248 



CIECUMNUTATION OF LEAVES. 



Chap. IS 



Dec. 5th at an angle of 13° beneath the horizon, but by 9.30 p.bi. 



the blade had straightened itself 



Fig. Ill, 



Petunia violacea : downward move- 

 ment and circumnutation of a 

 very young leaf, traced from 10 

 A.M. June 2nd to 9.20 A.M. June 

 6th. N.B. — At 6.40 A.M. on the 

 5th it was necessary to move the 

 pot a little, and a new tracing 

 was begun at the point where 

 two dots are not joined in the 

 diagram. Apex of leaf 7 inches 

 from the vertical glass. Temp, 

 generally 17J° C. 



SO much, which implies the 

 raising of the apex, that the 

 chord now stood at 37° above the 

 horizon, and had therefore risen 

 50°. On the next day similar 

 angular measurements of the 

 same leaf were made; and at 

 noon the chord stood 36° be- 

 neath the horizon, and 9.30 p.m. 

 3§° above it, so had risen 39J°. 

 The chief cause of the rising 

 movement lies in the straighten- 

 ing of the blade, but the short 

 petiole rises between 4° and 5°. 

 On the third night the chord 

 stood at 35° above the horizon, 

 and if the leaf occupied the 

 same position at noon, as on 

 the previous day, it had risen 

 71°. With older leaves no such 

 change of curvature could be 

 detected. The plant was then 

 brought into the house and 

 kept in a north-east room, but 

 at night there was no change 

 in the curvature of the young 

 leaves; so that previous expo- 

 sure to a strong light is appa- 

 rently requisite for the periodi- 

 cal change of curvature in the 

 blade, and for the slight rising 

 of the petiole. 



(21.) Wiyandia (Hydroleacese, 

 Fam. 149).— Professor Pfeffer 

 informs us that the leaves of this 

 plant rise in the evening ; but as 

 we do not know whether or not 

 the rising is great, this species 

 ought perhaps to be classed 

 amongst sleeping jjlants 



