324 MODIFIED CIECUMNUTATION. Chap. VIl. 



Im-ge hall, lighted only from the roof, did not sleep at night, 

 for in order to do so the leaves must he well illuminated during 

 the day. The cotyledons do not sleep. Linnaeus says that the 

 leaves of his Sida tibutilon sink perpendicularly down at night, 

 though the petioles rise. Prof Pfeffer informs us that the 

 leaves of a Malva, allied to M. sylv:stris, rise greatly at night; 

 and this genus, as well as that of Hibiscus, are included by 

 Linnseus in his list of sleeping plants. 



Anoda Wrightii (Malvacese). — The leaves, produced by very 

 young plants, when grown to a moderate size, sink at night 

 either almost vertically down or to an angle of about 45° beneath 

 the horizon ; for there is a considerable degree of variability in 

 the amount of sinking at night, which depends in part on the 

 degree to which they have been illuminated during the day. 

 But the leaves, whilst quite young, do not sink down at night, 

 and this is a very unusual circumstance. The summit of the 

 petiole, where it joins the blade, is developed into a pulvinus, 

 and this is present in very young leaves which do not sleep ; 

 though it is not so well defined as in older leaves. 



Qossypium (var. Nankin cotton, Malvacese). — Some young 

 leaves, between 1 and 2 inches in length, borne by two seedlings 

 6 and 7^ inches in height, stood horizontally, or were raised a 

 little above the horizon at noon on July 8th and 9th ; but by 

 10 P.M. they had sunk down to between 68° and 90° beneath 

 the horizon. When the same plants had grown to double 

 the above height, their leaves stood at night almost or quite 

 vertically dependent. The leaves on some large plants of 

 G. maritimum and Brazilense, which were kept in a very badly 

 lighted hot^house, only occasionally sank much downwards 

 at night, and hardly enough to be called sleep. 



Oonalis (Oxalidss).— In most of the species in this large genus 

 the three leaflets sink vertically down at night; but as their 

 sub-petioles are short the blades could not assume this position 

 from the want of space, unless they were in some manner ren- 

 dered narrower; and this is effected by their becoming more 

 or less folded (Fig. 127). The angle formed by the two halves 

 of the same leaflet was found to vary in different individuals of 

 several species between 92° and 150°; in three of the best 

 folded leaflets of 0. fragrans it was 76°, 74°, and 54°. The 

 angle is often different in the three leaflets of the same leaf. 

 As the leaflets sink down at night and become folded, theii 

 lower surfaces are brought near together (.see B), or even into 



