June, 1919] Variations in Transpiration 509 



even before the stomata are fully closed. The decreasing leaf 

 water deficit has no effect after the stomata are closed. 



Tobacco and mullein show a rhythm in the transpiration 

 curve in total darkness when preceded by a day of normal hght 

 conditions, while moth mullein under the same conditions does 

 not show this rhythm. This rhythm is expressed as a rise in 

 the rate of water loss at about the time the stomata usually 

 open in sunlight with a maximum about the middle of the fore- 

 noon and a decrease to the usual night rate by noon. This takes 

 place in total darkness under constant environmental condi- 

 tions. The rhythm in tobacco and mullein does not show up on 

 the second day in the dark-room, and in tobacco which is 

 placed in the dark-room about noon of the preceding day there 

 is no rhythm in the transpiration curve. The moth mullein does 

 not show this rhythm under the same conditions. It seems 

 therefore, that certain plants have this characteristic rhythm 

 while others do not. The cause of the rhythm is most likely 

 stomatal activity but because of the large errors in measuring 

 these movements as compared with the small movement nec- 

 essary to produce the slight increase in transpiration it has not 

 been found possible to verify it. 



LITERATURE CITED. 



1. Brown, H. T. and Escombe, F. Static diffusion of gases and liquids in relation 



to the assimilation of carbon and translocation in plants. Ann. Bot. 

 14 : 537-542. 1900. 



2. Lloyd, F. E, The Physiology of Stomata. Pub. 82. Carnegie Institution of 



Washington. 1908. 



3. Livingston, B. E., and Estabrook, A. H. Observations on the degree of stomatal 



movement in certain plants. Bull. Torrey Club. 39 : 15-22. 1912. 



4. Curtis, C. C. Some observations on Transpiration. Bull. Torrey Club. 



29 : 360-373. 1902. 



5. Darwin, Francis. Observations on stomata. Phil. Trans. Roy. Soc. London B. 



190 ::531-621. 1898. 



6. Livingston, B. E., and Brown, W. H. Relation of the daily march of transpira- 



tion to variations in the water content of foliage leaves. Bot. Gaz. 

 53 : 309-330. 1912. 



7. Livingston, B. E. The resistance offered by leaves to transpirational water 



loss. Plant World 16 : 1-35. 1913. 



8. Transeau, E. N. Apparatus for the study of comparative transpiration. Bot. 



Gaz. 52 : 54-60. 1911. 



9. Renner, O. Beitrage zur Physik der Transpiration. Flora, Bd. 100 : 451-548. 



