IOWA ACADEMY OF SCIENCES. 51 



moist, the southern and western slopes being drier and hence 

 more subject to fires, together with the fact that trees are more 

 common along streams where evaporation and heavy dews are 

 more abundant, have been urged with some reason in support 

 of the claim that the prairies are due to insufficient moisture. 

 During the growing season a lack of moisture means a lack of 

 food, and should prolonged winters, and long hot summers 

 alternate, the growing period becomes short, and during the 

 dry season the vitality of the tree is further diminished by 

 excessive transpiration. Deciduous trees have an advantage 

 over evergreens in the latter case because their transpiring 

 apparatus may be thrown off with the leaves during a dry 

 season. 



That dry, cold winters destroy trees has also been shown*, 

 and was amply demonstrated in Iowa last year. But all this 

 does not explain the alternation of prairie and forest in some 

 parts of eastern Iowa where the differences in humidity are 

 slight, or where greater local differences are not accompanied 

 by a corresponding variation in tree-growth, nor does it explain 

 why the bluffs on the Nebraska side of the Missouri river are 

 clothed with forests, while those on the Iowa side are mostly 

 treeless, f 



4. Temperature. — Of course no general differences in this 

 respect are noticeable in Iowa, but extremes of heat and cold 

 during different seasons, and especially rapid changes during 

 any one season, may do much injury to trees. For example, 

 in sheltered localities which have a southerly exposure trees 

 bud earlier and are often injured by frosts. This cause is 

 sufficient to prevent the cultivation of trees in many such local- 

 ities, and the same cause no doubt operated against the develop- 

 ment of native groves. 



That temperature alone is not sufficient to explain the pecu- 

 liar distribution of forest and prairie is, however, evident. 



*Thos. Meehan in Am. Nat., Vol. VII, p. 234. 1873; Aven Nelson, Bulletin 15, Wyo. 

 Exp. Sta.. 1893. 



t This is true at least south of Omaha and Council Bluffs. The Nebraska side, 

 north of Omaha, was not examined. 



