SOIL MOISTURE AND WILTING 123 



ration if the plants considered have been grown under different 

 evaporation conditions, and were exposed, at the time of wilting, 

 to the same rate of evaporation. Plants of Martynia and Physa- 

 lis, grown under a low rate of evaporation, indoors at Tucson, 

 Arizona, wilted immediatel}" when exposed to the high evapor- 

 rating power of the air outside; although both of these species 

 grow spontaneously during the dryest part of the year, in places 

 which are irrigated. This shows that the conditions under which 

 a plant is grown are very important in determining those under 

 which wilting occurs. 



The experiments here reported were carried out at the Desert 

 Laboratory of the Carnegie Institution of Washington, at Tucson 

 during the summer ol 1910. They deal with the relation exist- 

 ing between the evaporating power of the air and the percent of 

 water in the soil at the time of wilting, for similar plants grown 

 under similar conditions. This investigation was undertaken at 

 the suggestion of Professor B. E. Livingston, to whom the writer 

 is greatly indebted for valuable assistance during the progress 

 of the work. 



The evaporating power of the air at Tucson changes very greatly 

 during the day, so that plants wilting at different hours must do 

 so under very different rates of evaporation; but to secure still 

 greater variations four situations were selected in which the 

 evaporation conditions were quite distinct. These were: first 

 in the open, exposed to the direct rays of the sun; second, under 

 a lath shelter; third, in a room within a thick-walled adobe house; 

 fourth, in a glass chamber in the same room, the air in this cham- 

 ber being kept nearly saturated with moisture. The walls and 

 roof of the shelter were built of laths placed parallel to each 

 other, with openings equal to the width of a lath, about 3 cm. 

 Under this shelter there were less sunlight and wind than in 

 the open and consequently a lower evaporation rate. In the 

 house the evaporation rate was still lower, owing largely to the 

 still further exclusion of direct sunlight and wind. The shade 

 temperature in the open and in the room were, however, practi- 

 cally the same. The evaporation rate was, naturally, very low 

 in the nearly saturated atmosphere in the glass chamber. All 



