IOWA ACADEMY OP SCIENCES. 53 



quarter from which the heaviest blasts come. ' ' However, there 

 is little doubt that the physiological consequences far surpass 

 any merely physical effect. Baranetzky* established the fact 

 that shaking a plant increases the amount of transpiration for 

 a short time, but that this soon falls below the normal, the 

 diminution being due to the closing of the stomata. This 

 means that the process of respiration and assimilation are 

 checked, evidently by the temporary shock which the plant 

 has received. If this shaking should be violent and long con- 

 tinued, as in a successioa of strong winds, the plant would be 

 weakened, and in the end probably destroyed. The most exposed 

 trees would, of course, be in the greatest danger — hence those 

 upon exposed hilltops, or upon the windward side of a hill, or 

 upon open flat country. 



The winds which would produce the greatest effect are nat- 

 urally those which prevail during the spring and summer, 

 when the leaves are in full vigor. Winter winds could have 

 almost no effect of this kind, as the trees are then inactive. The 

 prevailing summer winds in practically all of Iowa are south- 

 westerly, and they increase in vigor and in frequency as we 

 go westward in the state. It, therefore, follows that trees on 

 flat areas or on southerly and westerly slopes are most exposed 

 to these winds, and that danger to trees from them increases 

 as we go westward. It is lurther true that these winds are 

 frequent during the growing period in later spring when tran- 

 spiration is greatest and most essential, and that in summer, 

 especially westward, they are commonly hot blasts which 

 weaken or destroy the guard cells, thus producing excessive 

 evaporation and leaving the plant in a weakened condition 

 with less energy for the initial growth of the following season. 

 Moreover, such winds modify the humidity and temperature of 

 the air and soil in a marked degree, and thus produce a direct 



Viewed in the light of these facts the distribution of forests in 

 Iowa becomes more intelligible. The accompanying map shows 

 that the region south and east of the Wisconsin lobe is most 

 heavily timbered, while the flat drift basin and the region west 

 of it are almost treeless. Most of the streams in the eastern 

 part of the state have a southeasterly course, and run for the 

 most part in rather deep or at least distinct valleys. The 

 effect on the physiological activity of the plant. 



• Bot. Zeltlng for 1873, p. 82. 



