In describing thermonastic curvatures Pfeffer says that "a special power of thermonastic response has been developed by various flowers, in which low temperatures produce closing movements, and high temperatures, opening ones. The flowers of Crocus vernus and Crocus luteus are specially responsive, as also those of Tulipa Gesneriana for these flowers perceptibly respond to a change of temperature of half a degree centigrade."[7]
We have hitherto studied the response of various organs to stimulus; we have now to deal with the effect of thermal variation. Does rise of temperature act like other forms of stimuli or is its action different? We have therefore to find:
(1) The physiological effect of variation of temperature.
(2) Whether thermonastic irritability is confined only to certain classes of organs, or is it a phenomenon of very wide occurrence?
(3) Whether variation of temperature induces in anisotropic organs only one type of response, or two types, positive and negative.
(4) The law which determines the direction of responsive movement.
EFFECT OF RISE OF TEMPERATURE.
As regards the effect of rise of temperature we have seen that, within normal limits, it induces expansion and acceleration of the rate of growth. Stimulus, on the other hand, induces precisely the opposite effect. Hence the physiological reaction of steady rise of temperature is, generally speaking, antagonistic to that of stimulus. This conclusion is supported by numerous experiments which I have carried out with various plant organs. Example of this will be found in the present and subsequent chapters.