Part 6 (1/2)

(2) Whether thermonastic irritability is confined only to certain cla.s.ses 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.

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 ill.u.s.tration 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 _Nymphaea_ 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 5C. there was an up-movement, or a movement of closure. Rise of temperature induced, on the other hand, a movement of opening.

[Ill.u.s.tration: 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.]

_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.

[Ill.u.s.tration: FIG. 110.--The Thermonastic Recorder. T, metallic thermometer attached to the short arm of the upper lever; the specimen of _Nymphaea_, 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_ Nymphaea: _Experiment 115._--Many of the Indian _Nymphaeaceae_ 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.

[Ill.u.s.tration: FIG. 111.--Negative thermonastic response of _Nymphaea_.

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 _Nymphaea_, 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.

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Specimen.

Effect of rise

Effect of stimulus.

of temperature.