CHAP. VII. 



SLEEP OF LEAVES. 



357 



Fig. 148. 



petioles to support them in a vertical position, so that each 

 night the main petiole became twisted, and all the packets were 

 extended horizontally, with the lower surfaces of the leaflets on 

 one side directed to the zenith in a most anomalous manner. 

 This fact is mentioned solely as a caution, as it surprised us 

 greatly, until we discovered that it was an anomaly. The 

 petioles are inclined upwards during the day, but sink at night, 

 so as to stand at about right angles with the stem. The amount 

 of sinking was measured only on one occasion, and found to be 

 39. A petiole was secured to a stick at the base of the two 

 terminal leaflets, and the circumnutating movement of one of 

 these leaflets was traced from 6.40 A.M. to 10.40 P.M., the plant 

 being illuminated from above. The temperature was 17 17^ C., 

 and therefore rather too low. During the 16 h. the leaflet moved 

 thrice up and thrice down, and as the ascending and descend- 

 ing lines did not coincide, three ellipses were formed. 



Desmodium gyrans (Tribe 6). A large and full-grown leaf of 

 this plant, so famous for the spontaneous 

 movements of the two little lateral leaflets, 

 is here represented (Fig. 148). The large 

 terminal leaflet sleeps by sinking vertically 

 down, whilst the petiole rises up. The coty- 

 ledons do not sleep, but the first-formed leaf 

 sleeps equally well as the older ones. The 

 appearance presented by a sleeping branch 

 and one in the day-time, copied from two 

 photographs, are shown at A and B (Fig. 

 149), and we see how at night the leaves are 

 crowded together, as if for mutual pro- 

 tection, by the rising of the petioles. The 

 petioles of the younger leaves near the sum- 

 mits of the shoots rise up at night, so as to 

 stand vertical and parallel to the stem ; 

 whilst those on the sides were found in four 

 cases to have risen respectively 46 , 36, 

 20, and 19 '5 above the inclined positions 

 which they had occupied during the day. 

 For instance, in the first of these four cases 

 the petiole stood in the day at 23, and at 

 night at 69-i above the horizon. In the 

 evening the rising of the petioles is almost 

 completed before the leaflets sink perpendicularly downwards. 



Desmodium gyrans: 

 leaf seen from 

 above, reduced 

 to one-half na- 

 tural size. The 

 minute stipules 

 unusually largft. 



