The Motor Mechanism of Plants

The recorded movement was due to the contraction of the vertical row of active cells in the line of sphygmographic contact C C ' (fig. 79) . The responsive movement is extremety small and will be expressed either in microns or in millionths of an inch. The former, represented by the S3Tmbol 4, is a millionth of a metre. One micron is equal to 40 millionths of an inch. it has been explained that under medium stimulation the sphygmographic response is due practical Ay to the con¬ traction oi the row of vertical cells l in the lower half of the pulvinus. After record of contraction, the measurement of this length was made as follows : a transverse microscopic

section of the pulvinus was made at the line which passed through C C'f the differential staining of which clearly brought out the actively contractile cells. ! he length l was measured by a microscope micrometer, and the number of Vertical length of contractile ceil.— The following results were obtained with the experimental pulvinus, which was of an average size. It may oe said in passing that the length varies in dilterent specimens from 0*45 to o*6 mm., and fue number of cells in the row from 19 to 24.

Experiment 72. Effect of feeble stimulation . The ; amplitude of the contractile response was yo mm., obtained under a magnification of 45°° times. Hence From data given 111 (1) (2) and (3) the following calcula tions have been made : Experiment 73. Effects of moderate and strong stimula¬ tion.- A record was taken under stimulation of moderate intensity, the magnification being 2000 times (fig. 81). Another record obtained under strong stimulation gave

Fig. 81. Record of diametric contraction of pulvinus of Mimosa, under moderate stimulation (magnification 2000 times). response of greater amplitude. The percentage of con¬ traction for feeble, moderate, and strong stimulation, cal¬ culated from the results of the experiments described, are given below : The active cells of the pulvinus are thus fourd to undergo very marked contraction under stimulation ; though the amount of contraction is beyond the power of the microscope to detect, yet the contraction of even an individual cell has

been measured with a considerable degree of accuracy. The rate of contraction is also very rapid, this being due to the presence of some ‘ active substance ' in the pulvinar cells of Mimosa. * Hitherto the pulvinus of sensitive plants like Mimosa have been alone regarded as sensitive. Is there any justification for this view ? The cortex of the pulvinus of Mimosa is continuous with that 111 the stem. Does the contractility of the cortex abruptly end at the pulvinar limit or does it also extend into the stem ? If so, what is the amount of contraction ? Is it of the same or of a very different order from that of the contractile cells in the pulvinus ? In other words, are all cortical cells sensitive, and do they respond to stimulation by contraction ?

The investigation was carried out with the identical specimen employed in the foregoing record of pulvinar contraction. The stem, at a short distance below the pulvinus, was mounted horizontally between the two con¬ tact-points of the Sphygmograph. A modification of the experiment enables the stem to be maintained in the* normal vertical position, in which case the sphygmographic contacts would be in a horizon¬ tal plane. It was, however, thought desirable to employ the same recording apparatus for measurement of the diametric contraction of the cortical cells in both the pul¬ vinus and in the stem, so that the results might be strictly comparable.

Since the excitability of radial stems is the same all round, the diametric contraction, if any, would occur on both sides of the stem. The sum of the depths of the cortex on two sides will be represented by /, and the number of cortical cells in the same row between the two sphygmographic con¬ tacts by n. The length l and the number n were found in the usual manner by microscopic examination of the section made after the experiment. In the present case,

Experiment 74. bffect of feeble stimulation. I he stem was subjected to electric stimulation of the same intensity as in the case of the pulvinus. This gave rise to a dia¬ metric contraction of the cortex, the amplitude of which was 18 mm. at a magnification of 4500. Tlrs proves that contractility is not confined to the pulvinus but is also characteristically present in the stem. — II *6 millionths of an inch Amount of contraction . . 2 per cent.

Effect of moderate stimulation. — The experiment was repeated under stimulation of stronger intensity ; the record is given in fig. 82. Comparison of figs. 81 and 82 will show how essentially similar are the responses of the cortical cells in the pulvinus and in the stem. The per¬ centage of contraction is of the same order ; the rate of contraction of the cortical, cells of the stem is less rapid on account of absence from them of ‘ active substance/

In the pulvinus the average diameter of an active cell is 0-024 nun., which in round numbers is a thousandth part of an inch ; its diametric contraction under feeble stimula¬ tion is 0 • 76 (a or about 30 millionths of an inch. The amount of contraction is 3*2 per cent. In the stem the average diameter of each cortical cell is 0-014 mm. or 6 ten-thousandths of an inch. The amount of contractidn is 2 per cent. It will be seen that the contractility of the cortex of the stem is of the same order as that of the pulvinus. The

Fig. 82. Record of diametric contraction of cortex of stem of Mimosa (magnification 2000 times). physiological mechanism is thus continuous in the stem and in the pulvinus ; the excitatory cellular reaction in the stem is essentially similar to that in the pulvmus. J he cortex of the pulvinus, owing to the presence of the ' active sub¬ stance/ reacts at a relatively quick rate. The difference is merely one of degree and not of kind. Though the cortical cells of the stem are relatively sluggish in their reaction, yet they may, on this very account, have developed some special activity such as the power of

repeated contraction. That there is some justification ior this surmise will be shown in a later chapter. The contractile reaction of the active cells in the pulvinus of Mimosa persists even when the leaf is prevented from movement. The fall of the leaf is a secondary conseou^nce of the contraction of the pulvinar cells. The size of a single contractile cell in the pulvinus is 0*024 mm. or about one thousandth of an inch in diameter. The diametric contraction under feeble stimulation is 0*76 p or 30 millionths of an inch, which is beyond the highest power of the microscope to detect. The measurement has been rendered possible by the employment of the Celi- Sphygmograph. The amount of contraction under feeble stimulation is 3*2 per cent., increased to 13*3 per cent, under strong stimulation.

There is an uninterrupted continuity between the cortex of the pulvinus, of the petiole, and of the stem ; the contract ile mechanism is essentially similar throughout. The contrac¬ tion of the cortical cells in the stem is of the same order as that of those in the pulvinus, being 2 per cent, under feeble stimulation. The diameter of each cell is 0*014 mm- or 6 ten-thousandths of an inch, the diametric contraction under feeble stimulation being 0*29 p or 11 millionths of an inch.

The presence of ' active substance * in the cortical cells of the pulvinus apparently enables them to contract more rapidly than other cortical cells. It has been shown in the previous chapter that the cortex of the pulvinus of Mimosa undergoes contraction under stimu¬ lation and that this can be detected in two different ways : namely, the diametric contraction of the active cells bv the Cell-Sphygmograph, and the responsive movement of the leaf by the Phytograph. The records obtained by the two methods were further shown to be in every way similar to each other. The cortex of the pulvinar leaf-joint is con¬ tinuous with that of the ,ptem ; it has been further shown

that the contractile mechanism is'also continuous : that is to say, that the cortex of the stem also exhibits contraction ander stimulation, its contractility not being of a different order from that of the ‘ sensitive ' pulvinus. We are immediately confronted with the follow1' ng questions : exhibit responsive movement similar to that of the pulvinus ot Mimosa ? 1 was able to discover (1901) the universal sensitiveness of all plants by the method of electric response, a detailed

account of which is given in Chapter XIII. I have recently succeeded in obtaining mechanical response of ordinary plants to stimulation by the employment of a highly sensitive method of recording cellular contraction. Experiment 75.- — A direct record cf the contraction of the cortical cells of a definite zone in the stem of fmpatiens Fig. 83. Effect of increasing intensity of stimulation on diametric contraction of stem of Impatiens recorded by Cell-Sphygmo- graph.

Diametric contraction recorded as a down-curve. Note increasing contraction under increasing intensities of stimulation s, s', s' with corresponding prolongation of period of recovery. was obtained by the Cell-Sphygrnograph. Stimulation by induction shock was sent along the length of the stem and the diametric contraction recorded in the usual manner. The amplitude of response is seen to undergo an increase under *ncreasing intensity of stimulation. After the cessation

of stimulation the contracted cells become re-expanded during recovery (fig. <83). The period of recovery, as in the case of Mimosa, becomes protracted after strong stimulation. The delicacy of the method of experimentation is greatly increased by the Optical Sphygmograph, described in a later chapter, by winch it is easy to produce a magnification of a million times. The optical method has the additional advantage of experimental demonstration before a large audience.

even ordinary plants, this sensitivity being gauged by the responsive contraction induced by an intensity of induction-shock which is below the threshold of human perception. ; of response. It will be remembered that a sub¬ tonic specimen of Mimosa responds to moderate stimulation by the abnormal positive or erectile response indicative of expansion. The tonic con¬ dition is found to be improved m consequence" of external stimulation, the abnormal positive being gradually transformed into normal negative or contractile response (p. 50). Is this reaction of universal occurrence, to be discovered even in ordinary cortical tissue ?

Experiment 75 a. Comparison of sensitivity of ordinary plants and of human subjects. — The secondary coil of the induction apparatus was placed at a sufficient distance from the primary, so that the shock couJd not be felt by a human subject. The experiment was next so arranged that the same sub-minimal induction-shock was passed through two human subjects and the stem of an Antirrhinum, ail included in the same electric circuit. The electric shock which could not be perceived by the human subjects produced, neverthe¬ less, a marked contractile response of the plant, it is a revelation to find plants reacting to stimulation which is so extraordinarily feeble.

Latent period. — In the pulvinus of Mimosa the rapidity of reaction, which is associated with the presence of f active ’ substance, is such that the latent period is only a fraction „ of a second, in the ordinary cortex the latent period is relatively long, of the order of 5 to 8 seconds. The . diametric contraction under stimulation is found to be universal. It is exhibited not only by herbaceous stems, such as those of Antirrhinum, Cosmos, Tomato, and Tradescantia, but also by the woody stems of the Rose and other shrubs. The sensitivity depends on the species, on the season, and on the age of the plant.

Experiment 75 b. Modifying effect of tonic condition on the sign of response.-— A dozen similar specimens of Antir¬ rhinum were taken, of which the first batch of six had recently been exposed to light, and the second batch kept in darkness for 24 hours. Under moderate electric stimula¬ tion every one of the light-exposed specimens responded by contraction ; the second batch of subtonic specimens responded to similar stimulation by expansion. 1 he abnormal posi¬ tive response by expansion was gradually transformed into normal negative or contractile response under continuous stimulation. The modifying effect of tonicity on responsive reaction, observed by different methods of experimentation, is thus found to be similar in ‘sensitive’ and in ordinary plants.

Reference has already been made that stimulation gives rise to two reactions A and 1) ; of these A is the more . persistent as observed in the positive lesponsc of subtonic tissue already described. It occurs through a block which arrests excitation D ( cf . Experiment 93a). It also lasts longer in a dying tissue than reaction D. Experiment 75 c. Effect of intense electric stimulation - This causes a violent contraction which proves to be the spasm of death, for subsequent stimulation induces no further contraction. It is very curious that a testing electric shock now causes a feeble expansion which disappears after a while.

I describe additional results on the effects induced by direct and indirect stimulation, the record being obtained by a different method. Experiment 76. — I carried out numerous experiments with leaves of different plants, the response being recorded Fig. 84. Response of ordinary leaf under transmitted excitation by the Phytograph. When an electric shock was sent along the length of the leaf-joint, it exhibited a contractile fall, similar to that of the leaf of Mimosa, though the extent and the rapidity of the movement was not so great. On the cessation of stimulation the leaf underwent gradual recovery of its normal position. The leaves of Impatiens, Vinca rosea, and Chrysanthemum coronarium were found suitable for this demonstration.

Experiment 77. Response to transmitted excitation. — The leaf of Mimosa gives a responsive fall not only to direct stimulation, but also to indirect stimulation o! the stem. The excitation is transmitted to the pulvinus by a definite tissue which functions as the nerve. Transmitted excita¬ tion also causes a fall in a number of ordinary leaves, as shown in the record obtained with the leaf of Vinca (tig. 84)- Such a conducting tissue occurs in a number of ordinary plants such as Vinca ; the response of its leaf to transmitted excitation is seen in fig. 84- Stimulation in this case was produced by thermal shock applied on the stem a short distance below the leaf. A similar effect is obtained with indirect electric stimulation.

Under diffuse stimulation the leaf of Mimosa and those of ordinary plants exhibit a movement of fall, which is attri¬ butable to the greater contraction of the more excitable lowrer half of the motor organ. Further insignt can only be obtained by means of a method for the quantitative determination of the differential excitability of the two halves of an anisotropic motor organ. For this purpose stimulation of the same intensity is applied first on the upper and then on the lowrer half of the organ. The amplitudes of the twro responses should give a measure of both the excitabilitv and the contractilitv of the twro halves of the organ. The contraction of the upper half causes an erectile movement of the leaf, shown in the record as a down-curve, while that of the lower half induces a down-movement shown as an up-curve.

The discovery of a suitable means of local stimulation presented some difficulty ; it was finally found in excitation produced by the polar action of a constant current. It has been shown in Chapter IX that under a. minimally effective current, excitation takes place only at kathode- make, there being no excitation at kathode-break, at u node-make, or at anode-hreak. Moreover, the excitation Two platinum pins are thrust into the cortex of the upper and lower halves of either the pulvinus or the leaf- joint. By suitable manipulation of the reversing key R, the upper and the lower halves of the anisotropic pulvinus or of the leaf- joint are locally excited in succession by being made kathode (fig. 85).

Experiment 78. Differential excitability of the pulvinus of Mimosa. — On gradually increasing the polarising current, Fig. 85. Diagrammatic representation of method of local excita¬ tion of lower and upper halves of the pulvinus of Mimosa by polar action of electric current from a battery. R, reversing key. Record taken on smoked-glass plate c. a point was reached wjien the lower half of the pulvinus alone exhibited contractile response, the uppe~ showing no such indication. This proves that the sensitivity of the lower half is very much greater than that of the upper. \\ hen the exciting current was raised to moderate intensity (4 micro-amperes) the upper half responded under kathode stimulation by a very feeble contraction. The response was so feeble that a magnification of 30 times had to be employed in obtaining the record. By reversing the direction of the current, the lower half was next subjected to the same intensity of stimulation, the result being a very great increase in the amplitude of response. The magnification in this case had to be reduced to 7-5 times to keep the record within the plate.

There are important differences in the characteristics of the two responses. The response of the upper half is very sluggish, the latent period for the initiation of contractile response being as long as 50 seconds ; the amplitude of the response is also very feeble, being about 3 mm, with magnifi¬ cation of 30 times. The response of the lower half exhibits a very great contrast. Associated with the presence of a large quantity of ' active substance ' the contractile reaction of the lower half is extremely rapid, tne response occurring almost instantaneouslv ; the maximum contraction was alsd attained in the course of about 2 seconds. The amplitude of response under magnification of 7-5 times was 60 mm. (sec fig. 22). a

These results are independently supported by others described on p. 37, where it is shown that the lower half of the pulvinus of Mimosa is much more excitable than the lower. The response of the upper half is sluggish, and the latent period is comparatively long. Experiment 79. Differential excitability of the leaf-joint. — Parallel results were obtained with the ordinary leaf-joint, of which the following results may be taken as typical. The minimal intensity of current which causes excitation of the lower half of the leaf-joint of Chrysanthemum corcnarinm was found to be ineffective for the upper. Under stronger current of i3 micro- amperes a feeble contraction was pro¬ duced in the upper half, the contraction of the lower being thirty times greater.