igig] CRIBBS—TILIA AMERICANA 267 



were obtained the tricolor paper was discarded as being more 

 difficult to prepare and less easily handled in the field. The plain 

 paper has the additional advantage of being somewhat slower, 

 and is thus more satisfactory when used at stations on the open 

 sands, for the change in color was found to take place so quickly 

 that even the paper with longer time values was frequently difficult 

 to read. The probability of error arising from short time periods 

 was largely ehminated by increasing the number of readings from 

 the usual 5 to about 8 or 10, and the time recorded in each instance 

 was the average of all readings made. 



The paper was applied to the leaf surface by means of the clip 

 devised and described by Livingston (8) . At each of the stations 

 leaves were chosen which were about i m. above the ground; 

 readings were taken from the same relative position on the dif- 

 ferent leaves; and so far as possible the same set of leaves was 

 employed in each subsequent day's work. Records were taken 

 from 2 leaves at each station, usually at hourly intervals, begin- 

 ning as soon as there was sufficient light to observe the color 

 change and continuing until darkness prevented further reading. 

 Records given here were taken from the abaxial (stomatal) side 

 only. 



The indirect method of determining the color change over the 

 standard evaporating surface, as suggested by Bakke (i), was 

 used during the second summer's work, thus making it unnecessary 

 to take the standard cup of Livingston and Shreve into the 

 field. 



Various devices were tried for heating the hygrometric paper 

 to force off the water of crystalhzation, for considerable difficulty 

 was experienced because of the prevalence of stong lake winds, 

 so a special lamp was devised and used for this purpose. The 

 chief difficulty with the acetylene lamp was to get a steady flame 

 for a long period of time. That which was finally employed con- 

 sisted of a railroad lamp, with a round wick, which had an oil 

 capacity of about o . 5 Hter. A tubular piece of tin was fashioned 

 so that it had a transverse dimension of about 8 cm. and a length 

 of 23 cm. This was fitted to the base by means of 3 guides so as 

 to leave a space of about 2 mm. below for ventilation. Into the 



