(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. DIFFERENT THERMONASTIC ORGANS. The only condition requisite for the exhibition of response is the differential excitability of an anisotropic organ. It is therefore likely to be exhibited by a large variety of plant organs, such as pulvini, petioles, leaves, and flowers, and my results show that this is actually the case. This particular sensibility, moreover, is not confined to delicate structures, but is extended to rigid trees and their branches. Before proceeding further, it is necessary to draw attention to the confusion which arises from the use of the common prefix '_thermo_' in thermonasty and thermo-tropism. With regard to this Pfeffer says "It is not known whether radiated and conducted heat exercise a similar thermotropic reaction."[8] I shall show that the reactions to radiant heat, and to conducted heat (rise of temperature) are of opposite character, radiation inducing contraction, and rise of temperature, expansion. It is therefore advisable to distinguish the thermal, or temperature effect, from the radio-thermal effect of infra-red radiation. [8] Pfeffer--_Ibid_, Vol. III, p. 177. TWO TYPES OF RESPONSE. As regards the effect of variation of temperature I shall proceed to show that there are two distinct types, which I shall, for convenience, distinguish as the _Positive_ and _Negative_. Positive thermonastic reaction is exhibited by organs in which the upper half is the more excitable. Response to rise of temperature is by _downward_ or _outward_ movement. In floral organs this finds expression by a movement of opening. In illustration of this may be cited the examples of the well known Crocus and also of _Zephyranthes_. Negative thermonastic movement is shown by organs in which the lower half is the more excitable. Here the response to rise of temperature is by an _upward_ or _inward_ movement. I shall show that an example of this is furnished by the flower of _Nymphæa_ which closes under rise, and opens during fall of temperature. POSITIVE THERMONASTIC RESPONSE.