306 BURTON E. LIVINGSTON AND EDITH B. SHREVE 



Table 2 is also employed in the standardization calculations 

 when the room-temperature where the tests are made (or the 

 water-temperature) is not 20.0°, but in this case the coefficient 

 given in the table is considered as inverted, and its reciprocal 

 is employed. For example, if a given slip is tested at 19.5° 

 and is found to have a period of change of 50 sec. (between 

 the times when it matches the more intense and the less intense 

 color standard, over the evaporating surface), the number 50 

 is divided by the coefficient for 19.5° (1.03, from table 2), to 

 give 48.5, the calculated time period for 20.0°. 



To illustrate the use of table 2 in connection with leaf tests, 

 a certain slip, with a time period of 44 sec. at 20.0°, is ap- 

 plied to a leaf when the air temperature near that leaf is 30.3°, 

 the thermometer being shaded from solar insolation, as is also 

 the leaf during the test. It is found that color change occurs 

 in 3 min. or 180 sec. We now find from table 2, that the 

 coefficient for 30.3° is 0.54, and we multiply 44 by this quan- 

 tity, thus obtaining 23.76 sec. the calculated time period 

 for the shp over the standard evaporating surface at 30.3°. 

 Finally, we divide 23.76 by 180, to obtam the index of trans- 

 piring power of the leaf surface tested, which is found to be 0.13. 



Slips may be standai'dized and used repeatedly, so long as 

 they are kept clean and free from color changes caused by slight 

 charring. Of course they must not be touched with the fingers; 

 a pair of clean, dry forceps is always employed for handling 

 them. If a slip becomes wet with liquid water or with any 

 exudation from the plant, or if it is otherwise soiled, it is to be 

 discarded. 



RANGE OF APPLICABILITY OF THE SLIPS 



It is obvious that any method depending for its results upon 

 a rate of change, is applicable only with a range of conditions 

 that allow the change dealt with to go on at a measurable rate. 

 In the present connection, the transpiring power of plant sur- 

 faces cannot be determined by means of hygrometric paper 

 unless these surfaces do give ofT moisture to the dried slips with 

 considerable rapidity, and unless this movement of moisture 



