The Royal Society, London. 
115 
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 M erg et 1 ) and Stahl. 2 ) These observers used hygro- 
scopic 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 resnlts. 
In my laboratory I have used, for some years, a hygroscope for 
demonstrating cuticnlar transpiration, in which evaporation is in- 
dicated by the nntwisting of the awn of Stipa pennata ; 3 ) my 
present Instrument is of the same general type, but the index is 
made of „Chinese leaf a , i. e., shavings of pressed and heated 
horn. 4 ) 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 degree of curvature is read off 
on a graduated quadrant, and in this way a numerical indication 
of the condition of the stomata is obtained. 
The instrument makes no claim to accuracy, but has proved 
extremely useful when used comparatively to indicate and localise 
-small changes in the transpiration of leaves, and therefore by 
implication, changes in the condition of the stomata. By observing 
under the microscope the uninjured leaf of Caltha palustris , and 
comparing the variations in the size of the stomata with the 
variations in the readings of the hygroscope, it is easy to convince 
one’s seif of the value of the method. It must be especially noted 
that though a fall in the hygroscope readings corresponds with a 
narrowing of the stomatal opening, it does not follow that zero 
on the hygroscopic scale means absolute closure of the stomata. 
This want of sensitiveness has one advantage, namely, that 
cuticular transpiration has no effect on the horn index, so that 
any movement of the index must depend on a stomatal tran¬ 
spiration. The hygroscope indicates well the gradual „closure“ 5 ) 
of the stomata that occurs as a plucked leaf withers. It is 
generally stated that marsh and aquatic plants do not close 
their stomata under these circumstances. I find that, although 
the phenomenon is much less marked than in terrestrial plants, 
yet that, in rnany species, partial closure of the stomata undoub- 
tedly occurs in the aquatic dass. 
The most interesting fact observed in withering leaves is that 
in rnany cases the „closure“ of the stoma is preceded by ternpo- 
rary opening, which may occur almost simultaneously with the 
severance ot the leaf from the plant. Thus the hygroscope readings 
q Comptes Rencius. 1878. 
2 ) Bot. Zeitung, 1894. 
8 ) Darwin and Acton, Practical Physiology of Plants. I. st edition. 1894. 
4 ) ^ also use the epidermis of a Yucca —a material which I owe to the 
kindness of Mr. Thiselton-Dyer. 
5 ) I use the word „closure“ to mean such a narrowing of the stomatal 
aperture as corresponds with zero on the hygroscope. 
8* 
