198 
FIFTEENTH REPORT. 
Ulmus fulva 
Celtis 
Morns 
Asimina 
Liriodendron 
Sassafras 
Platanus 
Crataegus 
Mains 
Prunus serotina 
Rhus copallina 
Rhus Vernix 
Ceanothus 
Vitis 
Acer saccharin inn 
Acer Negundo 
Co runs fiorida 
Xyssa 
< ’ephalanthus 
The usual explanation of the absence or scarcity of these plants in 
the Upper Peninsula would probably be that the climate, especially the 
minimum temperature, is simply too cold. This may be correct in some 
cases, but it would not be safe to make a sweeping assertion to that 
effect without experimental proof, for many species of trees can be 
cultivated successfully several hundred miles farther north than they 
grow naturally. And the extremely irregular boundary between the 
boreal conifer forests and the temperate hardwood forests, in New Eng- 
land for example, can hardly be explained by temperature alone. Many 
if not most of the species and genera just listed prefer rather rich 
soils, and perhaps the soils of the Upper Peninsula are simply deficient 
in some element that they need. A series of soil analyses would be a 
great help in the further study of the problem. 
