372 MODIFIED CIROUMNUTATION. Ouap. VII 



therefore their lower surfaces face but little outwards at night. 

 The petioles, which during the day are inclined only a little 

 above the horizon, rise at night in a remarkable manner, and 

 stand nearly or quite vertically. This, together with the 

 dependent position of the leaflets, makes the whole plant won- 

 derfully compact at night. In the two foregoing figures, copied 

 from photographs, the same plant is represented awake and 

 asleep (Fig. 155), and we see how different is its appearance. 



Cassia mimosuides. — At night the numerous leaflets on each 

 leaf rotate on their axes, and their tips move towards the apex 

 of the leaf; they thus become imbricated with their lower 

 surfaces directed upwards, and with their midribs almost 

 parallel to the petiole. Consequently, this species differs from 

 all the others seen by us, with the exception of the following 

 one, in the leaflets not sinking down at night. A petiole, the 

 movement of which was measured, rose 8° at night. 



Cassia Barclayana. — The leaflets of this Australian species are 

 numerous, very narrow, and almost linear. At night they rise up 

 a little, and also move towards the apex of the leaf. For instance, 

 two opposite leaflets which diverged from one another during 

 the day at an angle of 104°, diverged at night only 72° ; so that 

 each had risen 16° above its diurnal position. The petiole of a 

 young leaf rose at night 34°, and that of an older leaf 19°. 

 Owing to the slight movement of the leaflets and the consider- 

 able movement of the petiole, the bush presents a different 

 appearance at night to what it does by day ; yet the leaves can 

 hardly be said to sleep. 



The cireumnutating movements of the leaves of C. floribvmda, 

 calliantha, and pubescens were observed, each during three or four 

 days ; they were essentially alike, those of the last-named species 

 being the simplest. The petiole of C. floribunda was secured to 

 a stick at the base of the two terminal leaflets, and a filament 

 was fixed along the midrib of one of them. Its movements were 

 traced from 1 p.m. on August 13th to 8.30 a.m. 17th ; but those 

 during the last 2 h. are alone given in Fig. 156. From 8 a.m. oe 

 each day (by which hour the leaf had assumed its diurnal posi- 

 tion) to 2 or 3 P.M., it either zigzagged or circumnutated over 

 nearly the same small space; at between 2 and 3 p.m. the great 

 evening fall commenced. The lines representing this fall and 

 the early morning rise are oblique, owing to the peculiar manner 

 in which the leaflets sleep, as already described. After the 

 leaflet was asleep at 6 p.m., and whilst the glass filament hung 



