AND TRANSPIR-'VTION OF TREES 109 



The three series of 48 bottles, labelled and tightly corked, were 

 marked in ink on both label and cork: It, Ig, 2t, 2g, etc., and placed in 

 order in a shallow box of convenient size. The number in each case 

 stood for the reading of the experiment, and the letter indicated the 

 species of leaf, whether tremuloides or grandidentata. This system of 

 labelling not only indicated the species of leaf, but proved itself most 

 valuable in associating any cork always with its proper bottle, a device 

 to reduce the number of weighings. Each bottle with cork was then 

 weighed accurately, registered according to label, and arranged in series 

 according to number. Three such series were kept in readiness, each 

 designated by a special mark (check, comma, cross) to avoid any pos- 

 sible cork or bottle confusion. 



When the experiment was started, the first reading was taken by 

 removing three to six leaves (depending upon size) from Populus tre- 

 muloides, rolling them quickly, and corking them tightly in the bottle 

 marked It. Immediately the psychrometer was placed vertically with 

 its bulbs in the position formerly held by the leaves pulled off. The wet 

 and dry readings were tabulated. The photometer was placed in a 

 similar position and exposed to the light. The number of seconds re- 

 quired for suitable coloration was recorded together with the number 

 of the disk exposed. A similar photometric reading was taken immedi- 

 ately in the open, and the whole process repeated for Populus grandi- 

 dentata. 



After the readings were completed, the corked bottles with their 

 contained leaf rolls were weighed. The object of the tight cork was to 

 prevent the escape of evaporated water before weighing. All results 

 were tabulated and the corks were then removed and arranged in series 

 for future convenience. The 48 corkless bottles with their leafy con- 

 tents were placed in shallow boxes and put in the gas oven. In order 

 to prevent carbonization of the leaf tissues, the oven was regulated so 

 as never to exceed 100° C. They were kept in the oven until there 

 was no loss in weight, and this required on the average about 24 hours. 

 Upon removal from the oven, the bottles were again fitted with their 

 respective corks, and another weighing was taken, this time of bottle, 

 cork, and dried leaves within. 



The following data were thus obtained: 



Wl = weight of labelled bottle+ labelled cork 



W2 = weight of labelled bottle-j- labelled cork -f fresh leaves 



W3 = weight of labelled bottle-f labelled cork-f dried leaves 



