THE AMERICAN BOTANIST 35 



States at that time one of the prominent features of the vege- 

 tation was the abundance of pahns represented by the impre.s- 

 sions of leaves and much petrified wood. Among the pahns 

 are Thriuax, Bactrites and Palmetto, and a well-marked date- 

 palm is represented by characteristic seeds. Nutmegs are also 

 represented by fruits, as are the copaiba gum, the sapodilla. 

 and the carapa. The leaves preserved include mangrove, satin- 

 wood, citrus, stopper, button-wood, canna, sea grape, climbing 

 ferns {Lygodiiiui), tlie tropical marsh fern (Acrustic/niiii ) , 

 the cinnamon fern, and many other tropical types. There were 

 at least four separate times w4ien ice-sheets accumulated over 

 the land. Each of these lasted from 10,000 to 20,000 years, 

 and they were separated by long intervals of genial climate 

 known as the interglacial periods of thousands of years dura- 

 tion during which the floras spread northw'ard to even beyond 

 their present range. During the epochs of glaciation these 

 temperate forests retired southw^ard and gave way along the 

 ice front to arctic willows and dwarf birches. The post- gla- 

 cial amelioration of the climate, the opening to occupation of 

 areas that had been covered with glaciers and the mixing of 

 soil through ice action all combined to stimulate evolutionary 

 activity in the plant kingdom, particularly among herbaceous 

 forms. It seems probable that the characteristic Temperate 

 Zone herbaceous families date from about this period. 



