224 CIKCUMNUTAT1ON OF FLOWER-STEMS CHAP. IV. 



two smaller ellipses in the course of 21 h., yet the chief seat of 

 movement lies in the sub-peduncles, which ultimately bend 

 vertically downwards, as will be described in a future chapter. 

 The peduncles of Oxalis acetosdla likewise bend downwards, and 

 afterwards, when the pods are nearly mature, upwards ; and this 

 is effected by a circumnutating movement. 



It may be seen in the above figure that the flower-stem of 

 O.carnosa circumnutated during two days about the same spot. 

 On the other hand, the flower-stem of 0. sensitiva undergoes a 

 strongly marked, daily, periodical change of position, when kept 

 at a proper temperature. In the middle of the day it stands 

 vertically up, or at a high angle ; in the afternoon it sinks, and 

 in the evening projects horizontally, or almost horizontally, 

 rising again during the night. This movement continues from 

 the period when the flowers are in bud to when, as we believe, 

 the pods are mature : and it ought perhaps to have been included 

 amongst the so-called sleep-movements of plants. A tracing 

 was not made, but the angles were measured at successive periods 

 during one whole day; and these showed that the movement 

 was not continuous, but that the peduncle oscillated up and 

 down. We may therefore conclude that it circumnutated. At 

 the base of the peduncle there is a mass of small cells, forming 

 a well- developed pulvinus, which is exteriorly coloured purple 

 and hairy. In no other genus, as far as we know, is the peduncle 

 furnished with a pulvinus. The peduncle of 0. Ortegesii behaved 

 differently from that of 0. sensitiva, for it stood at a less angle 

 above the horizon in the middle of the day, than in the morning 

 or evening. By 10.20 P.M. it had risen greatly. During the 

 middle of the day it oscillated much up and down. 



Trifolium subterraneum. A filament was fixed vertically to 

 the uppermost part of the peduncle of a young and upright 

 flower-head (the stem of the plant having been secured to a 

 stick); and its movements were traced during 36 h. Within 

 this time it described (see Fig. 92) a figure which represents four 

 ellipses; but during the latter part of the time the peduncle 

 began to bend downwards, and after 10.30 P.M. on the 24th it 

 curved so rapidly down, that by 6.15 A.M. on the 25th it stood 

 only 19 above the horizon. It went on circumnutating in nearly 

 the same position for two da vs. Even after the flower-heads 

 have buried themselves in the ground they continue, as will 

 hereafter be shown, to circumnutate. It will also be seen in the 

 next chapter that the sub-peduncles of the separate flowers of 



