EVAPORATION AND CAUSAL FACTORS. 59 



the temperature of the air, the upper centimeter of soil, and at a 

 depth of 1 dm. during the course of series 34 are shown in figure 31. 



Since the temperature of the soil differs so much at various depths, 

 the effect upon shallow-rooted plants can not fully be determined, 

 but must play some part in the functioning of the roots. The tem- 

 perature of the superficial layers also has an effect upon the carpet- 

 weeds, like Portulaca oleracea and Amaranthus blitoides, as these 

 plants are subject to higher working temperatures than the more erect 

 plants. Deep-rooted plants, such as alfalfa, have their roots in soil 

 of uniform temperature and hence only seasonal changes affect them. 



To determine the reaction of the stomata to changes in soil- 

 temperature, a number of alfalfa plants were exposed to sunlight in 

 metal containers, containing the same soil with the same water- 

 content. Some of these were blackened to absorb more heat, some 

 left bright, and others were placed in moist earth for protection. A 

 chemical thermometer was placed in the soil of each container. The 

 results were indefinite, as sudden rises in temperature caused an 

 effect at lower temperatures than slow rises did. Low water-content 

 increased the effect or caused closure at lower temperatures, and 

 other factors which could only be guessed at entered into the experi- 

 ments. Closure was caused by temperatures ranging from 21 to 

 30 C. at different times. It was found, however, that a rise of tem- 

 perature in the soil would first cause stomatal closure and then wilt- 

 ing, even when the humidity was very high. 



The temperature of the leaves of a plant, while rarely the same, 

 usually differs but slightly from that of the air. This difference is the 

 result of equilibrium between the cooling effect of transpiration and 

 that of radiant energy. A number of readings of leaf-temperature 

 were made upon alfalfa, potato, and sugar-beet with a chemical ther- 

 mometer graduated to tenths of 1C. The temperature of the air 

 was read, then that of a leaf wrapped about the bulb, and the air- 

 temperature again. An unwilted alfalfa leaf did not vary more than 

 0.2C. above air- temperature, but a wilted leaf was at one time found 

 to be 1.9C. above the temperature of the air. Usually the leaves 

 with open stomata were found to be lower than the air, and those 

 with closed stomata during the day to be higher. Compared with 

 the results obtained by E. B. Shreve (1919A) with far more accurate 

 methods, these readings are too low, but are indicative of the effect 

 of stomatal movement upon leaf-temperature. 



EVAPORATION AND CAUSAL FACTORS. 



Evaporation is a product of several factors, the most important 

 of which are absolute humidity, air-temperature, barometric pres- 

 sure, wind, and sunlight. It is usually measured as the water-loss 

 from a free water-surface or from some type of evaporimeter, the 

 most important of which is the Livingston porous-cup atmometer. 



