PHYSIOLOGY 293 



the case of the leaflets of Omlis a path of |-li cm. is traversed in one 

 or a few seconds. Variations in illumination do not disturb these 

 movements. The autonomic variation movements of Trifolium and of 

 the Wood-Sorrel take place, on the contrary, only in darkness. Thus 

 the terminal leaflets of Trifolium pratense exhibit oscillatory movements 

 in the dark with an amplitude which may exceed 120, and are 

 regularly repeated in periods of 2-4 hours ; but on exposure to light 

 the leaflets cease their oscillations and assume a fixed light position. 



Paratonie Variation Movements are chiefly induced by variation 

 in the intensity of the light, by the stimulus of gravitation, and by 

 mechanical irritation (shock, friction). The pulvini of leaves may be 

 affected by several different stimuli ; the leaves of Mimosa pudica, for 

 example, are set in motion by the action of light, by the stimulus of 

 a shock, and sometimes also by gravitational stimuli ; in addition 

 they exhibit autonomic movements. 



A change from light to darkness, as from day to night, occasions 

 NYCTITROPIC MOVEMENTS or the so-called SLEEP MOVEMENTS. In the 

 day or light position, which is the same as that of diaheliotropic 

 foliage-leaves, the leaf-blades are perpendicular to the incident rays 

 of light. With the commencement of darkness the leaves or the 

 single leaflets fold either upwards with their upper surfaces inward, 

 or downwards with their lower surfaces together, and so remain until 

 the diurnal position is again assumed on recurring illumination. 



THE VARIATIONS IN TURGESCENCE IN THE OPPOSED HALVES OF THE 

 JOINT STAND IN INVERSE RELATION TO ONE ANOTHER ; OR IF OF THE 

 SAME NATURE ARE UNEQUAL IN DEGREE ( 9C ). 



As sleep movements are also manifested by plants growing in tropical climates, 

 where no injurious nocturnal fall of temperature occurs, the purpose of the sleep 

 position suggested by DAIIWIN, viz. the protection of the leaves from excessive 

 cooling, does not hold in all cases. STAHL seeks it also in the lessening of the 

 formation of dew, and the more energetic transpiration of the unwetted leaves 

 which would result. Sleep movements are particularly noticeable in Phaseolus, 

 Trifolium, Jtobinia, Acacia lophantha, Ainicin zygomeris (Fig. 236), Mimosa 

 pudica, Oxalis, Marsilia, Porlieria ; while less striking movements take place in 

 many other plants. 



Too intense light frequently causes the change from the diurnal position, and 

 a movement either towards or away from the nocturnal position. The leaflets of 

 the common Locust (Robinia pscudacacia) are folded downwards at night. In 

 ordinary diffuse daylight they assume their diurnal, outspread position ; but, if 

 exposed to the direct rays of the mid-day sun, they turn obliquely upwards. 

 In many plants ALTERATIONS IN THE INTENSITY OF THE LIGHT ALTER THE 

 GEOTROPISM OF THE MOTILE ORGANS ; the sleep movements are then accomplished 

 by the help of geotropic variation movement (Phaseolus, Lupinus) ( 97 ). 



The change from the diurnal to the nocturnal position continues 

 for a time to take place, even in constant darkness or prolonged 

 illumination. The leaves themselves seem to have a tendency to 



