Effect of thermal radiation: Experiment 114.—I stated that the effect of thermal radiation acts as a stimulus, inducing a reaction which is antagonistic to that of rise of temperature. In verification of this, I subjected the specimen to the action of infra-red radiation acting from all sides. The result is seen in the responsive movement of closure (Fig. 109 R). These experiments demonstrate clearly that the responses to rise of temperature and thermal radiation are of opposite signs.
As a movement of closure was induced by the diffuse stimulus of thermal radiation, it is evident that this must have been brought about by the greater contraction of the inner half of the perianth; hence the inner half of the organ is relatively the more excitable.
Fig. 110.—The Thermonastic Recorder. T, metallic thermometer attached to the short arm of the upper lever; the specimen of Nymphæa, N, has one of its perianth leaves attached to the short arm of the second lever by a thread. C, clockwork for oscillation of the plate.
NEGATIVE THERMONASTIC RESPONSE.
Response of Nymphæa: Experiment 115.—Many of the Indian Nymphæaceæ have their sepals and petals closed during the day, and open at night. I find that the perianth leaves of this flower are markedly sensitive to variation of temperature. The Thermonastic Recorder employed in this investigation is shown in figure 110. The record given in figure 111 shows that the perianth segment, subjected to a few degrees' rise of temperature, responded by an up-movement of closure, due to greater expansion of the outer half. The latent period was 6 seconds, and the maximum effect was attained in the further course of 21 seconds. This experiment shows that the thermonastic response of this flower is of the negative type.
Fig. 111.—Negative thermonastic response of Nymphæa. Application of warmth at the vertical mark induced up-movement of closure, but stimulus of electric shock at arrow induced rapid excitatory down movement of opening. Successive dots at intervals of a second.
Effect of stimulus: Experiment 116.—In the positive type of thermonastic organs, where rise of temperature induced a movement of opening, stimulus induced the opposite movement of closure (Expt. 114). We shall now study the effect of stimulus on the movement of Nymphæa, which undergoes closure during rise of temperature, as seen in the first part of the record in figure 111. Stimulus of electric shock was applied at the point marked with an arrow; the response is seen to be by a movement of opening. Here also we find the effects of rise of temperature and of stimulus to be antagonistic to each other. This will be clearly seen in the following tabular statement.
TABLE XXIX.—SHOWING THE EFFECT OF RISE OF TEMPERATURE AND OF STIMULUS ON THERMONASTIC ORGANS.