Fig. 164.  Strephium floribundum:  culms with leaves during the day, and when asleep at night.  Figures reduced.

Horizontally extended leaves arising from much inclined or almost horizontal culms, move at night so that their tips point towards the apex of the culm, with one lateral margin directed towards the zenith; and in order to assume this position the leaves have to twist on their own axes through an angle of nearly 90o.  Thus the surface of the blade always stands vertically, whatever may be the position of the midrib or of the leaf as a whole.

The circumnutation of a young leaf (2.3 inches in length) was traced during 48 h. (Fig. 165).  The movement was remarkably simple; the leaf descended from before 6.40 A.M. until 2 or 2.50 P.M., and then rose so as to stand vertically at about 6 P.M., descending again late in the night or in the very early morning.

* A. Brongniart first observed that the leaves of this plant and of Marsilea sleep:  see ‘Bull. de la Soc.  Bot. de France,’ tom. vii. 1860, p. 470. [page 392]

On the second day the descending line zigzagged slightly.  As usual, the ascending and descending lines did not coincide.  On another occasion, when the temperature was a little higher, viz., 24o — 26 1/2o C., a leaf was observed 17 times between 8.50 A.M. and 12.16 P.M.; it changed its course by as much as a rectangle six times in this interval of 3 h. 26 m., and described two irregular triangles and a half.  The leaf, therefore, on this occasion circumnutated rapidly and in a complex manner.

Fig. 165.  Strephium floribundum:  circumnutation and nyctitropic movement of a leaf, traced from 9 A.M.  June 26th to 8.45 A.M. 27th; filament fixed along the midrib.  Apex of leaf 8 1/4 inches from the vertical glass; plant illuminated from above.  Temp. 23 1/2o — 24 1/2o C.

ACOTYLEDONS.

Marsilea quadrifoliata (Marsileaceae).—­The shape of a leaf, expanded horizontally during the day, is shown at A (Fig. 166).  Each leaflet is provided with a well-developed pulvinus.  When the leaves sleep, the two terminal leaflets rise up, twist half round and come into contact with one another (B), and are afterwards embraced by the two lower leaflets (C); so that the four leaflets with their lower surfaces turned outwards form a vertical packet.  The curvature of the summit of the petiole of the leaf figured asleep, is merely accidental.  The plant was brought into a room, where the temperature was only a little above 60o F., and the movement of one of the leaflets (the petiole having been secured) was traced [page 393] during 24 h. (Fig. 167).  The leaf fell from the early morning till 1.50 P.M., and then rose till 6 P.M., when it was asleep.  A

Fig. 166.  Marsilea quadrifoliata:  A, leaf during the day, seen from vertically above; B, leaf beginning to go to sleep, seen laterally; C, the same asleep.  Figures reduced to one-half of natural scale.