Occasionally the leaflet stood still for a short time. There was no jerking movement, which is so characteristic of the little lateral leaflets. A sudden and considerable fall of temperature causes the terminal leaflet to sink downwards; thus a cut-off leaf was immersed in water at 95o F., which was slowly raised to 103o F., and afterwards allowed to sink to 70o F., and the sub-petiole of the terminal leaflet then curved downwards. The water was afterwards [page 360]
Fig. 150. Desmodium gyrans: circumnutation and nyctitropic movement of leaf (3 3/4 inches in length, petiole included) during 48 h. Filament affixed to midrib of terminal leaflet; its apex 6 inches from the vertical glass. Diagram reduced to one-third of original scale. Plant illuminated from above. Temp. 19o - 20o C. [page 361]
raised to 120o F., and the sub-petiole straightened itself. Similar experiments with leaves in water were twice repeated, with nearly the same result. It should be added, that water raised to even 122o F. does not soon kill a leaf. A plant was placed in darkness at 8.37 A.M., and at 2 P.M. (i.e. after 5 h. 23 m.), though the leaflets had sunk considerably, they had by no means acquired their nocturnal vertically dependent position. Pfeffer, on the other hand, says* that this occurred with him in from 3/4 h. to 2 h.; perhaps the difference in our results may be due to the plant on which we experimented being a very young and vigorous seedling.
The Movements of the little Lateral Leaflets .--These have been so often described, that we will endeavour to be as brief as possible in giving a few new facts and conclusions. The leaflets sometimes quickly change their position by as much as nearly 180o; and their sub-petioles can then be seen to become greatly curved. They rotate on their own axes, so that their upper surfaces are directed to all points of the compass. The figure described by the apex is an irregular oval or ellipse. They sometimes remain stationary for a period. In these several respects there is no difference, except in rapidity and extent, between their movements and the lesser ones performed by the large terminal leaflet whilst making its great oscillations. The movements of the little leaflets are much influenced, as is well known, by temperature. This was clearly shown by immersing leaves with motionless leaflets in cold water, which was slowly raised to 103o F., and the leaflets then moved quickly, describing about a dozen little irregular circles in 40 m. By this time the water had become much cooler, and the movements became slower or almost ceased; it was then raised to 100o F., and the leaflets again began to move quickly. On another occasion a tuft of fine leaves was immersed in water at 53o F., and the leaflets were of course motionless. The water was raised to 99o, and the leaflets soon began to move; it was raised to 105o, and the movements became much more rapid; each little circle or oval being completed in from 1 m. 30 s. to 1 m. 45 s. There was, however, no jerking, and this fact may perhaps be attributed to the resistance of the water.
Sachs states that the leaflets do not move until the surrounding air is as high as 71o - 72o F., and this agrees with our
* 'Die Period. Beweg.,' p. 39. [page 362]
experience on full-grown, or nearly full-grown, plants. But the leaflets of young seedlings exhibit a jerking movement at much lower temperatures. A seedling was kept (April 16th) in a room for half the day where the temperature was steady at 64o F., and the one leaflet which it bore was continually jerking, but not so rapidly as in the hot-house. The pot was taken in the evening into a bed-room where the temperature remained at 62o during nearly the whole night; at 10 and 11 P.M. and at 1 A.M. the leaflet was still jerking rapidly; at 3.30 A.M. it was not seen to jerk, but was observed during only a short time. It was, however, now inclined at a much lower angle than that occupied at 1 A.M. At 6.30 A.M. (temp. 61o F.) its inclination was still less than before, and again less at 6.45 A.M.; by 7.40 A.M.