Fig. 132.  Averrhoa bilimbi:  leaf asleep; drawing reduced.

they are motionless, but this may be due to the opposite ones being pressed together (Fig. 132).  The main petiole is in constant movement during the day, but no careful observations were made on it.  The following diagrams are graphic representations of the variations in the angle, which a given leaflet makes with the vertical.  The observations were made as follows.  The plant growing in a pot was kept in a high temperature, the petiole of the leaf to be observed pointing straight at the observer, being separated from him by a vertical pane of glass.  The petiole was secured so that the basal joint, or pulvinus, of one of the lateral leaflets was at the centre of a graduated arc placed close behind the leaflet.  A fine glass filament was fixed to the leaf, so as to project like a continuation of the

* Dr. Bruce, ‘Philosophical Trans.,’ 1785, p. 356.

** ‘Journal Linn.  Soc.,’ vol. xvi. 1877, p. 231. [page 331]

midrib.  This filament acted as an index; and as the leaf rose and fell, rotating about its basal joint, its angular movement

Fig. 133.  Averrhoa bilimbi:  angular movements of a leaflet during its evening descent, when going to sleep.  Temp. 78o — 81o F.

could be recorded by reading off at short intervals of time the position of the glass filament on the graduated arc.  In order [page 332] to avoid errors of parallax, all readings were made by looking through a small ring painted on the vertical glass, in a line with the joint of the leaflet and the centre of the graduated arc.  In the following diagrams the ordinates represent the angles which the leaflet made with the vertical at successive instants.* It follows that a fall in the curve represents an actual dropping of the leaf, and that the zero line represents a vertically dependent position.  Fig. 133 represents the nature of the movements which occur in the evening, as soon as the leaflets begin to assume their nocturnal position.  At 4.55 P.M. the leaflet formed an angle of 85o with the vertical, or was only 5o below the horizontal; but in order that the diagram might get into our page, the leaflet is represented falling from 75o instead of 85o.  Shortly after 6 P.M. it hung vertically down, and had attained its nocturnal position.  Between 6.10 and 6.35 P.M. it performed a number of minute oscillations of about 2o each, occupying periods of 4 or 5 m.  The complete state of rest of the leaflet which ultimately followed is not shown in the diagram.  It is manifest that each oscillation consists of a gradual rise, followed by a sudden fall.  Each time the leaflet fell, it approached nearer to the nocturnal position than it did on the previous fall.  The amplitude of the oscillations diminished, while the periods of oscillation became shorter.