The Motor Mechanism of Plants

Fig. 1 1 7. The Quadrant Method for determination of variation of electric resistance under photic scimulation. Two opposite quadrants of the leaf are shaded. Electric connexions ait made at the junction of the quadrants b p. with the battery and g g' with the galvanometer, v v' are opaque screens to shield opposite quadrants from light (Tropaeolum). the fourth is adjusted slightly to the right or to the left till , exact balance is obtained in darkness, when P Q = R S. One pair of opposite quadrants, say P and Q, is shaded by a double V-shaped screen V V'. Exposure of the leaf to light produces a variation of resistance, not of one, but of two opposite arms of the bridge, R and ^ the upsetting of the . balance is thus due to the product of the variations of resist- 9 ance in the two opposite quadrants. The sensitiveness of the method is very great, and a large galvanometric deilec- H tion in a particular direction indicates a diminution of the electric resistance of the quadrants stimulated by light. The result is further verified by a reversal experiment, when the double V-shaped screen is turned through 90° ; the quadrants

P and Q are then exposed to light, R and S being shaded from it. The resulting upset of the balance causes a galvano¬ meter deflection which is now in the opposite direction. The reliability and sensitiveness of the Quadrant Method may thus be tested by obtaining opposite responses on alternate illumination of the two pairs of quadrants. Experiment 113. — After securing a perfect balance in the dark, the double V-shaped screen is fixed, and the leaf is mounted in a rectangular dark chamber, closed except at the front which carries a photographic shutter by which

Fig. 1 1 8. Equal responses in opposite directions on alternate illumination of the two pairs of quadrants (Tropaeolum). one pair of quadrants can be exposed to light for a definite time, the source of light is an arc-lamp placed inside a lantern, the condenser of which sends out a parallel beam of light. A rectangular glass trough filled with alum-solution is interposed in the path of the light to absorb the heat-rays. The time of exposure is kept constant. Fig. 118 shows the

equal and opposite responses caused by alternate stimula¬ tion of the two pairs of opposite quadrants. Experiment 114. — It is possible to render the Quadrant Method extremely sensitive. 1 his is illustrated in the record obtained of the effect of a spark produced by the dis¬ charge of a Leyden jar, the time of illumination being of the Fig. 120. Effect of stimulus of light increasing in the ratio order of about a hundred -thousandth part of a second. This spark flashed at a distance of 15 cm. from the leaf ; the response is seen to consist of a preliminary positive twitch followed by a large negative response (fig. 119).

Experiment 115. — The arc-lamp was taken out of the lantern, and the responses to the diverging beam of light obtained at different distances from its source. As the in¬ tensity of light varies inversely as the square of the distance, suitable marks were made on a scale fixed on the table, so that the intensity of light incident on the leaf was increased in the proportion of i, 2, 5, 7 by bringing the lamp nearer to the leaf at the particular distances marked on the scale. The time of exposure was kept the same. The increasing intensity of light is seen to produce increasing amplitude of response (fig. 120).

Experiment 116. Effect of dilute vapour of chloroform on response to light. — This gives rise to a preliminary enhance¬ ment of response followed by decline and abolition, an effect similar to those already observed by the methods of mechanical and electromotive responses. I must anticipate certain results which are given in detail in a subsequent chapter. It will be shown that a sudden excitatory contraction of the plant takes place at the fatal temperature, normally at or about 6o° C. There is also produced at this critical point a sudden change ot galvano- metric negativity, indicative of intense excitation at death.

Experiment 117. Death-response and sudden diminution of resistance. — The electric resistance of the pistil of Hibiscus was found to undergo a sudden diminution at the critical temperature of about 6o° C. As stated in a previous chapter there are two different reactions induced by stimulation, a positive and a negative. The positive reaction of a subtonic tissue, or of an ordinary tissue under feeble stimulation, is shown externally : (a) bv expansion; ( b ) by an electromotive variation of gaivano-

metric positivity ; and (c) by an increase of the electric resistance of the tissue. The negative reaction, ori the other hand, is exhibited (a) by contraction, (6) by an electromotive change of galvanometric negativity, and (c) by a diminution of the electric resistance of the tissue. The following table shows parallel reactions manifested by diverse modes of response, mechanical and electric. Mechanical or electric stimulation induces a diminution of the electric resistance of the tissue.

The physiological character of the response by resistance- variation is demonstrated by the action of anaesthetics liKe chloroform, which produces a rapid diminution or the amplitude of response. The response of subtonic tissue is an increase of resistance, instead of a diminution. Strong stimulation gives rise to a series of multiple responses. A very sensitive means for detecting the effect of light is the Quadrant Method of resistance-variation, the quadrants of the lamina of a leaf serving as the four arms of a Wheat¬ stone Bridge.

The normal response to the stimulus of light is a diminu¬ tion of electric resistance. Increasing intensity ‘of light induces a corresponding diminution of resistance. A sudden diminution of resistance occurs at the critical point of death. Responses by mechanical movement, by electromotive change, and by resistance- variation, are different expressions of fundamental protoplasmic reactions induced by external stimulation. The various symptoms of death in plants, such as drooping, withering, uiscoloration, and the escape of coloured cell-sap, do not manifest themselves at the moment of death, but at a very much later period. Even after a plant has been sub¬ jected to a temperature in excess of the fatal degree, it continues for a time to appear fresh and living. How then is it possible to distinguish a living from a dead plant, and to determine the exact moment of transition ?

Such a discrimination is possible by watching the waning of some characteristic sign of the living condition, the death-point being indicated by its disappearance at the critical moment. I have shown that the electric response affords a criterion of the living condition of the tissue, the response disappearing after the death oi. the organ. Ire ideally perfect method, However, would be the discovery of a reaction which, at the moment of death, should undergo a sudden reversal to its opposite. 1 here would then be not even that minor degree of uncertainty which is inseparable from the determination of the vanishing point of a waning effect. Such a perfect method has been discovered by the detection of various spasmodic changes occurring in the plant at the moment of death.1

In order to obtain a record of the death-reaction it is necessary to bring about death in a gradual manner. Tor this, two different methods have been found suitable. 1 he first is to subject the plant to a continuous rise of tempera¬ ture till the fatal degree is reached. The second method, not so perfect as the first, is to apply a dose of dilute poison which proves fatal after a longer or shorter period, depending on the dose and on the virulence of the poison,

I will first show that a violent spasmodic movement occurs in Mimosa at the moment of death. The specimen of Mimosa is placed inside a double-walled bath ; the outer bath is filled with water, and gradual rise of temperature is produced by application of heat outside, the plant being placed in the internal chamber kept in proper humid condition. It is necessary to produce a gradual and continuous rise of temperature, for any sudden variation would produce an excitatory fall of the leaf.

Experiment ii8.- — A leaf of the plant was attached to the recording-lever in the usuai manner. The recording apparatus was of the oscillating type, in which the plate is made to move to-and-fro by an electro-magnetic, con¬ trivance, so that 'the record consists of a series of dots. The record of the movement of the leaf w as taken under a definite rise of temperature of i° C. per minute, from 25 ° C. upwards ; the down-curve represents expansion, the up- curve contraction (fig. 121). The curve shows an increasing expansive movement which was continued till the tempera¬ ture reached 60 0 C. After this a violent and spasmodic con¬ traction took place, the curve exhibiting a sudden inversion. The intensity and abruptness of the contractile movement was so V3ry great that the recording-lever was jerked off the recording-plate. In order to represent adequate^ the con¬ tractile portion of the curve, the intervals between successive dots had to be reduced so as to represent one-fifth of a degree.

The sudden contraction at 6o° C. is the death-spasm of the plant, for it is the last response given by the plant, as will be seen from what follows. Experiment 119. — In plants with thick stems, the attain¬ ment of the external fatal temperature by the interior of the plant is a slow process ; hence stout plants have to be sub¬ jected for a longer period to the fatal temperature to ensure death. It often happens that in such cases the temperature of the bath may rise one or two degrees above the critical

Successive dots in down or expansive part of curve lepresent rise of temperature of T C. per minute. Spasmodic contraction causing inversiou of curve took place at 6o° C. temperature of 60 0 ; hence thick organs, generaUy ^peaking, show a slightly higher death-point. Thin seedlings, how¬ ever quickly succumb to the action of the fatal temperature. Two’ batches of similar seedlings of Mimosa were placed in the same bath, the rate of rise of temperature being i C. per minute, The first batch was taken out of the bath at a few degrees below bo" and placed in water at ordinary temperature. The second batch in the thermal bath ex¬ hibited the spasmodic fall of the leaves at 6o°, after which

they also were placed in water at the temperature of the room. The first batch exhibited, after 2 hours, renewed signs of life and excitability, whereas the second batch never revived. The spasmodic movement of the fall of the leaf at 6o° C. may therefore be regarded as the death-spasm in plants, corresponding to the death-throe in animals. Experiment 120. — 7' he total abolition of response after exposure to the critical temperature is independently shown by the accompanying record, obtained with a particular specimen after the record of its death-spasm. The hot water in the bath was replaced by cold, and a record was

Fig. 122. Abolition of response to warming or cooling in specimen which had passed the death-point. taken of the effect on the plant of rise and of fall of tempera¬ ture. In the lower curve is shown the record of the effect of rise of temperature from 45 0 to 65°, and in the upper, the effect of cooling from 6o° to 450. It is seen that while the fiving plant exhibited a spasmodic contraction at 6o° there is no such effect in the present case (fig. 122). The very slight movement observable in the two curves is the physical effect of heating and cooling, which is quite negligible com¬ pared with the physiological erectile movement due to warming and the subsequent spasmodic contractile move¬ ment heralding the initiation of death-change.

Experiment 12 1. — As the death-spasm is a physiological response, a depression of vital activity may be expected to cause a dislocation of the death-point. This is found to be the case, for the critical death-point of a fatigued plant is found to be considerably lower than that of a fresh plant. In a particular instance, fatigue induced by tetanising induc¬ tion-shocks lowered the death-point from the normal bo° C. to 370 C., that is to say, by as much as 23° (fig. 123). Wound or injury to the tissue also lowers the death-point.

There is some instability in the death-response between the tempera¬ tures of 550 and 60 0 C. If the speci¬ men is suffering from injury, the death-point is found to be lowered by 3 or 4 degrees. A young plant dies a little earlier than an old one. t* or reasons already explained, the temperalure of death-spasm in stout ■ plants is often higher by 1 or 2 degrees. Fig. 123. Lowering of death - point under iar ig ue ; d ea ch - spasm took place at 370 C.

The excitatory response of the leaf of Mimosa at death is an active contraction of the pulvinus. The death-spasm is after a time followed by a post-mortem relaxation. This slow reaction is, however, quite different from normal erectile recovery from ordinary stimulation as exhibited by the living leaf. Moreover, the leaf, killed by high temperature, exhibits no further response to stimulation. The question that next demands attention is whether the sudden excitation at death occurs only in sensitive plants,

or whether it is exhibited by all plants and their different organs ? Should such an excitatory reaction occur at the critical temperature, how are we to detect it ? The characteristic excitatory reactions of different plant- organs have been described in a previous chapter. For convenience of reference I here classify them, beginning with the complex pulvinated and ending with the simple radial organ. 1. Pulvinated organs. — The lower half of the organ is more excitable than the upper half. Excitation causes the responsive fall of the leaf by the greater contraction of the lower half of the organ.

as an elongated pulvinoid, response to excitation being exhibited by a fall of the leaf. » 3. Anisotropic growing organs. — Many floral leaves ex¬ hibit differential excitability of their upper and under sides. Response to excitation is brought about b}^ the greater contraction of the more excitable side of the organ. 4. Organs with induced anisotropy ~ Straight radial organs have been shown to become curved and anisotropic under unilateral stimulation. The expanded convex side of the organ is then found to be more excitable than the contracted concave side ; hence excitation causes a straightening of the curved organ by the greater contraction of the more excitable convex side. This type of organ may be regarded as an intermediate link between a radial and a pronouncedly anisotropic organ.

5. Radial organs . — A radial organ, under excitation, exhibits a longitudinal contraction indicated by shortening of its length. 1 hese being the characteristic indications of excitatory reaction given by these different organs, it remains to ascertain if these indications are given by them when exposed to the critical temperature. Experiment 122. — The record of the pulvinus of the Bean-plant is shown in tig. 124. There is an increasing expansive movement as the temperature rises from 30°

Fig. 124. Thermo-mechanical curve indicating death-point at 60 0 C. of leaf of Bean (Phaseolus). onwards till a violent contractile spasm occurs at 6o°. It is significant that the critical temperature is the same in sensi¬ tive and in ordinary plants. The thermo-mechanical curve of the ordinary plant is, moreover, found to be essentially similar to that of Mimosa. Experiment 123. — As an example of a pulvinoid 1 cm- ployed the leaf of Water Hyacinth (Eiehliornia). The

up-movement of the leaf was slight during rise of tempera¬ ture, but the spasmodic contraction at the fatal temperature of 6o° C. was very pronounced (fig. 125). Fig. 125. Determination cf death-point of pulvinoid (Eichhornia). Death-spasm and inversion of curve at 6o° C. Experiment 124. Death-movement of flowers. — As already explained, the upper and under sides of each floral leaf are unequally excitable. The responsive movement at the critical temperature is one of curving up in certain cases and of curving down in others. In the case of the French Marigold grown in India, the florets of the ray curve up at the temperature of 590.

Experiment 125. Death-excitation of geotropically curved organs. — Physiological anisotropy has been shown to be induced in a radial organ by the unilateral action of the stimulus of gravity, the convex side of the curved organ being the more excitable. The thermo-mechanical record of such an organ exhibits a preliminary expansion of the convex side ; this expansion becomes converted into an abrupt and violent contraction at the critical temperature of 61 0 C. (fig. 126). ;]

Experiment 126. — It has been shown that a radial organ responds to excitation by longitudinal contraction. Records were taken with two specimens of the style of Datura alba from the same plant. The two curves were so similar that one could be taken for the other, ihe curve of increasing expansion was found to be suddenly reversed by the contractile spasm of death at 6o°. The thermo- mechanical curve of the radial organ is in every respect similar to that of the leaf of Mimosa.

Diverse organs of plants, both sensitive and ordinary, are thus found to exhibit the characteristic .spasm of death at the critical temperature, which is practically the same in all phanerogamous plants. It has been shown that excita¬ tory contraction can also be detected by two different electric methods; , namely, by galvanometric nega¬ tivity, and by diminution of electric resistance. Experiment 127. Electric spasm at death. — Suitable electric connexions were made with upper and lower halves of a pulvinated organ. Rise of temperature, up to the critical temperature, induced expansion and galvanometric positivity of the more effective lower half of the organ. At 6oc C. there was an abrupt inversion of the curve from electro¬ positivity to electro - negativity (fig. 127).

A sudden diminution of resist¬ ance is ^lso found to take place at this critical death-temperature. The life-and-death curves of dif¬ ferent organs are seen to be essentially similar to each other. The following table shows that though the methods of determination are so widely Fig. 127. Deter¬ mination of the death- point by elec¬ tromotive variation. The electromotive variation at the critical point is a change from electro¬ positivity to electro¬ negativity; the point of inversion of the curve is at 6o° C.