On a Method of Studying Transpiration. 



273 



bulb thermometers were in the upper part of the jar, while the branch 

 had leaves in both lower and upper regions. I did not find this to be a 

 serious source of error, and it is one which might be avoided by fitting 

 an apparatus by which the air in the bell jar could be stirred and thoroughly 

 mixed, as indeed was done in some of the later experiments. 



The rate of transpiration was estimated by a potometer, not one of the 

 type formerly used by me,* in which an air bubble is timed as it passes 

 rapidly along a narrow capillary tube, but one in which the free end of 

 the water-column is timed with a stop-watch as it passes along a horizontal 

 tube of about a millimetre internal diameter.f It is, in fact, like Kohl's 

 potometer, or that figured in Pfeffer's ' Physiology,' though the method of 

 bringing the column back to zero is not identical with either. I have 

 not thought it necessary to give the actual quantities of water absorbed 

 by the plant per hour, but merely a series of numbers proportional to the 

 rate of absorption. 



In all experiments (except No. 8) the plants were placed close to the north 

 windows of the laboratory ; the action of the stomata was in all cases 

 excluded by a coating of grease, transpiration taking place only by incisions, 

 as above described. 



In the following tables T means temperature, i|r stands for relative 

 humidity : — 



Experiment 1. — November 6, 1909. P. kmrocerasus. Fig. 1, Cut branch in 



potometer. 



Time. 



Period. 



Rate. 



T. 



+■ 









° C. 



per cent. 



10.13 A.M. 



i 



36 



13 -8 



74 



10.39 „ 



ii 



38 



13 -6 



74 



10.49 „ 



iii 



36 



13 -6 



74 





Bell-jar over plant. 







11.16 „ 



iv 



16 



14 -2 



91 



11.28 „ 



V 



20 



34-7 



92 



11.37 „ 



vi 



14 



15 -0 



93 



11.51 „ 



vii 



17 



15 2 



94 



transpiration curve. Experiment 7, in which the last reading was taken at sunset, gives, 

 nevertheless, a good straight diagonal, as seen in fig. 7. A number of experiments were 

 made (like Experiment 8) in the dark room. I cannot see that they differ as a whole 

 from those illustrated in the present paper. 



* F. Darwin and E. Phillips, ' Camb. Phil. Soc.,' 1886, vol. 5 ; see also E. Darwin and 

 Acton, ' Physiology of Plants,' 1901, 3rd Edit., p. 79. 



t In all the later experiments the diameter was either - 95 mm. or 1 - 1 mm. 



x 2 



