580 ECOLOGY 



other, as in the night closing of the bean leaflets. The rapid closing of 

 Mimosa by contact is due to the sudden escape of water from the cells 

 into the adjoining air spaces, while subsequent opening is due to its slow 

 reentrance ; the slower photeolic movements are made possible by a 

 change in the permeability of the cell membranes occasioned by a change 

 in illumination. The leaflets of the hydroph)rte, Marsilea, close if 

 emersed, but not if floating, possibly because water contact preserves 

 the necessary turgescence, or possibly because water may mechanically 

 inhibit closing. 



The advantages of leaf motility. — The most obvious advantage which 

 plants derive from leaf motility is protection from excessive transpi- 

 ration, by reason of the reduced surface in the closed position that is 

 assumed as a result of desiccation. In some leaves (as in those which fold 

 along the midribs) the exposed surface may be reduced one half, and it 

 may be reduced almost as much in a plant like Mimosa, where the leaflet 

 segments closely overlap one another. In other cases, closing means 

 rather a changed orientation, as in the drooping leaflets of the bean, 

 or in the erect leaflets of the beach pea, which assume a temporary 

 profile position, suggesting the permanent profile position of Lactuca 

 and Silphium. 



Night closing is a much more common phenomenon than is drought 

 closing, but its advantage is not obvious. Protection from cold has 

 been suggested, but this view has no supporting evidence; motile leaves 

 often are open on cool days and closed on warm nights. The facilitation 

 of nocturnal transpiration also has been suggested, it being supposed that 

 verticality prevents wetting by dew ; but there is no apparent advantage 

 from increased nocturnal transpiration. Perhaps nocturnal closing is 

 quite useless, as is pretty certainly the case with such extraordinary 

 movements as those of Desmodium gyrans and the contact movements 

 of Mimosa. 



Leaves are subject to passive movements through the action of wind or vpater. 

 The leaves of the reed {Phragmites) are attached to the stem in such a manner 

 as to swing around in the wind like a weather vane, their ready yielding prevent- 

 ing injury. Compound leaves, as in the coconut and in many submersed aquat- 

 ics (as the water milfoil), offer but little resistance to wind or water currents, and 

 hence escape injury, while large, simple leaves, as those of the banana (fig. 846) 

 or of the water lilies, are shredded when similarly exposed. The lateral flatten- 

 ing of the petiole of the aspen and of other species of Populus results, even in 

 the lightest breezes, in an almost constant trembling of the leaf, a phenomenon with- 

 out obvious advantage. 



