Chap. Vil. 



SLEEP OF LEAVES. 



335 



Fig. 136. 



32° C., 14 oscillations of a few degrees occurred m 19 m. On 

 the other hand, an oscillation may be much slower ; thus a leafloJ 

 was observed (temperature 25° 0.) to 

 rise during 40 m. before it fell and 

 completed its oscillation. 



Poiiieria hygrometrica (Zygophyllese) 

 — The leaves of this plant (Chi!i,in 

 form) are from 1 to IJ inches in length, 

 and bear as many as 16 or 17 small 

 leaflets on each side, which do not 

 stand opposite one another. They are 

 articulated to the petiole, and the 

 petiole to the branch by a pulvinus. 

 We must premise that apparently two 

 forms are confounded under the same 

 name : the leaves on a bush from Chili, 

 which was sent to us from Kew, bore 

 many leaflets, whilst those on plants 

 in the Botanic Garden at Wdrzbnrg 

 bore only 8 or 9 pairs ; and the whole 

 character of the bushes appeared some- 

 what different. We shall also see that 

 they differ in a remarkable physio- 

 logical peculiarity. On the Chilian 

 plant the petioles of the younger leaves 

 on upright branches, stood horizontally 

 during the day, and at night sank 

 down vertically so as to depend parallel 

 and close to the branch beneath. The 

 petioles of rather older leaves did not 

 become at night vertically depressed, 

 but only highly inclined. In one 

 instance we found a branch which had Polkria 

 grown perpendicularly downwards, 

 and the petioles on it moved in the same 

 direction relatively to the branch as 

 just stated, and therefore moved up- 

 wavds. On horizontal branches the 

 younger petioles likewise move at night 

 in the same direction as before, that is, 

 towards the branch, and are consequently then extended hori- 

 zontally; but it is remarkable that the older petioles on tho 



hygrometrioa : rir- 

 cumnutation and nycti- 

 tropic movements of pe- 

 tiole of leaf, traced from 

 9.35 A.M. July 7th to 

 about midnight on the 

 8th. Apex of leaf 7J 

 inches from tho vertical 

 glass. Temp. 19J°-20i°C. 



