(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.

+---------------------------------------------------------------+

Specimen.

Effect of rise

Effect of stimulus.

of temperature.

+------------------+---------------------+----------------------+

_Zephyranthes_

Movement of opening

Movement of closure.

(positive type).

_Nymphaea_

Movement of closure

Movement of opening.

(negative type).

+---------------------------------------------------------------+

In _Nymphaea_ 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_.

In the _negative_ type, rise of temperature induces a movement of closure. Here the outer half of the organ is the more excitable. The flower of _Nymphaea_ belongs to this type.

The effect of stimulus is antagonistic to that of rise of temperature.

In positive thermonastic organs stimulus induces a movement of closure; in the negative type it induces a movement of opening.

XXIX.--ON PHOTOTROPISM

_By_

SIR J. C. BOSE.

In different organs of plants the stimulus of light induces movements of an extremely varied character. Radial organs exhibit tropic movements in which the position of equilibrium is definitely related to the direction of incident stimulus. Nastic movements under the action of light are, on the other hand, regarded as curvatures of the organ which show "no relation to the stimulus but is determined by the activity of the plant itself".[9] There are thus two cla.s.ses of response to light which seem to be unrelated to each other. Returning to the directive action of light, radial stems often bend towards the light, while certain roots bend away from it. It may be thought that this difference is due to specific difference of irritability between shoot and root, the irritability of the former being of a positive, and of the latter, of a negative character. But there are numerous exceptions to this generalisation. Certain roots bend towards the light, while a stem, under different circ.u.mstances, moves towards light or away from it.

Again an identical organ may exhibit a positive or a negative curvature.

Thus the leaflets of _Mimosa pudica_ acted on by light from above fold upwards, the phototropic effect being _positive_. But the same leaflets acted on by light from below exhibit a folding upwards, the phototropic effect being now _negative_. Effects precisely the opposite are found with the leaflets of _Biophytum_ and _Averrhoa_. They fold downwards whether light acts from above or below. Finally, a radial organ in found to exhibit under light of increasing intensity or duration, a positive, a dia-phototropic, or a negative phototropic curvature.

[9] Jost--_Ibid_, p. 428.

In these circ.u.mstances the theory of specific positive and negative irritabilities is untenable; in any case, it throws no light on the phenomenon of movement. The difficulties of the problem are thus clearly stated by Pfeffer: "When we say that an organ curves towards a source of illumination, because of its heliotropic irritability and we are simply expressing an ascertained fact in a conveniently abbreviated form, without explaining why such curvature is possible or how it is produced.... Many observers have unfortunately devoted their attention to artificially cla.s.sifying the phenomenon observed, and have entirely neglected the explanation of causes underlying them."[10]

[10] Pfeffer--_Ibid_, Vol. II, p. 74.