LEAVES 



579 



Motile leaves. The phenomena of leaf motility. In many Legu- 

 minosae (as in the clovers and locusts) and in Oxalis the leaflets of the 

 compound leaves close by night (especially in cool weather) and open by 

 day ; such closing movements have been called photeolic movements or, 

 less correctly, sleep movements or nyctitropic movements. In many 

 legumes desiccation causes the leaflets to assume the closed position ; 



in Mimosa contact also 

 produces the same effect 

 (figs. 831, 832), and in 

 Desmodium gyrans there 

 are movements apparently 

 without external cause 

 (fig. 684). While most 

 motile leaves are com- 

 pound (figs. 833, 834), 



831 



832 



FIGS. 831, 832. Leaf motility in the sensi- 

 tive plant (Mimosa pudica): 831, an open leaf; 

 832, a leaf whose leaflets (/) have been closed by 

 mechanical impact ; note also that the petiole (p) 

 has dropped; s, stipule; m, pulvinus. 



FIGS. 833, 834. Leaf motility 

 in Oxalis: 833, open leaves as seen 

 by day; 834, closed leaves as seen 

 by night. 



some simple leaves exhibit motility, as in Portulaca oleracea (figs. 686, 

 687) and in Euphorbia polygonifolia, those of the latter closing along 

 the median line like a book. 



Most leaf movements are due to changes in the turgescence of the 

 delicate cells that make up the body of the enlarged base (known as the 

 pulvinus} of the petiole or petiolule (leaflet stalk) ; the central position 

 of the delicate conductive bundle and the absence of mechanical cells 

 facilitate motility. Closing may be due to a decrease of turgescence, as 

 in drought, or to an increase, which is greater on one side than on the 



