The Motor Mechanism of Plants

the predominant influence of temperature on the diurnal variation of excitability. Effect of physiological inertia. — The experimental results already described showed that the change of excitability lags behind the inducing cause. This is further illustrated by the diurnal curve [cf. iig. 57) ; the minimum temperature was attained shortly after 5 a.m., but excitabilitji was not reduced to a minimum till about 3 hours later. I will now discuss in greater detail the diurnal variation of excitability in Mimosa, taking the typical case illustrated in fig. 57. The favourable temperature was here almost constant from noon to 5 p.m. ; the condition of light was also favourable. Hence the excitability was found to be constant and at its maximum between these hours. 1 he temperature began to fall after 6 p.m., and there was, in addition . the depressing effect of gathering darkness. Owing to th 3 time-lag, the fall of excitability did not commence immediately at 6 p.m., but an hour afterwards, and continued till the next morning. During this period there was cumula¬ tive effect not only of darkness but also of depression caused by the falling temperature. The turgor was also at its maximum early in the morning, which tends to produce depression of moto-excit ability. On account of the com¬ bined effects of these different factors, and of the phenomenon of lag, the period of minimum excitability was reached at about 8 a.m. The excitability was then gradually and con¬ tinuously increased under the continued action of light and of rising temperature, till the maximum was attained shortly after noon.

As has been shown, temperature exerts a predominant influence in inducing variation of excitability. The diurnal period is, therefore, characteristically modified according to the season. In winter and in early spring the night temperature falls very low, resulting in a complete abolition of excitability, which persists for a considerable period in the morning. In summer the prevailing high temperature causes a characteristic modification. As the night is warm the fall of excitability is not so great as in spring. I he minimum temperature early in the morning in summer is 25*5° C., instead of 19-5° C. in spring. Hence the excitability in the morning exhibits depression but not total abolition. After the minimum in the morning the excitability is restored in a staircase manner as in spring.

During daytime, again, the temperature often rises to 38° C. between 1 and 3 p.m., after which there is a mitigation of heat, the temperature returning towards the optimum. The record (fig. 61) shows the excitability to have been higher between 4 and 6 p.m. than between 1 and 3 p.m Experiment 54. — An interesting variation is seen in the following record, obtained in summer, in which the nyctitropic movement was very pronounced. The periodic variation of excitability exhibited here is practically the same as that

in other summer-specimens (cf fig. 6i). variations, noticeable alter 5 all'd 9 teristic differences in +he curve of recovery. On account of the rapid diminution of light there was a sudden excitatory fall of the leaf, exhibited by the displacement of the base-line upwards, which persisted till 9 p.m. On this account the recover}- after each stimulation appears to be incomplete. The extent of the responsive fall is also seen to have reached a limit, for the leaf cannot fall beyond the maximum. After 9 p.m. there was a rapid increase of turgor, which reached its maximum about 6 A.M., causing a rapid erectile movement of the leaf, shown by the down-movement of the base-line. After each stimulation there now appeared an overshooting of the line of recovery (fig. 62).

The moto -excitability of Mimosa was gauged every hour of the day and night by the amplitude of the response to a constant testing stimulus. This was effected by means of automatic devices for periodic stimulation and for record¬ ing the resulting responses. The diurnal record shows that the excitability of the plant is not the iame throughout the day, but undergoes characteristic variation at different hours. In a typical case in spring the excitability attained its maximum value after miciuay and remained constant for several hours. There was then a continuous fall of excitability, the minimum being reached at about 8 in the morning. The plant was at this time practically insensitive. The moto-excitability was then gradually restored in a staircase manner till it again reached its maximum at 12 noon.

Lhe diurnal variation of excitability is primarily due to the hourly variation of temperature. The effect is modified in a less degree by the variation of light. It was stated in a previous chapter that stimulation of the plant can be effected by the application of a constant electric current. A feeble current has the unique advantage of producing local stimulation ; this is a matter of great importance in certain investigations of a crucial character, such as one described in a subsequent chapter.

It is well known that an electric current causes specific excitatory reactions in animal tissues. Excitation is characteristically produced at the sudden starting or make of the current ; it may also be produced at the stoppage or ‘ break ’ of the current. Thus when a feeble current is passed by means of two electrodes through a muscle, an excitatory contraction is initiated on the sudden starting of the current at the kathode where the current leaves the tissue, no excitation being produced at the anode, that is at the point of entrance of the current. At the break of the current, excitation is produced neither at the kathode nor at the anode. When the intensity of the electric current is increased the excitation due to the make still takes place at the kathode only ; whereas at the break of the current excitation takes place at the anode.

The characteristic polar effects of current on Protozoa have been found by different observers to be, generally speaking, opposite to' those on animal tissues. Hence it has been supposed that the laws of polar reaction in unhbril- lated protoplasm must be different from those in highly differentiated animal tissues. My experiments on the polar action of electric current on plants prove, on the contrary, that the reactions of the undifferentiated protoplasm of the plant-body are identical with those of the animal tissues. This is demonstrated by the following experiments with the primary pulvinus of Mimosa pudica.

One electrode from a battery is applied on the pulvinus, and the second electrode attached to a distant indifferent ITg. 63. Commutator, for causing make and break of the cur *ent, point. Four cycles of operation can be performed with the commutator by which the current in either direction can be made or broken. Fig. 63 illustrates the position after sudden make of the current by tilting the commutator to the left, the pulvinus being made the kathode. By proper manipulation of the commutator, the pulvinus may be sub¬ jected (1) to kathode-make, (2) to kathode-break, (3) to anode-make, and (4) to anode-break.

Experiment 55. Effect of feeble current.— I used a leaf of Mimosa which was in a moderatelv sensitive condition. The electric resistance between the two points of contact was found to be a million ohms. An E.M.F. of 8 volts was found to be effective in inducing the excitatory fall of the leaf, the pulvinus being made the kathode. Hxcitation is induced on sudden variation of the current and not during the continuance of it. Hence, if after the excitatory fall the current is continued, the leaf re-erects itself.

Experiment 56. Effect of moderate current. — I he E.M.b. of the acting current was next increased to 12 volts, in carrying the experiment through the usual cycle of opera¬ tions, it was found that the kathode excited at make and not at break. The effect of the anode, on the other hand, was that it excited at break and not at make. The automatic record by the plant is given in fig. 64. The normal signal- line below indicates no current ; the up-line, kathode-make ; return to horizontal, kathode-break ; down-line, anode-make \ and return to the horizontal represents anode-break. It will be noted that under 8 volts excitation took place only at kathode-make, represented by the symbol Km. Under 12 volts there is a stronger response at kathode-make and none at kathode- break ; no excitation was produced at anode-make, but it occurred at anode-break Ab. The excitation formula under the action of moderate current is therefore KmAb.

The universality of these characteristic polar excitations is proved by the results of experiments which I carried out with the sensitive organs of various other plants, such as the leaflets of Mimosa, of Biophytum, of Neptunia oleracea, of A verrhoa Carcwibola and of A very ho a Bilwibi. Fig. 64. Response under 8 volts ex¬ citation at Km ; under 12 volts excitation at Km and at Ab. effective intensity of the current depends on the excitability of the particular specimen, a vigorous specimen being excited by a relatively feeble current. It also depends on the relative excitability of different moto** organs, the pulvinules of the leaflets being, in general, more sensitive than the main pulvinus. As an example of the response

Fjg. 65. frustration to the left shows excitation of leaflets of Biophytum induced at kathode-make, that to the right the effect at anode- break ; moderate current. of the leaflets, I describe an experiment with Biophytur ; sensitivum, employing the Bipolar method. The two elec¬ trodes were applied at two points on the petiole, which carries numerous pairs of motile leaflets. I give a sketch (fig. 65) showing the application of the bipolar method in excitation of leaflets of Biophytum, where K is the kathode and A the anode.

Experiment 57. Effect of feeble current (Biophytum). — This caused excitation only at kathode-make and not at break. No excitation was produced at anode on either make or break. The form ula for the excitatory reaction is thus Km. Experiment 58. Effect of moderate current (Biophytum). Excitation took place only at kathode-make and not at anode-break ; the figure to the left shows the excitation initiated at the kathodic point at make, which did not remain localised but was conducted in both directions. After the recovery of the leaflets the circuit was broken and excitation was initiated at the point of anode-break (fig. 65). The formula for moderate intensity of current is KmAb.

Numerous experiments were carried out with different plants which give similar results. In the two tables (below) are given the effects of feeble and of moderate current. The value of the current given is what was found to be minimally effective for highly excitable specimens. The specific reactions of the plants to electric current are thus in every way similar to those in animal tissues. Excitation at kathode-mak > and at anode-break. Formula KmAb.

From the results given above it is seen that under a gradual increase of current the first type of response, Km, is transformed to the second type KmAb. Can any further transformation be discovered under still greater intensity of current ? The minimum current for a highly excitable specimen of Mimosa for the exhibition of the first type of response Km, was found to be 2 micro-amperes, and 4 micro-amperes for Fig. 66. Effect of increasing intensity of current in transforming response KmAb of Type II to response KmAmAb of Type I1T.

the second type KmAb. It must be borne in mind that the minimally effective current depends on the excitability of the tissue, and therefore varies with the tonic condition of the plant. Experiment 59- Transformation of type under increase of current . The following shows the gradual transformation of the type of response in an identical specimen under gradual increase of current. The specimen was moderately sensitive, and the minimum current for excitation only at Km was found to be 3*5 micro-amperes. When the current was increased to 5-6 micro-amperes, excitation occurred at kathode-make and at anode-break, the formula being KmAb. When the current was further increased to 6*3 micro-amperes, a new

and unexpected type of reaction made its appearance. Excitation was produced not only at kathode-make and anode-break, but also at anode-make. The excitation formula is therefore KmAmAb. I reproduce a record of this transformation under in¬ creased intensity of current. Application of a current of 6 volts had given rise to excitation at kathode-make and at anode-break ; but current increased up to 10 volts gave rise to execution at kathode -make, anode-make and anode-break (fig. 66). I

Experiment 6o. Excitation with stronger current The experiment was continued with the above specimen with this difference, that the current was further increased to 12 • 7 micro-amperes. 1 his gave rise to the fourth type of reaction, namely, excitation at make and oreak of -Kathode, and at make and break of anode (fig. 67). the excitation formula of the fourth type is thus KmKbAmAb. 1 The following is a tabular statement of typical results obtained with a moderately sensitive pulvinus of Mimosa under increasing intensity of current.

Table VIII. — Polar Action of Current of Increasing Intensity on Pulvinus of Mimosa. In order to avoid complications which might possibly arise from fatigue induced by protracted experiments carried out with the same specimen, I repeated the experi¬ ments with fresh specimens which fully confirmed the previous results. The value of the effective current for each type is to a certain extent modified by the sensitivity of the organ, by its age and by the season ; a highly excitable specimen as a rule requires a feebler current than a les^ excitable one.

Experiment 61. — The characteristic reactions to current of increasing intensity are exhibited not merely by the leaf of Mimosa but also by the leaflets of all sensitive plants such as those of Biophytum and Averrhoa. 1 give below the results obtained with leaflets of Mimosa, which ma) be taken as typical of the others. Pfluger’s Law of 'Polar Excitation of animal tissues covers only the first two types, and is therefore not a complete statement of the polar action of current. It is necessary to include the following :

(IV) Under still stronger current, excitation occurs at kathode- make, at kathode-break, at anode-break, and at anode-make. The existence of Types III and IV has been denied and attempts made to explain them by the assumption of produc¬ tion of secondary poles bv the strong currents employed. The meaning of secondary poles will be understood from the experiments of Engelmann and Biedermann on the ureter of the rabbit. They observed that, in an insulated specimen , a moderate' current induced excitation only at kathode- make and at anode-break, the excitation formula being the normal KmAb. But the polar reactions were found to be reversed into AmK.b when the specimen, instead of being insulated , was placed on a good conducting support such as sa.lt clay, file kathode now caused excitation not at msxe but at break, and the anode at make instead of at break. This reversed action AmKb was explained by the fact that the conducting clay, on which the uninsulated specimen was placed, gave rise to diffusion of the current and resulting

How is the reversal of normal reaction to be explained ? Why did the kathode fail to excite at make but caused excitation at break ? Why again did the anode cause excitation at make and not at break ? It is the production of secondary poles that is supposed to be responsible for the reversal of normal reactions. The anode excited at make because there were numerous secondary kathodes produced in the neighbourhood The kathode failed to excite at make because of the depressing action of the secondary anodes.

The excitatory reactions of the Types Ill and IV cannot however, be explained by any assumption of secondary poles for the following reasons : secondary poles by a strong current does not explain the observed phenomena, for the induc¬ tion of secondary poles would have produced excitation only at anode-make and none at • kathode-make. But under strong current, excita¬ tion occurs not only at kathode -make but also at kathode-break Excitation is, moreover, produced by the anode both at make and break. The con¬ ditions of the experiment, again, made the pro¬ duction of secondary poles impossible, as explained below.

making a complete loop round the pulvinus, which was thus equally and throughout its circumference kathode or anode as the case might be. The organ, like the insulated ureter, was perfectly isolated, so that the conditions for the production of secondary poles wrere entirely absent. connexion was made by thrusting a platinum pin into the sensitive cortex, which was thus directly acted upon by the current. Under these condi¬ tions there could be no possibility of the production of secondary poles. Nevertheless the four types of reaction were observed under current of in¬ creasing intensity.

perfectly normal. But when with the same specimen and v/ith identical connexions the intensity of the current was increased, the excitatory reactions became gradually transformed and in definite sequence, first into Type III, i.e. excitation at kathode-make, at anode-make and anode-break ; and subsequently, under still stronger current, into Type IV with excitation at kathode make, kathode- break, anode-make and anode-break. The results given above fully establish the supplementary laws of polar action with strong current.

The characteristic polar reactions are exhibited not only by sensitive plants but by all plants. I give below the summary of results obtained with Balsam, which may be taken as typical, the response being recorded by the move¬ ment of the leaf. Experiment 62. — The excitability of ordinary plants' being less than that of sensitive plants, the minimum current foi each type of reaction has to be proportionately increased. Thus in Balsam (Impatiens) a minimum current of 8-5 micro-amperes was found necessary for the first type x£m, the responding leaf exhibiting a contractile fall. When the current was increased to 12-5 micro-amperes, excitation occurred at kathode-make and at anode-break. Further increase of current to 18 micro-amperes gave rre to reactions of Type III ; still stronger current, i.e. 20 micro-amperes, caused transformation to t ype IV.

Table X. — Effects of Current of Increasing Intensity on Ordinary Plants (Impatiens). Protoplasmic excitation is induced in plants by the polar action of a constant current. The characteristic reactions are exhibited not only by sensitive but also by ordinary plants. The phenomena may therefore be regarded as universal. The following Laws of Polar Excitation in Plants under feeble and moderate currents have been established : The polar reactions of the undifferentiated protoplasm of the plant-body are thus identical with those ef highly differentiated animal tissues.

With stronger current, two other types of reaction are manifested. The Supplementary Laws of Polar Excitation in Plants are : (III) With a strong current, excitation takes place at kathode make, at anode-make, and at anode-break. (IV) With still stronger current, excitation occurs at kathode-make, at kathode-break, at anode-make, It was shuwn in a previous diopter that the irritability of the leaf ot Mimosa could be maintained constant even when borne on a stem detached from the parent plant. Could we proceed still further and, getting rid of all unnecessary appendages, obtain the contractile response of the pulvinus Uself 7 An isolated muscle, kept under proper conditions, continues to exhibit its contractility unchanged foi many hours. Would it be possible to secure an isolated pulvinus which, like the isolated muscle, would respond to stimu¬ lation by contraction ?

In order to reduce the shock-effect of operation, the pulvinus was benumbed by local application of ice-cold water. It was then cut off at its junction with the stern, leaving only a strip of epidermis attached to the upper half of the pulvinus. The petiole was left as a convenient handle for manipulation. The sub-petioles may be allowed to remain or may be removed ; in the latter case the cut end of the petiole is closed by flexile to prevent rapid loss of water by evaporation.