304 Mr Darwin, Observations on Stomata [May 10, 



In 1894 Stahl* introduced his cobalt method, in which he 

 took advantage of the fact that paper impregnated with cobalt 

 chloride is blue when dry, and becomes pink in damp air. Thus 

 if dry cobalt paper is placed on each surface of a hypostomatal 

 leaf, the paper rapidly changes to pink on the lower side, but 

 remains blue on the upper surface. Whereas when the stomata 

 are shut no such difference is seen. With this method Stahl 

 has made out a number of most interesting facts. It is moreover 

 a method especially applicable to demonstration and lecture 

 purposes, since the effect is rapidly obvious, and clearly visible 

 from a little way off. 



In the Practical Physiology, 1894, of Mr Acton and myself, 

 I described a rough hygrometer in which the index was the 

 awn of Stipa, and which I used to demonstrate that transpiration 

 is ordinarily stomatal rather than cuticular. 



The method which I have to describe is like Merget's and 

 Stahl's papers, and like my Stipa hygrometer, an indirect one, 

 depending on the yield of watery vapour from the stomata. 



Two or three years ago I came across a toy consisting of the 

 figure of a man cut out of a thin sheet of horn prepared in such 

 a way as to be extremely hygroscopic. When placed on a damp 

 surface it writhes and twists, curving away from the source of 

 moisture. The toy had lain hidden in an old house for something 

 like 100 years; it was made for fortune-telling purposes, the 

 degree of movement on the hand indicating the disposition of 

 the patient. The material was described as Chinese sensitive 

 leaf; it is mentioned in Holtzappfel's Turning and Mechanical 

 Manipulation, Vol. I., p. 123, 1846, as made from pressed horn. 

 Similar toys in the form of fishes were described by Maria 

 Edgworth, 1808, and appear to have come from Japan. Figures 

 of women and also of fish, made apparently of gelatine or gold- 

 beaters' skin, are still sold for fortune-telling. But these are not 

 so good for my purpose as horn sheets. Being unable to obtain 

 any more of my original material I made use of shavings of 

 horn and for a long time despaired of making them sufficiently 

 sensitive. At last a chance observation showed me that the 

 horn shavings are rendered sensitive by heat. 



The method of using the material is as follows. A strip of 

 horn sheet 8 or 9 mm. long by 3 or 4 wide is fastened by 

 one end to a small block of cork so that the horn lies flat on 

 any surface on which the instrument is placed: the horn bears 

 at its free end a bristle to serve as an index. When it is placed 

 on a damp surface the index instantly rises to an angle of 20° 

 to 50° or even more, whereas it remains flat on a dry surface. 



* Bot. Zeitung, 1894. 



