154 Neiv Method of Estimating the Aperture of Stomata. 



independent of transpiration. The poronieter is accordingly superior to the 

 hygroscopic methods (e.g., Stahl's cobalt test and the horn hygroscope), from 

 which variations in aperture can only be inferred from increased transpira- 

 tion ; (2) the porometer indicates the behaviour of a group of living stomata 

 which can be continuously studied for hours or even days. In this it 

 compares favourably with Lloyd's microscopic method, which, however, has 

 the advantage of giving the actual size of the stomatal pore, instead of 

 merely a series of readings from which changes in the size in the stoma can 

 be inferred. 



As a test of the porometer we have selected two of the principal factors 

 that influence stomatal aperture, namely, illumination and water supply ; 

 we have shown that the known effects of these agencies are well demonstrated 

 by our method. 



In the case of leaves severed from the plant we have confirmed a state- 

 ment made by one of us, viz., that the first effect of withering is the opening 

 of the stomata, which is, however, followed by closure. 



The section on the causal relation between stomatal aperture and 

 transpiration is a single illustration of the conclusion, which we hope to 

 justify when the results of a considerable series of experiments already made 

 are published. 



We desire to thank Mr. F. F. Blackman for many useful suggestions made 

 to us in the course of our research. 



LITERATURE REFERRED TO. 

 Miiller, N. J. C, " Die Anatomie und die Mechanik der Spaltoffnung," ' Jahrb. fiir 



wiss. Botanik,' 1873, vol. 8, p. 75. 

 Stahl, E., " Einige Versuche liber Transpiration und Assimilation,'' 'Bot. Zeitung,' 1894, 



p. 117. 



Darwin, F., "Observations on Stomata," 'Phil. Trans.' B, 1898, vol. 165, p. 531 ; "A 

 Self-recording Method applied to the Movements of Stomata," ' Botanical Gazette, ' 

 February, 1904, p. 81. 



Lloyd, F. E., 'The Physiology of Stomata,' Carnegie Institution, 1908. 



