— 146 — 



instance by closure of the stomata can be avoided by making the 

 experimentation time short. Ordinarily I used only 15 minutes. 

 The jars were weighed twice, after 7 and after 15 minutes from the 

 beginning of the experiment the two estimations checking each 

 other. Also the width of the openings of the stomata was investigated 

 before and after the experiment (cf. below.). With so short experi- 

 mentation times it seems ordinarily not to be necessary to cut the 

 branches off below water, the width of the stomata being the same 

 before and after the experiment. On the other hand this time is 

 sufficient to make the estimations sufficient accurate. The weight 

 of the jars could be estimated with an accuracy of zb 0.01 g., and 

 the loss of weight during the 15 minutes was at least 0.25 g. The 

 weighings were made in a small wooden shed, build for that purpose. 

 During the experimentation time the plants were placed in the 

 open air. 



The leal area was determined in the following way. 1 — 2 g. 

 of leaves were weighed and drawn on paper, whose weight was 

 determined pr. 1 cm 2 . By weighing the cuttings the total area 

 (upper + lower surface) of 1 g of leaves was determined and from 

 the weight of the whole lot of leaves from the experimentation 

 plant the leaf area was calculated. The determination of the leaf 

 area of 1 g. of leaves was carried out at different times of the year, 

 but not in every single experiment. The means of the leaf areas 

 (upper + lower surface) pr. 1 g. were the following:. 



Vacciniam Vitis idaea 64.6 cm 2 



Vaccinium uliginosum 114 „ 



Betula 119 „ 



In Empetrum the determination of the leaf area was more 

 difficult. The area of a single leaf was estimated by measuring the 

 length and the breadth of the leaf this being considered as a flat, 

 rectangular body. Hence the area of 1 g of leaves was determined 

 by counting the leaves and multiplying the number with the area 

 of a single leaf. The mean area of 1 g of leaves was 80 cm 2 . 



The temperature and the moisture of the air were measured 

 with an August's Psychrometer. 



Since the surrounding factors, as pointed out of Livingston 

 (1906), influence the evaporation from a surface of water almost in 

 the same manner as the transpiration from leaves*) the evapor- 



*) According to Briggs and Shantz (Journ. of agric. research 9, p. 277, 

 1917) a close correspondence between the transpiration of Medicago saliva 

 and the evaporation seems not to exist. 



