414 Mr. F. Darwin. 



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 impli- 

 cation, 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 self 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 sensitive- 

 ness 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 transpiration. The hygroscope indicates well 

 the gradual " closure "* 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 circumsbances. I find that, although 

 the phenomenon is much less marked than in terrestrial plants, yet 

 that, in many species, partial closure of the stomata undoubtedly 

 occurs in the aquatic class. 



The most interesting fact observed in withering leaves is that in 

 many cases the " closure " of the stoma is preceded by temporary 

 opening, which may occur almost simultaneously with the severance 

 of the leaf from the plant. Thus the hygroscope readings rise at 

 first, and subsequently sink to zero. The interest of this fact is the 

 demonstration of the interaction between the guard cells and the 

 surrounding epidermis. The phenomenon is best seen in plants with 

 milky juice, but is not confined to this class. The preliminary 

 opening of le stomata occurs in the early morning, but not in the 

 evening a fact which is of importance in relation to the mechanism 

 of the nocturnal closure of the stomata. 



A diminution of the stomatal transpiration can also be brought 

 :about by compressing the stem of the plant in a vice, a process which 

 is known to diminish the water supply. f The stomatal closure is here 

 probably an adaptive response to the lowering of the water-supply 

 of the leaf, but this is not quite certain. 



A series of experiments were made on the comparative effect of 

 moist and dry air, from which it is clear that the stomata " close " 

 before any visible signs of flaccidity occur in the leaf. When leaves 

 are exposed to air dried by H 2 S0 4 , " closure " is preceded by a remark- 



* I use the word " closure " to mean such a narrowing of the stomatal aperture 

 as corresponds with zero on the hygroscope. 



f F. Darwin and R. Phillips, ' Camb. Phil. Soc. Proc.,' 1886. 



