PHYSIOLOGY 



271 



tension, and, in consequence of the resulting superior pressure of the 

 lower half, the leaf is raised again to its diurnal position. 



According to Darwin, the leaves are protected from too great a 

 loss of heat by radiation by the assumption of the nocturnal position. 

 This loss of heat, may sometimes be very considerable, so much indeed, 

 that nyctitropic leaves, forcibly retained in their diurnal position, 

 were frozen, while adjacent '' sleeping " leaves sustained the night 

 temperature without injury. As sleep movements are also manifested 

 by plants growing in tropical climates, where no injurious nocturnal 

 diminution of temperature occurs, the advantage accruing from the 

 sleep position in the previous instance is not explanatory of the 

 nyctitropic behaviour of leaves in all cases. Sleep movements are 

 particularly noticeable in Phaseolus, Trifolium, Robinia, Acacia lophantha, 

 Amicia zygomeris (Fig. 212), Mimosa pudka, etc. 



Too intense light frequently causes the change from the diurnal 



Fig. 212. — Amicia zygomeris, showing diurnal and nocturnal position of leaves. 



position, and a movement either towards or away from the nocturnal 

 position. The leaflets of the common Locust (Robinia pseudacacia) 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 change the 

 geoteopism OF THE motile okgans ; the sleep movements are then accomplished 

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



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 

 pass at regular intervals from one condition to the other. The daily 

 periods are the result of the stimulus imparted by the light, the 

 periodic action of which induces the regular changes of position. If, 

 however, the external stimulus ceases to operate, the internal disposi- 



