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.

Inspection of the record exhibits another very interesting char­ac­ter­is­tic. We saw that light retarded growth by inducing an incipient contraction. In the Desmodium leaflet the contractile reaction of light is exhibited by the char­ac­ter­is­tic modi­fi­ca­tion of its pulsations. The duration of application of light is represented by the horizontal line. In Fig. 86 the up-curve represents up-movement of diastolic expansion, and the down-curve of systolic contraction. The contractile reaction of light is seen to counteract the normal expansion, with diminution of diastolic limit of pulsation.

EFFECT OF RISE OF TEMPERATURE ON PULSATION.

It has been shown that while rise of temperature up to an optimum enhanced the rate of growth, the effect of light was to retard it. Hence the effects of light and warmth are antagonistic.

Fig. 87.—Effect of rise of temperature on pulsation of leaf­let of Desmodium gyrans. Hori­zon­tal line repre­sents the dura­tion of gradual rise of tem­pera­ture from 30°C. to 35°C. Note the ex­pan­sive effect of rise of tem­pera­ture in re­duc­tion of sys­tolic limit of pul­sa­tion.

Effect of rise of temperature on pulse-record: Experiment 93.—In studying the effect of rise of temperature on the pulsation of leaflets of Desmodium, we discover similar antagonistic reactions of light and warmth. The leaflet was placed in a plant-chamber with an electric arrangement for gradual rise of temperature. The first two records were taken in the normal temperature of the room, which was 30°C. The temperature was gradually raised to 35°C, the record being taken all the time. It will be seen (Fig. 87) that the effect of warmth is diametrically opposite to that of light. The record in [Fig. 86] exhibited the contractile effect of light by reducing the diastolic limit of expansion. In the present case the expansive reaction of warmth is exhibited by the reduction of systolic limit of contraction. The temperature of the plant chamber was now allowed to return to 30°C., and we observe the gradual restoration of normal systolic limit of contraction.

SUMMARY.

Two different effects are found in the action of the stimulus of light alike on the autonomous activity of leaflet of Desmodium gyrans and of growing organs. In condition of sub-tonicity light renews pulsation of Desmodium and enhances the activity of growth. In normal tonic condition the effect induced is the very opposite, light causing an arrest of pulsation and retardation or arrest of growth.