Dec, 1909.] Evaporation in a Bog Habitat. 19 



Livingston's experiments (3) with the atmometer at Tucson 

 have shown that the evaporating power of the air, aside from its 

 indirect effect upon soil moisture, is an important factor in plant 

 development. Several species of plants were grown in soil which 

 was kept as nearly as possible at its optimum moisture content, 

 and their behavior in relation to the rate of evaporation was 

 studied. Two species which were able to transmit water to the 

 leaves faster than it was lost b}^ transpiration, grew vigorouslv, 

 even during a period of drought. Several other varieties were 

 unable to provide the excess water for growth during the period 

 of drought, but remained quiescent, and resumed their growth 

 upon the return of the season of lower evaporation. Other 

 plants not only failed to provide the excess water for growth 

 during the drought, but did not respond even on the coming of 

 the season of lower evaporation and soon died. It is concluded, 

 from these experiments, that the evaporating power of the air 

 controls desert vegetation to a great extent, for it inhibits the 

 growth of plants which are not able to adjust themselves to the 

 low evaporation rate, and thus plays an important part in the 

 determination of centers of plant distribtition. 



Further work (4, 0,) has brought out the value of the atmom- 

 eter in the differentiation of habitats. While the amount of 

 rainfall, through its eft'ect upon soil moisture, is effective upon 

 vegetation over large areas, the evaporating power of the air 

 may vary greatly within these areas, and within neighboring 

 habitats. Data taken in the Missouri Botanical Garden showed 

 that the average ratio of the evaporating power of the air in the 

 open field, and in the shade of a coppice was approximately as 

 2.5 to 1. About the same ratio was apparent in an open straw- 

 berry patch, and beneath a shade tent. 



Atmometer readings taken at Tucson, and at different alti- 

 tudes in the Santa Catalina Mountains indicate a gradual decrease 

 in the rate of evaporation with altitude. Considering as unity 

 the standard Tucson atmometer at 2412 feet, the relative loss of 

 the instruments at 6000, 7500, and 8000 feet, was .8, .5 and .4 

 respectively. These conclusions with reference to the decreasing 

 rate of evaporation at higher altitudes have been corroborated by 

 similar experiments conducted by Shaw in the Selkirks (6). 



Transeau (7) continued the study of the relation of plant 

 societies to evaporation. He placed instruments in different 

 plant habitats about Cold Springs Harbor, Long Island, com- 

 paring all readings with that of a standard instrument in the 

 Carnegie Garden. He reported an evaporation of 100% on an 

 open gravel slide, and showed that the partial invasion of the 

 slide by vegetation produced a decrease of 40% in evaporation. 

 The rate in a forest habitat varied from 50% in the open wood to 

 10% in the swamp forest. In the light of these data, it is easy to 

 see why plants, accustomed to the swarap environment, cannot 

 succeed in an open woods with a rate of evaporation five times as. 



