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Show 248 CIRCUMNUTATION OF LEAVES. CHAP. IV. Dec. 5th at an angle of 13° beneath tho horizon, but by 9.30ur. tho blade had straightened itself Fig. 111. so much, which implies tho 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 vertienl glass. Temp. generally 17i° C. raising of the apex, that tho chord now stood at 07° above the horizon, and had therefore rison 50°. On the next clay similar angular measurements of the same leaf were made; and at noon the chord stood 36° 00. neath tho horizon, and 9.30 P.M. 3!0 above it, so had risen 39t0 • The chief cause of tho rising movement lies in tho straighten· ing of the blade, hut the short petiole rises between 4° and 5°. On the third night tho chord stood at 35° above tho horizon, and if tho leaf occupied tho s::tme position at noon, as on the previous d::ty, it had rison 71°. With older loaves no such change of curvature could bo detected. Tho pl::tnt was then brought into the house and kept in a north-cast room, but at night thoro was no change in the curvature of the young leaves; so that previous expo· sure to a strong light is app~· rcntly requiRitc for the pcriodi· cal chango of curvature i~ ~ho blade, and for tho slight l'JS!llg of tho petiole. (21.) Wigandia (IIydrolcacero, Fam. 149).-Profcssor Pfeff~r informs us that tho leaves of thiS plant rise in tho evening; but as we do not know whether or ~ot tho rising is groat, this speCies ought perhaps to he classed amongst sleeping plants. CHAP. IV. DICOTYLEDONS. 249 (22.) Petunia violacea (Solanero, Fam. 157).-A very young leaf, only! inch in length, highly inclined upwards, was observed for four days. During tho 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 fiO'uro on p. 248 (Fig. lll), shows that this was effected by modified circumnutation; and during the latter pn.rt of the time thoro 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 ro. e a little each evening; but this upward tendency appeared to be almost conquered by the leaf striving to become more and more horizon tal as it grew older. Tho angles which two older leaves formed together, were measured in the evening and about noon on 3 successive days, and each night the anglo decreased a little, though inegularly. Acanthus mollis : circumnutation of young leaf, traced fi·om 9.20 A.~t. June 14th to 8.30 A.M. 16th. Apex of leaf 11 inches from the vertical glass, so movement considorably magnified. Figure here red ueed to onehalf of original scale. Temp. 15°-16!° C. (23.) .Acanth~ts mollis (Acanthacere, Fam. 168).- The younger of two ~eaves, 2.t inches in length, petiole mcluded, produced by a seedling plant, was observed during 47 h. Early on each of the three mornings, the apex of the leaf fell · and it continued to fall till 3 P.rr:., on the two afternoons when observed. Afte~ 3 P.M. it rose considerably, and con~mued to rise on the second night until the early morning. But on the first night it fell instead of rising, and we have little doubt that this was owing to the leaf being very you~g and becoming through epinastiC growth more and more horiz? ntal; for it may be seen in the diagram (Fig. 112), that tho leaf stood on a higher level on the first than on th~ second clay. Tho loaves of an allied species (A. spinosus) certainly ro~e every night ; and the rise between noon and 10.15 P.M., w en measured on one occasion, was 10°. This rise was chiefly |