OH. VIIl] MIMOSA, SLEEP. 205 



is well to examine the nocturnal positions of a trifoliate 

 Oxalis, such as 0. aceiosella (in which the leaflets point 

 nearly vertically downwards at night), and of Marsilea 

 quadrifoliata (in which the four leaflets rise and arrange 

 themselves in a vertical packet). The nyctitropism of 

 Melilotus, with its curious right and left-handedness', of 

 Cassia, in which the leaves sink and twist, and of Des- 

 modium gyrans, in which the vertical droop of the larger 

 leaflets is particularly striking, should also be studied. 

 Movements not produced by means of a pulvinus, but by 

 the growth of the leaf-stalk should be examined ; for 

 instance the nocturnal rise of the young leaves of Nico- 

 ticma glauca or of the cotyledons of the cabbage and 

 radish {Brassica oleracea and Raphanus sativus). 



In all these cases note that the nocturnal is more 

 nearly vertical than the diurnal position, and that when 

 there is close contact between neighbouring leaflets it is 

 generally the upper surface of the leaf that is protected. 



(256) Nyctitropic movements: Mimosa. 



In order to study the sleep movements of leaves more 

 closely we employ a self-recording method. Fig. 40 is 

 a copy of a tracing^ made by the main petiole of Mimosa 

 pudica from 4 p.m. Aug. 16, until noon of the following 

 day. The tracing was made by means of the hanging 

 writer described in experiment 203, which recorded the 

 position of the petiole at intervals of half-an-hour on the 



^ Power of Movement in Plants, p. 346, fig. 140. 



^ To diminish the horizontal extension of the diagram, the horizontal 

 lines drawn by the writing index are reduced. The engraving is more- 

 over reduced by J from the drawing so prepared. 



