168 GENERAL BOTANY 



organ. By a contraction of the lower, and expan- 

 sion of the upper side of the pulvinus, the blade is 

 made to turn down, and by the reverse action, the 

 blade would be brought up. 



Many plants with simple leaves, and with leaves 

 or leaflets that have no pulvinus, act in much 

 the same way. Thus, the leaves of SIDA CARPINI- 

 FOLIA (fig. 32) rise at sunset, to an angle of 

 about 50 to the horizontal, i.e. more than half 

 way to the vertical position. Those of PHYLLAN- 

 THUS NIRURI, which is very common among grass 

 (fig. 7), fold along the axis and face sideways. Those 

 of EUPHORBIA ROSEA, another common little herb,, 

 move towards the stem and the edges roll back a 

 little. In INDIGOFERA ENNEAPHYLLA (fig. 9), another 

 very common herb which has pinnate leaves, the 

 leaflets rise slightly and each folds up along its mid- 

 rib. In OXALIS, each leaflet folds up along its mid- 

 rib, and these fall down so as to face sideways. 



A large number of similar cases can be seen without 

 difficulty on any night after sunset, among trees, 

 shrubs, our garden flowering herbs, and the smaller 

 herbs that make up the ordinary Indian ' grass '. 

 The movements are very regular, the sleep position 

 being taken up each evening, and the day position on 

 each morning at definite hours. 



These curious movements must be of some use to 

 the plant, and that they are of a protective nature 

 is known, because it has been found that leaves or 

 leaflets which are artificially prevented from assuming 

 their natural 'sleep position', suffer. It is certain 

 that a horizontal blade loses far more heat bv radiation 



