CHAP. VII. SLEEP OF LEAVES. 347 



one leaflet, which had originally faced east, was observed after 

 9 days to face west. The seat of both the twisting and bending 

 movement is in the pulvinus of the sub-petioles. 



We believe that the leaflets, especially the two lateral ones, 

 in performing the above described complicated movements 

 generally bend a little downwards ; but we are not sure of this, 

 for, as far as the main petiole is concerned, its nocturnal move- 

 ment is largely determined by the position which the leaf 

 happens to occupy during the day. Thus one main petiole was 

 observed to rise at night 59, whilst three others rose only 7 

 and 9. The petioles and sub-petioles are continually circum- 

 nutating during the whole 24 h., as we shall presently see. 



The leaves of the following 15 species, M. officinalis, suaveolens, 

 parviflora, alba, infesta, dtntata, gracilfs, sulcata, elegans, cccrulea, 

 petitpierreana, macrorrhiza, Italica, secundiflora, and Taurica, 

 sleep in nearly the same manner as just described; but the 

 bending to one side of the terminal leaflet is apt to fail unless 

 the plants are growing vigorously. With M. petitpierreana and 

 sccundiftora the terminal leaflet was rarely seen to bend to one 

 side. In young plants of M. lialica it bent in the usual manner, 

 but with old plants in full flower, growing in the same pot and 

 observed at the same hour, viz., 8.30 P.M., none of the terminal 

 leaflets on several scores of leaves had bent to one side, though 

 they stood vertically ; nor nad the two lateral leaflets, though 

 standing vertically, moved towards the terminal one. At 

 1030 P.M., and again one hour after midnight, the terminal 

 leaflets had become very slightly bent to one side, and the 

 lateral leaflets had moved a very little towards the terminal one, 

 BO that the position of the leaflets even at this late hour was far 

 from the ordinary one. Again, with M. Taurica the terminal 

 leaflets were never seen to bend towards either of the two lateral 

 leaflets, though these, whilst becoming vertical, had bent towards 

 the terminal one. The sub-petiole of the terminal leaflet in 

 this species is of unusual length, and if the leaflet had bent to 

 one side, its upper surface could have come into contact only 

 with the apex of either lateral leaflet; and this, perhaps, is the 

 meaning of the loss of the lateral movement. 



The cotyledons do not sleep at night. The first leaf consists of 

 a single orbicular kaflet, which twists at night so that the blade 

 stands vertically. It is a remarkable fact that with M. Taurica, 

 and in a somewhat less degree with M. macrorrhiza and petit- 

 pierreana, all the many bmall and young leaves produced during 



