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.
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.
Response of Zephyranthes: Experiment 113.—Viewed from the top, the inner side of the petal of a flower is the upper side. The Crocus flower under rise of temperature opens outwards by expansion of the inner side, which must be the more excitable. As Crocus was not available in Calcutta, I found the flower of Zephyranthes (sometimes called the Indian Crocus) reacting to variation of temperature in a manner similar to that of Crocus, that is to say, the flower opens under rise and closes with a fall of temperature. For obtaining record all the perianth segments but one was removed. This segment was attached to the recording lever. On lowering of temperature through 5°C. there was an up-movement, or a movement of closure. Rise of temperature induced, on the other hand, a movement of opening.
Fig. 109.—Thermonastic and radionastic responses of petal of Zephyranthes C, closing movement due to cooling, and H, opening movement due to warming; R, closing movement due to heat-radiation. Note opposite responses to rise of temperature and to thermal radiation.