THE GEOLOGIC HISTORY OF PLATANUS 3 



worthless, but enough remain which are based upon an abundance 

 of absohitely characteristic leaves, in some cases accompanied by 

 typical fruits, to render it certain that in Middle Cretaceous 

 times, ancestral plane trees were an abundant element in the flora 

 of North America, and that later in the Cretaceous they had 

 spread to South America and the Arctic region. They may have 

 continued across the latter region into Europe although the 

 records are not entirely convincing as regard the Cretaceous but 

 are more ample in support of such a migration in the Tertiary. 



These early ancestors had somewhat elongated rhomboidal 

 leaves, with irregularly and remotely toothed margins, decurrent 

 on the petiole, which was conspicuously enlarged at the base. 

 There was a tendency, not especially pronounced, toward palmate 

 trilobation. The floral axes were already shortened and aggre- 

 gated and the fruiting heads were racemose as is indicated by the 

 predominancy of this habit in the existing species and its frequent 

 occurrence in forms like Platanus occidentalis that normally have 

 but one fruiting head to a peduncle. These ancestral leaf char- 

 acters are deduced from the form of the earliest species and from 

 the substantial agreement between them and the leaves of modern 

 seedlings and adventitive shoots from old stumps both of which 

 are supposed to exhibit more or less reversionary characters. 

 Several of these are figured in the present connection. 



Reacting to the genial influences of the Cretaceous climate 

 these early forms soon broadened their leaves, which also became 

 lobate, so that Platanus Kmnmeli Berry of the Magothy Forma- 

 tion is scarcely distinguishable from the leaves of the existing 

 species, especially Platanus orientalis. Its leaves are exceedingly 

 abundant and the clays in places are packed with the remains of 

 its fruits — true midcretaceous " buttonballs. " From that re- 

 mote age to the present time Platanus leaves have all shown a 

 very strong generic likeness so that they are relatively easy of 

 determination. 



A quarter of a century ago Professor Ward wrote a paper on 

 the paleontologic history of the genus Platanus^ in which he ad- 



iWard, Proc. U. S. Natl. Mus., 11: 1888, pp. 39-42, pis. 17-22. 



