GYMNOSPERMyE. 



45 



barely attains three feet when full grown. The fruits somewhat resemble those of Podo- 

 carp. No fossil has been referred to it, though some fossil foliage bears a great resem- 

 blance to it. Ginkgo is the only genus that has been recorded from the British Eocenes. 



Ge7ius — Ginkgo. 



The genus, better known as Salisburia, possesses a single species. Its leaves are 

 adiantoid, leathery, very variably lobed, and of all sizes up to an extreme of five inches 

 across, and deciduous. The fruit, about an inch in diameter, is drupaceous, on a slender 

 foot-stalk, composed externally of a fleshy layer, and internally of a hard light-coloured 

 shell, somewhat unsymmetrical owing to the abortion of one of the seeds. The foliage, 

 though like that of the maidenhair fern, may when fossil be recognised by its stout 

 petiole, often three inches long and distinctly articulated at the base. Again, however 

 irregularly the leaf may be lobed, its primary bilobation is discernible. 



Though so singularly restricted a genus now, its ancestry is perhaps more venerable 

 than that of any other forest tree. The Carboniferous fruits, Trigonocarimm and Noegge- 

 ratJiia, are believed by both Hooker and Saporta to have belonged to some allied form. 

 Psygmophyllmi of Schimper approaches nearly to Ginkgo, and even true Ginkgo has been 

 said to be found in the Carboniferous. Baieria, beyond doubt a close ally, appears in 

 the Permian, and the bilobate Jeanpaulia of the Rhsetic of Bayreuth is probably a true 

 Ginkgo ; but an even more definite species has been found in the same system in 

 Australia. The group, however, did not reach its maximum development until the 

 Jurassic period. Heer's 'Jurassic Flora of Eastern Siberia'^ contains a most important 

 contribution to their past history. Five genera are placed in the group : — Ginkgo, 

 Baieria, TricJiopitgs, Phoenicojjsis, and CzekanowsJcia, the two latter of which seem, 

 however, to have little affinity with Ginkgo. The most aberrant form, obviously 

 belonging to the group, is Trichopitys of Saporta, in which the parenchyma is reduced to 

 a narrow expansion bordering the veins, yet with the characteristic bilobation and petiole 

 preserved. Its affinity is best traced through G. concinna, which is similar, but the 

 segments of the leaves are more expanded and receive two or three veins each. 



The Arctic Ginkgos have been subdivided by Heer into too many species : the 

 leaves of the existing species are so excessively variable under diff'erent conditions of 

 growth that their varieties would more than embrace all the supposed Jurassic species. 

 The second genus, Baieria, possesses a larger and more palm-like leaf averaging nearly five 

 inches in radius, its bilobation and venation connecting it closely with Ginkgo. The 

 persistence, throughout the whole group, of characters which would hardly have been 

 suspected to possess a morphologic value is a peculiarly interesting fact. 



^ ' Flor. Foss. Arctica,' vol. iv. 



