1 30 Journal of Agricultural Research voi. v. No. 3 



the two lower lines of the graph, is seen to be very small as compared 

 with the day transpiration. 



The second graph for wheat (July 14, 191 2) was selected to show the 

 effect of cloudiness in the afternoon, beginning at 3.30 p. m. The change 

 in the transpiration rate is seen to occur soon after this, and the tran- 

 spiration between 5 and 6 p. m. is very low compared with that on a 

 clear day, as shown by the first chart. The transpiration during the 

 night of July 14 was higher than during the night of July 2. Automatic 

 measurements with a wet-bulb instrument show that the air contained 

 less moisture during the night of July 14 than during the night of July 

 2, which would account for the increased transpiration. The tempera- 

 ture on the two days was practically the same. 



The third chart shows a record of a pot of alfalfa, taken outside the 

 inclosure. The plants were freely exposed to the wind, which ranged in 

 velocity from 7 to 14 miles per hour during the morning and from 2 to 

 5 miles during the afternoon. Over 8 liters of water were transpired 





TO SPffWG /i^OTO^ 



TO B/^LL OfPOf'Pef? 



TO f^ecoRoeR 





Fig. 17.— Wiring diagram of the electric circuits of the automatic transpiration scale. 



during the day, and it is of interest to note how closely this loss is con- 

 fined to the daylight hours. 



The transpiration recorded on the three record sheets reproduced in 

 figure 16 is plotted in rectangular coordinates in figure 18, showing for 

 each pot of plants the transpiration rate in grams per hour for each hour 

 of the day. It may be added that the pots used were equipped with 

 sealed covers, so that the loss of water by direct evaporation from the 

 soil was practically eliminated. 



SUMMARY 



This paper describes an automatic transpiration scale of 200 kgm. capac- 

 ity and 5 gm. sensibility, designed for use in connection with the large 

 culture pots employed by the writers in water-requirement measurements. 

 The apparatus is so constructed that the plants may be freely exposed to 

 wind and weather. Steel balls are used as weights, as in Anderson's 

 balance, each ball corresponding to a change in weight of 20 gm. A 

 spring motor is provided to lift the beam positively when a ball is dropped, 

 which is an essential feature when plants are exposed to wind. The 

 apparatus works very satisfactorily except in the presence of whirlwinds 

 or sudden gusts, which lift the plants and tend to give a transpiration 



