Observations on Stomata. 413 



IX. " Summary of the principal Results obtained in a Study of the 

 Development of the Tuatara (Sphenodon punctatum) ." By 

 Professor A. DENDY. Communicated by Professor HOWES, 

 F.R.S. 



X. " Tables for the Solution of the Equation 



By W. STEADMAN ALDIS, M.A. Communicated by J. J. 

 THOMSON, F.R.S. 



XI. " The StomodaDum, Mesenterial Filaments, and Endoderm of 

 Xenia." By J. H. ASHWORTH, B.Sc. Communicated by 

 Professor HICKSON, F.R.S. 



The Society adjourned over the Long Vacation to Thursday, 

 November 17fch, 1898. 



" Observations on Stomata." By FRANCIS DARWIN, F.R.S. 

 Received May 31, Read June 16, 1898. 



(Abstract.) 



The method described depends on the fact that in adult leaves 

 transpiration is stomatal rather than cuticular, so that, other things 

 being equal, the yield of watery vapour depends on the degree to 

 which the stomata are open, and may be used as an index of their 

 condition. In principle, it is the same as the methods of Merget* and 

 Stahl.f These observers used hygroscopic papers impregnated with 

 reagents which change colour according as they are dry or damp, and 

 Stahl, who employed paper soaked in cobalt chloride, has obtained 

 excellent results. In my laboratory I have used, for some years, a 

 hygroscope for demonstrating stomatal transpiration, in which evapo- 

 ration is indicated by the untwisting of the awn of Stipa pennata ; J 

 my present instrument is of the same general type, but the index is 

 made of " Chinese leaf," i.e., shavings of pressed and heated horn. If a 

 strip of horn is placed on a dry substance, e.g., the astomatal surface of 

 a leaf, it does not move, but on the stomatal surface, it instantly curves 

 strongly away from the transpiring surface. In the hygroscope the 



* ' Comptes Rendus,' 1878. 

 f ' Bot. Zeitung,' 1894. 



Darwin and Acton, ' Practical Physiology of Plants/ 1st edition, 1894. 

 I also use the epidermis of a Yucca a material which I owe to the kindness 

 of Mr. Thiselton-Dyer. 



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