DEPARTMENT OF BOTANICAL RESEARCH. 73 



index of transpiring power rests in the assumption that the slips are 

 at the temperature of the surrounding air. The type of thermo-couple 

 described in another section (page 80) has been adapted to test the 

 temperature of the cobalt paper as it rests upon the leaf under its 

 glass cover. Tests have been made on widely different types of plants, 

 growing both in the greenhouse and in the open. In all cases the 

 temperatures were read every 10 seconds until the cobalt test was 

 completed. While the temperature of the leaf-surface itself frequently 

 differs several degrees from that of the surrounding air, the tempera- 

 ture of the cobalt slip covered by glass does not reach so great a 

 difference during the time required for a cobalt test. The temperature 

 of the slip departs from the air temperature by different amounts, 

 according to the time of day, the season, the amount of cloudiness 

 present, the conditions of the plants, etc., the departure being from zero 

 to 1.5 degrees. The last amount is for temperatures above 30° C. only. 

 Follo"v\ang is a list of plants for which the indices of transpiring 

 power have been calculated,* using first, the temperatures as read 

 from a shaded mercury thermometer standing in the foliage of the 

 plant, and second, the temperature obtained by making a correction 

 embodying the maximum difference from air temperature obtained 

 for that plant for that time of day: 



Encelia farinosa. Phaseolus sp. Amaranthiis palmeri. 



Prosopis velutina. Rumex hymenosepalus. Bahia absinthifolia. 



Fouquieria splendens. Nasturtium sp. Strepthanthus arizonicus. 



Lantana sp. Ricinus officinale. Malva borealis. 



Poinsettia sp. Tradescantia sp. 



In no case does the second result differ from the first more than 8 

 per cent, and in the majority of cases it is under 2 per cent. The maxi- 

 miun difference between the indices of 10 different leaves of the same 

 apparent age on the same plant is frequently over 20 per cent and 

 never under 8 per cent. Consequently this work has established the 

 fact that the temperature of the air may be safely used for the tem- 

 perature of the cobalt shp. 



It is of course just as important to know whether or not the air 

 temperature may be safely used in the calculations for the standard- 

 ization of the slips over a porous evaporating surface. With a slight 

 change in the apparatus the temperatures of the slips as they rested 

 over the standard evaporating surface were taken, the tests being 

 made at 1° intervals between 20° and 30° C. The results show defi- 

 nitely that if the standardization is made within 2° or 3° of 20° C. 

 the error introduced is smaller than that obtained from consecutive 

 readings on the same slip at any given temperature. The above 

 statement does not hold for temperatures above 24° C. All the 



* For method, see Livingston, B. E., and E. B. Shreve, Improvements in the Method for 

 Determining the Transpiring Power of Plant Surfaces by Hygrometric Paper. The Plant World, 

 vol. 19, No. 10. 



