1903] BOG PLANT SOCIETIES 4*3 



favorable for a wide-spreading zone of boreal societies. In 

 Pennsylvania the high relief of the Appalachians and consequent 

 low temperature also afforded exceptional opportunities for the 

 spread of these societies far to the south. Here too the cold 

 water of the glacial drainage pouring down the numerous tribu- 

 taries of the Allegheny, Susquehanna, and Delaware rivers may 

 have had a marked influence by lowering the temperatures of 

 the narrow valleys, just as the streams which flow from Mount 

 Katahdin and the glaciers of Mount Hood (Cowles) and Mount 

 Shasta (Merriam) 19 affect the temperature of their adjacent 

 valleys today. The presence of many such northern forms as 

 the white pine, spruce, and hemlock in areas of the southern 

 Appalachians has long been attributed to the glacial period. 20 

 In the Ohio valley the streams flowing from the south would aid 

 in maintaining equable temperatures and preserving the broad- 

 leaved mesophytes as far north as the Ohio River. Beyond the 

 Mississippi the conditions must have resembled those now 

 prevalent in the Saskatchewan basin. Bessey 21 reports the 

 occurrence in Nebraska of deposits of "well defined branches, 

 twigs and occasionally tree trunks" at depths varying from 

 twenty to fifty feet below the surface, and concludes that in 

 recent geological times there must have been extensive conifer 

 forests throughout the state. The present distribution of trees 

 in Nebraska shows outliers of the western yellow pine (Pinus 

 ponderosa scopulorum) in the central part of the state far removed 

 from the main area of its occurrence. 



Now as to the bog plants : since under favorable conditions 

 they may occupy other habitats than undrained depressions, 

 they probably existed on the borders of the heavily loaded 

 streams, in ravines and moist situations generally along the 

 whole ice front. It is to be noted that practically all of the 



J 9 MERRIAM, C. H., Results of a biological survey of Mt. Shasta, California. 

 North Americana Fauna no. 16. 1899. 



20 GRAY, A., Forest geography and archaeology. Amer. Journ. Sci. and Arts III. 

 16:85. 1878. HOOKER, J. B., The distribution of North American flora. Amer. 

 Nat. 13 : 155. 1879. 



21 BESSEY, C. E., The forests and forest trees of Nebraska. Ann. Rep. Neb. 

 State Bd. of Agri. 1888 : 93. 



