ECOLOGICAL PAPERS. 119 



EVAPORATION AND PLANT SUCCESSION ON THE 

 SAND DUNES OF LAKE ]\IICHIGAN. 



By George D. Fuller, 



University of Chicago. 



The porous cup atmometer as used today was devised by Dr. 

 B. E. Livingston in 1906. It consists of a hollow cup of porous 

 clay 12.5 cm. high, with an internal diameter of 2.5 cm. and a 

 thickness of wall of about 3 mm. It is filled with pure water and 

 connected by means of glass tubing to a reservoir usually consist- 

 ing of a wide mouthed glass bottle of one-half liter capacity. The 

 water, passing through the porous walls, evaporates from the sur- 

 face, the loss being constantly replaced from the supply within 

 the reservoir. Readings are made by refilling the reservoir from 

 a graduated burette to a certain mark scratched upon its nec2. 

 For convenience in handling a portion of the base of the cup is 

 coated with some impervious substance and before being used in 

 the field the instrument is standardized by comparing its loss of 

 water with that from a free water surface of -45 sq. cm., exposed 

 under uniform conditions. As a further check against error this 

 standardization is repeated at intervals of six to eight weeks 

 throughout the season. 



The instrument thus briefly described is designed to be used 

 by ecologists in measuring the evaporating power of the air in 

 plant habitats. This power varies with changes in temperature, 

 humidity, and rate of motion of the atmosphere, and with the 

 intensity of the illumination. The readings of the atmometer, there- 

 fore, express a summation of the various atmospheric factors 

 which combine in making demands upon the water contained in 

 the aerial portion of plants. By careful experiments it has been 

 found that there is a close relationship between transpiration and 

 this evaporating power of the air. The atmometer, therefore, gives 

 a convenient and accurate means for the quantitative determina- 

 tion of those atmospheric factors which affect the water supply 

 of plants, or in other words, it affords a means of exactly meas- 

 uring the comparative xerophytism of plant habitats in so far 

 as it is determined by atmospheric conditions. The importance 

 of such measurements may be imagined when it is recalled that 

 ecologists are agreed that water is by far the most important 

 factor in determining the character and extent of the various 

 plant associations. 



