COMBINATIONS OF STIMULI. 



637 



position again. This suggests the question whether there is any light-stimulus at all 

 in the matter. We have here, however, to do simply with an after-eflfect of the 

 previous light-stimulus, which after a few days ceases in constant darkness just as it 

 does also in constant light. 



If, on the other hand, the Field Clover (Trifolium prateme) is used in the above 

 experiment, or T. incarnatum,- Oxalis acetosella and some other plants, they would 

 be found to be in continual niovement, so that at any time after the course of 

 a few hoiirs an apparent nocturnal position alternating with an apparent diurnal 

 position occurs. ' In this' ease, however, we have to do with a movement independent 

 of the variation of light, and therefore not to be regarded as an after-effect of it. From 

 internal changes,, as yet not understood, the leaves make up and down movements in 

 periods of time of a few hours. Since these leaves also are sensitive to light, 

 however, and have true sleep-movements, this independent so-caUed spontaneous 

 or autonomous periodicity is hardly noticed under ordinary circumstances, simply 

 because the influence of the light is stronger than the spontaneous movement. 

 Nevertheless the converse occurs also ; this in a very striking degree, for ex- 

 ample, in the case of another elover-like plant 

 (Hedysarurn gyrans), the leaf of which is here 

 figured. The two small lateral leaflets of this 

 make in the course of a few minutes periodic 

 oscillations, it matters not whether they are in 

 light or darkness, if only the temperature is 

 rather high — at least 22° C. 



These spontaneous movements, not caHed' 

 - forth by external changes, must therefore be 

 sharply distinguished in the first place from the- 

 sleep-movements, with which they are more or 

 less combined, since the point to be established 

 is that the daily periodicity of sleep-movements 

 is due to variations in the Kght, and is thus causally essentially different from the 

 spontaneous movements. 



But we have to do with yet other complications, not less calculated to lead 

 to error in the study of daily periodicities and their causes. The leaves in question, 

 or rather their motile organs, are also helio tropic, i.e. dependent on light in quite 

 another way : in the case of that stiftiulation of light which produces waking and 

 sleeping, the stimulus lies in the variations of the intensity of the light — it is not 

 the light as a constaiit force which effects these movements, but the varying 

 intensity; the increasing intensity in the morning induces the waking and exten- 

 sion of the leaves, the decrease of the light in the evening the nocturnal position 

 or closing. In the heliotropic curving of the motile organs, on the contrary, it is the 

 constant influence of the light which effects the curving, just as in the case of 

 heliotropic stems and roots. If one of the above-named plants stands for some time 

 undisturbed at a window, all the laminae become turned towards the light ; if the plant 

 is turned round, the motile organs make other curvatures until the laminae again 

 turn their upper sides to the light. The movements of waking and sleeping go on 

 undisturbed in such heliotropically curved organs. A further great difference between 



s s 2 



FIGi 365. A leaf of Hedysarurn gyrans (nat. 

 size). 



