COMBINATIONS OF STIMULI. 627 



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-effect 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 {Tnfolium pratense) is used in the above 

 experiment, or T. wcarnaium, Oxalis aceiosclla and some other plants, they would 

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

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

 position occurs. In this case, 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-called 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 clover-like plant 

 {Hedysar 11711 gy rails), the leaf of which is here || .fW 



figured. The two small lateral leaflets of this (|| _,^^;#^*^^X 

 make in the course of a few minutes periodic m^^^^^/i \' \h>\ 

 oscillations, it matters not whether they are in 'I -^[^ u / V--^^\ 

 light or darkness, if only the temperature is ^ \| //\"--i\ 



rather high — at least 22° C. \ /\^\ 



These spontaneous movements, not called V /\^\ 



forth by external changes, must therefore be M /Vx\ 



sharply distinguished in the first place from the M ( / V^] 



sleep-movements, with which they are more or \j [\v) 



less combined, since the point to be established 

 is that the daily periodicity of sleep-movements 

 is due to variations in the light, 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 heliotropic, i.e. dependent on light in quite 

 another way : in the case of that stimulation 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 constant 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 



