1897.] by a Neiv Method. 307 



The existence of periodicity may be best demonstrated by the 

 method employed by Pfeffer in observing the " sleep " of flowers, 

 namely, by seeing that they behave differently to a given change 

 of temperature at different times of day. The case is the same 

 for stomata. In the morning a much longer period of darkness 

 is required to close the stomata than is necessary at night : on 

 the other hand stomata which are closed, open in the light much 

 more easily in the morning than in the late afternoon. 



8. The well-known result that the stomata close when a leaf 

 is cut from the plant and allowed to wither is strikingly 

 demonstrable with the hygroscope, the readings sinking gradually 

 to zero. 



In this way the fact observed by Stahl can be confirmed, 

 namely, that the leaves of marsh and water-plants do not, as a 

 rule, close their stomata on withering. 



The closure of the stomata produced by watering plants with 

 dilute NaCl solution has also been confirmed *. 



9. In some cases the process of withering is strikingly 

 different. In Pointsettia, Tropaeolum, Campanula vidalii and 

 some other plants the closure of the stomata is preceded by 

 well-marked opening — as the following figures will show. 



The result is of some importance as bearing on the question of 

 mechanism. 



The effect of cutting off the water supply must be to 

 diminish turgescence — and diminished turgescence of the guard 

 cells must mean closure. Therefore the opening of the stomata 

 must be due to the loss of turgor in the other elements of the leaf, 

 especially of the ordinary epidermic cells — the final closure 

 being due to the withering of the guard cells + This effect is 

 especially well seen in plants with milky juices, e.g. Campanula 



* See Schimper, Indo-Malayische Strand Flora, 1891. 

 t On this subject see Leitgeb, loc. cit. 



