160 TREE ANCESTORS 



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 remote 

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

 strong generic likeness so that they are relatively easy of deter- 

 mination. 



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

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

 vocated the probable origin of the modem stipules from basilar 

 leaf lobes. This suggestion was based on the basal lobes of several 

 early Tertiary species and the occasional occurrence of comparable 

 lobes in the modern Plaianus occidentalis . This suggestion while 

 interesting has not met with a ready acceptance. At the time that 

 Ward wrote much less was known of the paleontologic history of 

 the genus, especially its earlier manifestations, than is known today. 

 At the present time the major outlines of this history can be sketched 

 in with a good deal of certainty. 



With the dawn of the Eocene, Platanus is abundant in North 

 America, its original home. It would seem that the Eocene v/it- 

 nessed the greatest specific differentiation of the genus for no less 

 than sixteen different species have been desciibed. North America 

 is still the home of the majority of these but the genus had un- 

 doubtedly spread into Asia and it is exceedingly common in the 

 Arctic regions in the so-called Arctic Miocene which is really much 

 older than Miocene. From these far northern and now boreal 

 lands Platanus has been recorded in Siberia, Greenland, Iceland 

 and Spitzbergen. At this period it appears to have been too warm 

 in low latitudes, for Platanus is absent from the Eocene iloras of 

 southeastern North America where the remains of tropical strand 

 floras are found, and from the south European Eocene. The 

 abundant American species occur for the most part in the low 

 hilly country which marks the site of the present Rocky Moun- 

 tains. From some of these basins, which at that time enjoyed a 



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



