1903] BOG PLANT SOCIETIES 413 
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 (Merridm) 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.” 
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” 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 occurrenee. 
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 
7 MERRIAM, C. H., Results of a biological survey of Mt. Shasta, California. 
North Americana isin, no. 16. 1899. 
™” GRAY, A., Dest geography and archaeology. Amer. Journ. Sci. and Arts III. 
16:85. 1878. Bic R, J..B., The distribution of North American flora. Amer. 
Nat. 13: 155. 1879. 
** BessEy, C. E., The forests and forest trees of Nebraska. Ann. Rep, Neb. 
State Bd. of a 1888 : 93. 
