On another occasion the petiole of a leaf was secured to a little stick close beneath the leaflets, and a filament tipped with a bead of sealing-wax was affixed to the mid-rib of one of them, and a mark was placed close behind.  At 7 P.M., when the leaflets were asleep, the filament depended vertically down, and the movements of the bead were then traced till 10.40 P.M., as shown in the following diagram (Fig. 129).  We here see that the leaflet moved a little from side to side, as well as a little up and down, whilst asleep.

* Sachs in ‘Flora,’ 1863, p. 470, etc; Pfeffer, ‘Die Period.  Bewegungen,’ etc., 1875, p. 53. [page 327]

Fig 129.  Oxalis acetosella:  circumnutation of leaflet when asleep; traced on vertical glass during 3 h. 40 m.

Oxalis Valdiviana.—­The leaves resemble those of the last species, and the movements of two leaflets (the main petioles of both having been secured) were traced during two days; but the tracings are not given, as they resembled that of O. acetosella, with the exception that the up and down oscillations were not so frequent during the day, and there was more lateral movement, so that broader ellipses were described.  The leaves awoke early in the morning, for by 6.45 A.M. on June 12th and 13th they had not only risen to their full height, but had already begun to fall, that is, they were circumnutating.  We have seen in the last chapter that the cotyledons, instead of sinking, rise up vertically at night.

Oxalis Ortegesii.—­The large leaves of this plant sleep like those of the previous species.  The main petioles are long, and that of a young leaf rose 20o between noon and 10 P.M., whilst the petiole of an older leaf rose only 13o.  Owing to this rising of the petioles, and the vertical sinking of the large leaflets, the leaves become crowded together at night, and the whole plant then exposes a much smaller surface to radiation than during the day.

Oxalis Plumierii.—­In this species the three leaflets do not surround the summit of the petiole, but the terminal leaflet projects in the line of the petiole, with a lateral leaflet on each side.  They all sleep by bending vertically downwards, but do not become at all folded.  The petiole is rather long, and, one having been secured to a stick, the movement of the terminal leaflet was traced during 45 h. on a vertical glass.  It moved in a very simple manner, sinking rapidly after 5 P.M., and rising rapidly early next morning.  During the middle of the day it moved slowly and a little laterally.  Consequently the ascending and descending lines did not coincide, and a single great ellipse was formed each day.  There was no other evidence of circumnutation, and this fact is of interest, as we shall hereafter see.

Oxalis sensitiva.—­The leaflets, as in the last species, bend vertically down at night, without becoming folded.  The much elongated main petiole rises considerably in the evening, but in [page 328] some very young plants the rise did not commence until late at night.  We have seen that the cotyledons, instead of sinking like the leaflets, rise up vertically at night.