme, Forst., Scohpendrium nigripes, 

 Hook., Linisaya reniformis, Dry., and others. Por many years the 

 fossil leaves of Ginkgo were regarded by Brongniart and other 

 authors as species of ferns, and usually included in the genus 

 Cyclopteris. 



It was Heer who first drew attention to the probable generic 

 identity of certain Mesozoic leaves fi'om the Arctic regions and 

 Northern European localities with the fern - like leaves of the 

 maidenhair-tree.' The recognition of these fossil leaves as species 

 of Ginkgo rather than as ferns was based on the form of the petiole, 

 which, Heer pointed out, agrees exactly with that of Ginkgo hiloha, 

 but more especially on the discovery of male flowers practically 

 identical with those of the recent species, and of seeds and short 

 foliage shoots very similar to those of the maidenhair - tree. 

 There are but few recent plants possessing leaves which might 

 be mistaken for those of Ginkgo ; a few ferns, such as those 

 already mentioned, have leaves in form and venation very similar 

 to the maidenhair-tree ; but the form of the petiole and the 

 frequent association of the reproductive structures with fossil 

 leaves, afford confirmatory evidence which enables us to speak with 

 certainty as to many of the fossil forms. Among the Angiosperms 

 Ginkgo-Yik.B leaves are very rare, and in such a plant as Hakea 

 Baxter i, R. Br., the reticulate venation precludes confusion with 

 the fem-like leaf of Ginkgo. 



From evidence afforded by fossil leaves and flowers it has been 

 possible to draw up a history of the Ginkgoaoese, which demon- 

 strates the extreme antiquity and wide geographical distribution of 

 Ginkgo and other genera. It is true that some of the leaves 

 referred to Ginkgo may be ferns, and on the other hand certain 

 so-called fern species might equally well be referred to the 

 Ginkgoacese. 



In 1881 Heer' published an interesting paper on the history of 

 Ginkgo-VLke trees, in which he summarized the available data. 



1 Heer (81=). 



