XI] MECHANISM OF MOVEMENTS 113 



casia, &c.) and Gymnosperms ( Abies) present such a curious 

 appearance if one illuminates them suddenly at night, by 

 the electric light for instance. In some cases the petioles 

 are found to have become more erect, and the laminae to 

 have folded up one over the other e.g. Trifolium, Vicia, 

 Strephium, &c. In others e.g. Lupin, Robinia, Oxalis, 

 Averrhoa, &c. the laminse become pendent from the 

 erect, or slightly depressed leaf-stalks. Others, again, 

 undergo displacements which direct the plane of the 

 lamina, whether of leaflet or leaf, with its lateral edges 

 upwards and downwards e.g. Coronilla, Arachis, &c. 

 and other positions occur. 



There can be little doubt that Darwin's explanation of 

 these nocturnal positions of leaves, brought about by 

 sleep-movements (nyctitropism), is the right one : they 

 are adaptations to reduce the danger of cooling by radia- 

 tion to a minimum. 



In all these cases the mechanism of the movements 

 depends on changes of turgidity in the cells of the pul- 

 vinus found at the base of each petiole or petiolule ; and 

 exact measurements show that the gradual erection or 

 depression of the leaf-stalk or lamina concerned, as it 

 passes from the diurnal to the nocturnal position, or con- 

 versely, is effected in jerks, so that the observed inclination 

 at any moment is a variable one oscillating above and 

 below a mean. The differences in the turgidity of the cells 

 which bring about these erections or depressions are due 

 to complex changes, resulting in variations in the amounts 

 of osmotic substances and water in the cells of the pulvinus. 

 That these changes are frequently induced by variations 

 in intensity of the light cannot be doubted. But whether 

 the abstraction of water (resulting in the diurnal position) 

 is merely correlated with the increased transpiration in 

 daylight, and the increased turgescence of the pulvinus at 



w. n. 8 



