20 The Botanical Survey of Nebraska 



phenomena in the first two inches of soil in the two habitats. The 

 high temperature of the soil and its fluctuations coupled with a 

 relatively high saturation deficit of the air, and these two condi- 

 tions frequently coincident with a deficiency of soil moisture in 

 the first 2 inches clearly present a difficult situation for the van- 

 guard of invaders to solve. A frequently fluctuating soil tem- 

 perature coupled with occasional, dangerously high extremes are 

 probably of considerable significance in determining the direction 

 of succession as has been indicated by Weaver's investigations in 

 Washington.^ 



EVAPORATION 



Studies of the evaporating power of the air at the various sta- 

 tions were also continued in connection with the quantitative 

 determination of the other environmental factors. It has already 

 been conclusively shown by Weaver and others that evaporation 

 plays a very important role in the determination of the course of 

 succession and in the composition of plant communities. The 

 data herewith presented still further support and corroborate the 

 conclusions drawn from earlier investigations in this regard. 



The data on evaporation were secured by the now well-known 

 Livingston porous-cup atmometer method. Standardized cylin- 

 drical cups were used and these were fitted to containers both 

 with and without the greatly improved rain-correcting type of 

 apparatus. Thus the cups were run in duplicate, but everything 

 surrounding the work pointed conclusively to the greater re- 

 liability of the data secured from the rain-correcting instruments 

 and so these are the only data considered in this discussion. The 

 authors feel that data secured by means of the older type of ap- 

 paratus without the rain-correcting features are at best poor ap- 

 proxim.ations of the actual evaporating power of the air in the 

 habitats in question. The rain-correcting instrument should cer- 

 tainly be used in all investigative work of this sort. All other 

 apparatus for the study of evaporation based upon the principle 

 of the porous-cup should be abandoned for investigative work. 



3 "A Study of the Vegetation of Southeastern Washington and Adjacent 

 Idaho," J. E. Weaver, Univ. Nebr. Studies, XVIL, No. i, 1917. 



26 



