The petiole carrying the leaflet is mounted water-tight in the short arm of an U-tube filled with water; for producing internal hydrostatic pressure in the plant the height of water in the longer arm is suitably raised. The U-tube holding the specimen may be adjusted up and down, and laterally. A hinged support also allows the specimen to be placed at any inclination. The movement of the leaflet, it is to be remembered, does not always take place in a vertical direction. The object of the mechanical adjustments is to place the specimen at such an angle that its up and down movements when in a straight line should be vertical, or have its long axis vertical when the movement is elliptical. It is important that the specimen should be illuminated equally from all sides; for one-sided illumination causes a bending over of the leaflet towards light. The pulvinule of the leaflet acts like the pulvinus of _Mimosa_, that is to say, the leaflet undergoes a sudden fall to down position by the contraction of the more effective lower half of the pulvinule; the ‘up’ position denotes recovery and expansion of the more effective half. The up-and-down movements of the leaflet correspond to the diastolic and systolic movements of the animal heart. There is, indeed, as I have shown elsewhere[W] a very close resemblance between the activities of rhythmic tissue in the plant and in the animal. [W] BOSE--Irritability of Plants--p. 295. EFFECT OF DIFFUSE LIGHT ON PULSATION OF _DESMODIUM_. _Experiment 92._--For the study of effect of light on _Desmodium_, I first obtained record in darkness. A horizontal beam of divergent light from an arc lamp placed at a distance of 200 cm. was made to act diffusely on the leaf from all sides. This was done by means of three inclined mirrors, the first throwing the light vertically downwards, the second vertically upwards, and the third horizontally forward from the side opposite the lantern. The effect of light is seen demonstrated in Fig. 86. [Illustration: FIG. 86.--Effect of light on pulsation of _Desmodium_ leaflet. Duration of application of light is represented by the horizontal line. Up-curve represents diastolic expansion and down-curve systolic contraction. Note contractile effect of light in diminution of amplitude and reduction of diastolic limit of pulsation.] Light was applied at the second pulsation. It will be seen that light retards or arrests the autonomous activity. On the cessation of light the normal activity was found to be gradually restored. It is of much interest to note here the similarity of action of light on autonomous activity of the leaflet of _Desmodium_ and of a growing organ. In both, we find that while in the sub-tonic condition of the tissue the effect of light is to enhance or renew the autonomous activity of growth and pulsation, in normal condition the effect is to retard it.