212 NATUEAL HISTOEY BULLETIN 



temperature and topography. McAfee (1876), Fernow (1892), 

 and Baker (1908) placed emphasis upon dry winds and Geikie 

 (1898) referred to the influence of wind-action in general terms. 

 Todd (1878 and 1880) called attention to the exposure of tree- 

 less areas to prevailing winds and considered this factor espe- 

 cially in its relation to the relative humidity of air and soil. The 

 writer (1900) also placed especial emphasis upon this factor in 

 its relation to the absence of trees on exposed surfaces. "Whit- 

 ney (1876) however did not consider this factor important, but 

 declared (p. 582) that "if the force of the wind were essentially 

 inimical to the growth of trees we should find them thriving, 

 if anywhere, in the sheltered nooks, and to the leeward of the 

 northwesters, that being the quarter from which the heaviest 

 blasts come. ' ' He evidently considered only the mechanical force 

 of winds and did not view the question of wind-influence in its 

 broader aspects. But even the mechanical force of the wind 

 should cause greater damage during the summer when the 

 broader foliage-surfaces are exposed than in winter when most 

 of our trees are stripped of their leaves and hence expose neither 

 their greatest surfaces nor their most tender tissues. 



Evaporation 



Evaporation has also received some attention in connection 

 with the prairie problem. Engelmann (1862) thought that it 

 chilled the soil and thus prevented the development of a forest 

 flora. More frequently it has been discussed in its relation to 

 the humidity of the air, as by Anders (1882), Whitney (1882), 

 and others. Todd (1878) considered the variable humidity of 

 air and soil of greatest importance, and the writer (1911) has 

 recently discussed the influence of unequal evaporation from 

 exposed and sheltered surfaces in its relation to prairie and 

 forest. 



When we consider the influence which evaporation exerts 

 upon the transpiration of plants we can scarcely over-estimate 

 its importance as a factor in our problem. Around it we must 

 group the great climatic factors: rainfall which determines the 

 amount of available moisture, and temperature and wind, which 

 cause or facilitate evaporation together with such local or acces- 



