downwards as almost to form a semicircle.  The chord—­that is, a line drawn from the apex of the blade to the base of the petiole—­of a young leaf, 4 3/4 inches in length, stood at 2.50 P.M. on [page 248] Dec. 5th at an angle of 13o beneath the horizon, but by 9.30 P.M. the blade had straightened itself so much, which implies the raising of the apex, that the chord now stood at 37o above the horizon, and had therefore risen 50o.  On the next day similar angular measurements of the same leaf were made; and at noon the chord stood 36o beneath the horizon, and 9.30 P.M. 3 1/2o above it, so had risen 39 1/2o.  The chief cause of the rising movement lies in the straightening of the blade, but the short petiole rises between 4o and 5o.  On the third night the chord stood at 35o above the horizon, and if the leaf occupied the same position at noon, as on the previous day, it had risen 71o.  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 exposure to a strong light is apparently requisite for the periodical change of curvature in the blade, and for the slight rising of the petiole.

(21.) Wigandia (Hydroleaceae, 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 plants.

Fig. 111.  Petunia violacea:  downward movement 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 17 1/2o C. [page 249]

(22.) Petunia violacea (Solaneae, Fam. 157).—­A very young leaf, only 3/4 inch in length, highly inclined upwards, was observed for four days.  During the whole of this time it bent outwards and downwards, so as to become more and more nearly horizontal.  The strongly marked zigzag line in the figure on p. 248 (Fig. 111), shows that this was effected by modified circumnutation; and during the latter part of the time there was much ordinary circumnutation on a small scale.  The movement in the diagram is magnified between 10 and 11 times.  It exhibits a clear trace of periodicity, as the leaf rose a little each evening; but this upward tendency appeared to be almost conquered by the leaf striving to become more and more horizontal as it grew older.  The angles which two older leaves formed together, were measured in the evening and about noon on 3 successive days, and each night the angle decreased a little, though irregularly.

Fig. 112.  Acanthus mollis:  circumnutation of young leaf, traced from 9.20 A.M.  June 14th to 8.30 A.M. 16th.  Apex of leaf 11 inches from the vertical glass, so movement considerably magnified.  Figure here reduced to one-half of original scale.  Temp. 15o — 16 1/2o C.