472 



PLANT RESPONSE 



or will record themselves on a sensitised photographic film 

 wrapped round the revolving drum. 



In carrying out such a continuous record, certain pre- 

 cautions are necessary. Owing to transpiration, the float on 

 which the plant is mounted will become lighter, causing an 

 ascensional movement of the record. In order to obviate 

 this, it is only necessary (i) to weight the float to such an 

 extent that the variation caused by transpiration is negli- 

 gible ; and (2), in addition to this, the diameter of the cylin- 

 drical float may be so increased as to reduce still further the 

 ascensional movement due to loss of weight by transpiration. 

 By these means the error from this source may be reduced 



to any extent desired. In the 

 figure of the apparatus which is 

 here given (fig. 188), the specimen 

 was a young seedling of Oryza 

 sativa, in v/hich transpiration was 

 relatively little. I shall presently 

 give photographic records obtained 

 in the manner described. 



Continuous photographic re- 

 cord of periodic variations of 

 transpiration. — By a somewhat 

 similar method we are enabled to 

 determine the periodic variation of 

 the rate of transpiration. In this 

 case, the plant is mounted with its 

 roots in a test-tube, which acts like 

 a float, and is partially filled with 

 water, but not so full as to make it 

 sink. The test-tube containing the 

 plant is attached to one arm of the 

 Optic Lever, and the outflow from 

 the outer cylinder so adjusted that 

 the ascensional movement of the test-tube, due to its loss of 

 weight by transpiration, is exactly balanced by the subsidence 

 of the water-level of the water that buoys it up. In order 



Eig. 189. Photographic Record 

 showing Variation of Rate 

 of Transpiration in Cucur- 

 bita, from 3 P.M. to 12 P.M. 



The rate is seen to undergo 

 enhancement till about 

 11.30 p.m., after which 

 there is a rapid fall of the 

 rate of transpiration. 



