i THE NATURE OF TRANSPIRATION n 



hermetically to the cut end of the branch, or by directly 

 weighing the amount of water transpired. In the latter 

 case, which was found to be the more satisfactory, the 

 branch, inserted through a caoutchouc cork into a test- 

 tube containing water, was hung from one arm of a 

 balance. The arrangement is shown in Fig. 1. In this 

 figure b is a tower ' containing calcium chloride, and 

 c is a sulphuric acid bulb for drying the gas supplied. 

 Before passing through the drying materials, the gas enter- 

 ing at e is led into an inverted flask d, which is provided, in 

 addition to the tubes of entry and exit, with a U-tube, /, 

 filled with oil. The supply of gas is adjusted until the 

 oil in the longer arm of the tube / is brought to a certain 

 level. By this means the pressure, and consequently the 

 flow, of gas through the apparatus, can be adjusted and 

 compared. When vapours are to be supplied, the liquid 

 from which the vapour is derived is placed in a sulphuric 

 acid bulb, like that in the figure, but inserted in the train 

 between the air supply and the flask d. In each case 

 the supply was passed into the apparatus from a cylinder 

 containing the compressed gas. 



The chamber containing the branch was exposed only 

 to a very feeble light so that the stomata were closed 

 throughout the experiments. 



The first experiments made were with the index method 

 of estimating the rate of transpiration. A modification 

 of the apparatus, as figured, which is readily understood, 

 was then used, and the branch, sealed hermetically to a 

 capillary tube containing the index, was inserted from 

 below into the receiver. 



With these arrangements there soon appeared to be a 

 marked difference in the rate of transpiration in oxygen 

 and carbon dioxide. Thus, to quote the mean of a number 

 of observations with a branch of Cytisus laburnum in 

 carbon dioxide the index moved 1 cm. in 38 sec. ; with 

 the same branch in oxygen it moved 1 cm. in 28 sec. 



