[Illustration: 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. +---------------------------------------------------------------+ | Specimen. | Effect of rise | Effect of stimulus. | | | of temperature. | | +------------------+---------------------+----------------------+ | _Zephyranthes_ | Movement of opening | Movement of closure. | | (positive type). | | | | | | | | _Nymphæa_ | Movement of closure | Movement of opening. | | (negative type). | | | +---------------------------------------------------------------+ In _Nymphæa_ it is the outer side of the perianth that is relatively the more excitable since diffuse electric stimulus induces a movement of opening due to the greater contraction of the outer side. It is by the greater expansion of this more excitable side that the movement of closure is effected during rise of temperature. From the results of experiments given above we arrive at the following:-- LAW OF THERMONASTIC REACTION RISE OF TEMPERATURE INDUCES A GREATER EXPANSION OF THE MORE EXCITABLE HALF OF AN ANISOTROPIC ORGAN. SUMMARY. Thermonastic movements are induced by the differential physiological effect of variation of temperature on the two halves of an anisotropic organ. Rise of temperature induces greater expansion, and enhancement of rate of growth of the more excitable half of the organ; lowering of temperature induces the opposite effect. Two types of thermonastic movements are met with, the _positive_ exhibiting a movement of opening during rise of temperature; in these the inner half of the organ is relatively the more excitable. Example of this is seen in the _Crocus_ and in _Zephyranthes_.