iQ2o] FREEMAN— EVAPORATION AND TRANSPIRATION 205 



We may infer from this that the guard cells of the stomata become 

 more turgid, making these openings larger, and thus increase the 

 evaporation coefficient of the surfaces. 



The use of this formula in interpreting the results of compara- 

 tive tests of the transpiration of different varieties or species of 

 plants may be illustrated by two examples. The first may be 

 taken from seven experiments made upon a peach tree (Prunus 

 per ska) and a large greasewood shrub (Covillea mexicana), which 

 were growing in close proximity upon well cultivated and watered 

 soil on the grounds of the University of Arizona. The data of 

 this experiment are given in table V, which includes the tempera- 

 ture within the cylinder, the dewpoint of the external air, the dew- 

 point of the air coming out of the cylinders, the relative humidity 

 of the same, and the number of square centimeters of leaves used. 

 There is then placed in the last column a calculated "standard dew- 

 point, " which is found by means of the formula, and is the dewpoint 

 of the outgoing air which would have been given by a physical sur- 

 face of the same evaporating potentiality as the given leaves (at 

 the time when the experiment was made), but having an area 

 of 100 sq. cm., a cylinder (air) temperature of 30 C, and an 

 external dewpoint of io° C. The wind movement in all cases 

 was the same, being approximately 85 liters per hour. It will 

 be noted in column e that the dewpoint of the escaping air 

 from the greasewood cylinder was higher three times and lower 

 four times than the dewpoint of the air escaping from the peach 

 cylinder. When, however, each is calculated to the standard 

 dewpoint, that of the air from the greasewood cylinder was always 

 higher. It is surprising to find that the leaves of the greasewood 

 (a strictly xerophytic plant) show such a uniformly higher tran- 

 spiration rate than those of the peach (a deciduous mesophyte). 

 It is interesting to note, therefore, that, whereas 100 sq. cm. of the 

 greasewood leaves weighed 3.93 gm., a similar surface of peach 

 leaves weighed only 1.92 gm. When the calculations were based 

 upon equal weights of herbage, the peach showed an equally 

 uniform greater transpiration per 100 gm. green weight foliage. 

 The greasewood experimented upon was growing in the orchard 

 where it received an abundance of irrigating water. Its leaves 



