206 NATUEAL HISTOEY BULLETIN 



adapted to dry regions can survive. The more or less frequent 

 recurrence of such periods which are fatal to the mesophytes of 

 the forest is sufficient to wipe out or rather prevent the develop- 

 ment of a forest flora on those surfaces which are most exposed 

 to evaporation. Forest trees are perennial and must exist 

 through all the varying conditions of succeeding seasons. Any 

 period, no matter how short, which is fatal to trees is sufficient 

 to prevent the development of a forest even though the greater 

 part of each season be favorable to tree-growth, and the failure of 

 the trees of course results in the failure of the minor forest flora 

 which in our territory is essentially mesophytic. 



Moreover it should be remembered that trees are tall and lift 

 the transpiring leaf surfaces to a considerable height. In this 

 position the leaves are not only more exposed to the direct rays 

 of the sun, but they are much more exposed to strong winds. 



The well-known fact that wind-velocity increases with height 

 above the ground which was demonstrated by Stevenson^* 

 and has been frequently verified since, increases the danger to 

 taller plants and makes more difficult the development of forest 

 trees. This increased exposure to evaporation at greater heights 

 should also be kept in mind when comparing the results of the 

 observations at Missouri Valley, for evaporation was there de- 

 termined at, or near the surface of the ground, therefore in posi- 

 tions most favorable to dry areas. 



It should not be assumed however that increased evaporation 

 necessarily means greater loss of water by the plant. Experi- 

 ments which are now being conducted in the plant physiology 

 laboratory in the State University of Iowa indicate that trans- 

 piration is stimulated and increased by somewhat greater wind 

 velocity, but that when the velocity is increased beyond a cer- 

 tain maximum, which is variable for different plants, transpira- 

 tion is diminished, the activity of the plant being evidently 

 checked by the violence of the wind. The increased loss of water 

 at the optimum velocity must not be set down as disadvantageous 

 to the plant, for it merely indicates greater vigor and activity on 

 the part of the plant. 



"Journal of the Scot. Meteorological Society, New Series, Vol. V, 1880, 

 p. 348. Also cited in Scliimper's Plant Geography (English edition), 

 1903, p. 76. 



