56 IOWA ACADEMY OP SCIENCES. 



larger groves in which the trees furnish mutual protec- 

 tion. In the latter cases the border trees, especially 

 on the west, are frequently stunted and generally lacking 

 in vigor; cottonwoods, however, usually being an exception. 

 In many localities in that section of the state, rows or narrow 

 bands of trees of some extent, which cross elevations and 

 depressions, well illustrate the effect of winds. On the wind- 

 ward slopes, and the tops of the ridges, the lines and bands 

 are interrupted, the trees being smaller or entirely extinct, 

 while the most vigorous trees are found in the lee of the south- 

 westers. Yet a larger number of trees planted to form a 

 grove will often thrive, even on the hilltops. It is also a well 

 known fact that individual trees, or small parts of groves, 

 which are mere remnants of larger groves upon higher or more 

 exposed grounds, soon perish after isolation by the destruction 

 of their companions. 



To summarize briefly, wind must be regarded as one of the 

 most of the important agencies which are concerned in Iowa 

 in checking tree growth, for the following reasons: 



1. Winds, especially whea violent, or frequent, or hot and 

 dry, affect trees unfavorably, both mechanically and physiolog- 

 ically. 



2. During the season of the year when the physiological 

 effect would be most keenly felt, the prevailing winds in Iowa 

 are southwesterly, and being commonly both hot and dry, they 

 are especially injurious to trees. 



3. The distribution of our native forests is in harmony with 

 the character and direction of the winds, taken in conjunction 

 with the topography and direction of the river valleys. 



4. The experieoces of those who have planted trees, espe- 

 cially in the western part of the state, testify to the power of 

 wind as a restraining factor of tree growth. 



5. Wind is one of the most general of the agencies which 

 are held to account for the development of prairies, both in 

 frequency of occurrence and in the extent of the area over 

 which it may operate, and hence, would produce substantially 

 the same effect in kind, though not necessarily in degree, over 

 large areas. 



It must not be assumed, however, that individual localities 

 may not furnish seeming contradictions, for the various 

 agencies which have been discussed will affect the problem, 

 more or less, locally. The differences between the eastern 



