196 BOTANICAL GAZETTE ' [September 



that described by Nichols (4), and a final correction made on their 

 being collected in the autumn. By the coefficients thus obtained, all 

 readings were reduced to the standard adopted by Livingston (5), 

 the directions he gives being so closely followed that it is unneces- 

 sary to detail further the methods used in operating the instruments. 

 Three or more stations were always maintained in each association, 

 the mean of the various readings being taken as giving the true 

 measure of the evaporating power of the air for that association. 



In most instances no correction has been made for errors caused 

 by rainfall, although during showers some water undoubtedly 

 passes into the porous cup and into the reservoir, because it was 

 thought that the amount of variation thus produced would be the 

 same for all stations within so limited an area, and hence the com- 

 parative relation of results would remain unchanged. This assump- 

 tion has been largely verified by Brown (6), using an atmometer 

 with a rain-correcting valve, and also by the experience of the 

 writer by maintaining control instruments with the valves. The 

 record for the prairie association for 191 1 was wholly, and that for 

 the oak-hickory forest for 19 12 was partially, made with rain- 



5 



atmometers 



comparisons 



and to exhibit the progress of the evaporation rate during the 

 entire season, the average water-loss per day between the weekly 



r 



xpressed 



with ordinates representing the number of cubic centimeters lost 

 per day by a standard atmometer, the abscissas being the intervals 

 between the weekly readings. The readings included within each 

 calendar month are indicated at the top of the diagram. For 



numbered 



nninir 



that is 



with those of the cottonwood dune. 



PLANT ASSOCIATIONS AND STATIONS 



The stations established in the plant associations upon the 

 sand dunes were all upon an area of dunes lying between the little 



VI 



The shore at this point is gradually encroaching upon the lake 



