The Motor Mechanism of Plants

It is to be borne in mind that an identical reaction may be described from different points of view, from the responsive movement or from the chemical change that underlies it. For example, a plant in hypertonic or highly excitable condition (due to absorption of energy from out¬ side! exhibits on stimulation an impulsive contractile fall or negative response of its leaf. This is associated with chemical breakdown, a process of catabolism. Now isola¬ tion of the plant from environmental stimuli reduces it to a subtonic condition which is characterised by a reversal oi response from normal negative to abnormal positive. Under stimulation the subtonic leaf becomes erected against the force of gravity, the potential energy of the system being thereby increased. Unlike the predominant catabolic or breakdown process in the normal tissue after stimulation, the subtonic tissue exhibits a chemical change which must be of opposite sign — namely, of anabolic or building-up process.

The results of experiments described show that the sub¬ tonic tissue becomes energised by the impinging stimulus ; the tonic level becomes raised, and the negative contractile response becomes increasingly pronounced till the pre¬ dominant negative masks the positive. The existence oi the two processes, and the predominance of the one or the other under different conditions, are clearly exhibited in the record given in figs. 31-34. Among the diverse manifestations of protoplasmic irritability, as previously stated, are contractility, con ductivity, and rhythmicity. It is interesting to lind that

stimulation produces parallel changes in these different manifestations. The normal activity is maintained when the tissue is in favourable tonic condition by absorption of energy from the environment. In the subtonic condition, on die other hand, there is a depletion of energy, with the result that (i) the power of contraction disappears in a con¬ tractile tissue ; (2) the power of transmission of excitation is arrested in a conducting tissue ; and (3) the automatic powe- of repeated contraction in a rhythmic tissue comes to a state of standstill, as will be shown later. Stimulation rex *V(:S l*iesc different manifestations 111 a staircase manner.

Stimulations, individually ineffective, become effective on repetition. Within limits, the number of additive stimulations for effective excitation varies inversely as the intensity of the stimulus employed. Ihe contractile response of the motor organ of Mimosa exhibits characteristics similar to those of animal muscle. 1 he responses show fatigue under conditions ol incom¬ plete recovery from excitation. The anomalous erection, after preliminary fall of the leaf of Mimosa under continuous stimulation, is explicable on the common characteristics of response in animal and plant tissue s. In both, contraction is reversed to relaxation under continuous stimulation.

The responsive reaction in Mimosa is not of one but of two kinds: positive and negative. The positive A is associated with accession, and the negative I) with run-down of energy, the resultant effect being A - D. The contractile mil of the leaf is associated with evolution of energy which had previously been stored in the tissue. The plant isolated from environmental stimulation falls into a subtonic condi¬ tion, when its power ol contraction disappears.

Stimulation effects both internal and external work, the relative values of which undergo progressive change. In the condition of subton city the energy of ir ident stimulus is utilised in raising the tonic, level and in increasing the functional activity of the tissue ; in this stage the response to stimulation is purely positive, indicative of absorption of energy. In the intermediate stage the response is diphasic, positive followed by negative, with staircase increase of the negative response. In the optimum condition attained by absorption of energy from external sources the negative response becomes disproportionately larger than the impinging stimulus that occasions it. The ‘ trigger action ’ is only a particular event in the tonic cycle.

The active movement of life has been shown to lx* brought about by expenditure of energy that had previously been stored in the organism. It is the energy-content of the organism that has been conveniently designated as its tonic condition. The experiments described in the previous chapter conclusively proved that the contractility of the pulvinus and the resulting fall of the leaf disappear when the tissue passes into the subtonic condition. The essential condition for motility is therefore a favourable tonic condition, as already defined. t

There are other peculiarities exhibited by the motor organs ; thus pulvini of different species of plants show characteristic differences in their sensitiveness a well as in the rate and extent of their contractile movements. Three types of pulvini may conveniently be distinguished— * Active/ ‘ Semi- Active,’ and Inactive.’ Mimosa represents the first, Neptunia the second, and Erythrina the third type. Erythrina may be taken as representative of the relatively inactive pulvini of many leguminous plants such as the Bean-plant (Phaseolus).

The active pulvinus of Mimosa responds to an electric stimulus as feeble as o*i unit, while, the minimal stimulus necessary to initiate response in that of Neptunia is 2 units, and in that of Erythrina it is about 4 Hints. On application of the maximal stimulus in the three cases, tne extent oi the responsive movement is found to be very different. The normal outspread position of the leaf of Mimosa is from I0 J to is° above the horizon ; on stimulation it executes an angular fall of nearly ioo° in the course of about 1*5 second, which may be taken as the average value. In Neptunia the maximum fall of 15 is attained in the course ol 180 seconds. Finally, in Erythrina the extent of the. move¬ ment is through only o-8°, necessitating a moderately high magnification in order to obtain a suitable record ; the maximum fall in this case is attained in the course of 480 seconds.

Table III. — Rates of Contractile Reaction of Active, Semi-Active, and Inactive Pulvini. The rate of the responsive movement of Mimosa is seen to bv. about 800 times more rapid than that of Neptunia, and 30,000 times than that of Erythrina. What can be the cause of this difference ? In consider¬ ing responsive movements, two different elements have to be taken into account — first, the power of contraction, and second, the rapidity of contraction. The power of con¬ traction has been shown to be essentially dependent on the tonic condition, since a subtonic tissue exhibits no con¬ traction. But however favourable the tonic condition may be, it cannot make a senn-active or inactive tissue contract as rapidly as the active tissue of Mimosa. The rapidity of contraction must be due to some other hitherto unsuspected factor.

Fig. 35. Longitudinal section uf petiole and pulvinus of Mimosa passing through the upper and lower vascular bundles. rt, the corticid cells of the petiole which remain un¬ stained. Lower p to the right indicates contractile cells of pulvinus which be¬ come deeply stained. A longit udmal section of the petiole and the pulvinus of Mimosa is sh^wn in fig. 35. There are four main vascular bundles in the petiole, which are there separate from each other, but converge and meet in the pulvinus. The pith becomes thereby reduced and the cortex increased proportionately. The two bundles, upper and lower, in the section are seen to approach each other in the pulvinus.

It is remarkable that though the rapid movement of the leaf is effected by the contractile cells of the pul\ inus, yet these active cells present no special features of structure by which they can be distinguished under the micro¬ scope. They do not differ in any important respect from the relatively inactive cells of the pulvinus of the Bean -plant. Again, in the pulvinus of Mimosa itself it is impossible to detect, by microscopic examination, the place where the active cells begin or end, and how they are dis- tributed.

It appeared probable that the iapidity of the contractile reaction might be due to some special modification of the protoplasmic contents of the active cells. If so, the differ¬ ence between the active and inactive cells might be revealed by the action of suitable chemical stains and developers. Experiment 24. Localisation of actively cor tractile cells by Hcematoxylin and Safranin developer.-- As previously stated, it is impossible, even under microscopic examination, to f.nd the line of demarcation between the inactive cortical cells of the petiole and those of the actively contractile pulvinus. Double-staining of the longitudinal section of the pulvinus of Mimosa produced a most remarkable result (cf. fig. 35). It appeared as if a hand had picked out, with the utmcr t care, every contractile cell and painted its protoplasmic contents deep red. The contents of the cortical cells of the petiole remained unstained. The line of demarcation between the active cells of the pulvinus and inactive cells on both sides of the petiole is now well defined.' Under careful application of the reagents, the depth of staining of the lower half of the pulvinus appeared to be greater than that of the upper half. The very deeply stained cells in the lower half of the pulvinus were numerous and appeared compactly arranged, whereas in the upper half they were relatively few and scattered in their distribution.

Experiment 25. Safranin. — Safranin may be employed alcne. Prolonged application of a dilute solution stains the protoplasmic contents of the active ceils deep red. This stain brings out certain interesting characteristics of the cell-contents to be presently described. Safranin and Light Green developer. — These also produce a great contrast between the active and inactive cells. While the protoplasmic contents of the active cells alone are stained deep red, the inactive cortical cells of the petiole (or rather their cell-walls) are stained green.

Experiment 26. Methylene Elite and Erythrosin. — The sections are immersed in strong alcoholic solution of methylene blue; after washing in absolute alcohol they are immersed in erythrosin dissolved in olive oil. The ctam- ing has to be stopped at the right moment. The contents of the active cells aie stained deep blue, while the inactive cortical cells are stained red. Experiment 27. Hcematoxylin and Bismarck Brown . - A deep brown staining discriminates the active from the inactive cells.

Experiment 28. Iodine Green and A luni-C aniline. — \ sharp contrast is brought about between the active and inactive cells of the cortex, the former being stained vivid green, while the latter become pink. The various stains thus discriminate the active from the ’^active cells in the same petiole-pulvinar preparation, and indicate that the difference depends upon the presence of a substance in the former which is absent in the latter. Experiment 29. — Application of stains to the longitudinal section of the pulvinus of the semi-active Neptunia brought out a significant difference between active and semi- active motor organs. In Mimosa pudica the active cells which become stained are compact and extend almost to the central vascular strand. But in Neptunia the stained cells extend barely to one-third the thickness of the cortical tissue of the pulvinus. There are, moreover, large patches of cells hi the pulvinus which remain unstained.

Experiment 30. — No staining occurred in the pulvinar cells of the inactive Bean-plant and of Erythrina, the substance which undergoes staining being here altogether absent. Experiment 31* — In Mimosa Spegazzinii the primarv pulvinus of the leaf is very feebly sensitive, whereas the secondary pulvini of the leaflets are highly excitable. J Inst tried the specific action of the dyes on sections of the primary pulvinus ; only a few cells here and there became stained. Application of the reagents to the secondary pulvinus, how¬ ever, produced deep staining of all the cortical cells of the motor organ. The high sensitiveness of the leaflets is therefore associated with the abundance of the stainable substance in the active cells and their greater number.

I carried out experiments with the object of discovering the order of excitability in different pulvini, from the highly active to the inactive through intermediate gradations. The different types can be placed under four headings, viz. the active, the semi-active, the feebly active, and the inactive. The change from one group to the next is not abrupt, but gradual. The most active is (1) the primary pulvinus of Mimosa. The next in order are the secondary pulvini (2) of Mimosa Spcgazzinii, (3) of Biophytum, and (4) of Neptunia. Under the semi-active class come (5) the primary pulvinus of Neptunia and (6) the secondary pulvinus of Averrhoa. The feebly active is represented by (7) the primary pulvinus of Mimosa Spegazzinii. The in¬ active organ? include the primary pulvim (8) of Averrhoa, (9) of Erythrina, and (10) of the Bean-plant (Phaseolus).

It is highly significant that the distribution of the stainable substance closely follows the above order (see Table IV). Inasmuch as there is shown to be a direct relation between the activity of the pulvinus and the amount of stainable substance in its contractile cells, it mav be concluded that this stained substance is directly related to the rapidity of contraction, that it is, in fact, an active substance. 1 reproduce photo-micrographs of portions of the lower half of the pulvinus of Mimosa, of Neptunia, and 01 the Bean-plant (fig. 36).

G&inular character of protoplasm. — Another modification, probably of much significance, is the granular character of Fig. 36. Reproduction of photo-micrographs of portions of the pulvini of : (a) active Mimosa, (b) semi-active Neptunia, and (c) inactive Bean-plant. The stained content of the contractile cells represented dark. the contents of the active cells observed iicatron in the safranin-stained tissue. observe a large number of grains in the protoplasmic contents of the * active' cells in which they occur.

In regard to the semi-active puivinus, the distribution of the active cells is not only scattered, but the granular character of the protoplasm appeared to be less marked In the inactive puivinus the protoplasm is not granular and, as already stated, remains unstained. It is remarkable that the powerful contraction of animal muscle is also dependent on the presence of a certain plas¬ matic substance — the granular sarcoplasm. This is well Fig. 37. Transverse sections of pectoral muscles of (a) the falcon, (b) the goose, and (c) the domestic fowl. The relative amount of granular substance (shown as shaded) varies lirectly with the bird’s power of sustained flight (after Knoll).

seen in the transverse sections of the pectoral muscles of the falcon, the goose, and the domestic fowl, in which the relative amount of the granular substance varies directly with the bird's power of sustained flight (fig. 37). Inquiries into tlie character of the ‘ active substance ' in plants aie still in progress * several questions require further elucidation. I here describe tlie results which have already been secured. The presence oi the active substance in large quantity is associated with the extraordinarily rapid acceleration of the contractile reaction. The contraction is probably brought about through one or more intermediate processes, ending ultimately in oxidation, combustion, and production of

carbon-dioxide.1 It xv ill be shown that the contractile reaction takes place only in the presence of oxygen, and becomes arrested in an atmosphere of carbonic acid gas (Experiment 38, p. 73). The active substance would appear 4'0 be highly oxidisable, such as would absorb oxygen from ail available sources. It should, therefore, be capable of reducing oxides. Experiment 32. Reaction with Osmic acid. — This par¬ ticular oxidising agent was rapidly reduced by the proto¬ plasmic contents of the active cells into a lower oxide which produced a dark stain. The contrast between the active and inactive cells is as strongly marked as that produced by double-staining.

The staining could still be obtained after treatment of the prepared section with cold or warm alcohol or ether. The active substance ’ cannot, therefore, be a fat or a lipoid ; it still contained a powerful reducing group, characterised by double or triple bonds. This is justified by the fact that no staining was obtained after bromination, which converts an unsaturated into a saturated compound. The activity of the modified protoplasm is therefore dependent upon the presence of a highly reactive and oxidisable substance, an unsaturated compound with double or triple bond-combinations.

The contractile reaction is associated with run-down of energy. It can only take place wdien the tissue is in a 1 The contraction of the animal muscle is a very' complex phenomenon, supposed to take place through two distinct processes. ' The energy required for the second process is afforded by a reaction in which some substance, carbohydrate or fat, is oxidised. Much oxygen is used and carbo. -dioxide given off.' — Bayliss, Principles of General Physiology, p. 461.

favourable tonic condition, by which is meant its previous absorption of energy from external sources. An atonic tissue loses for the time being its power of contraction. There is an additional factor which determines the rapidity of contraction. This is the presence of an activ;. substance in the contractile cells. The rapidly contractile cells are demarcated from others by the differential action of stains. It is shown that the distribution of the stainable active substance affords a measure of the rapidity of contraction of the motor organ. In the semi-active Neptunia the active cells are more sparsely distributed than in Mimosa. The inactive pulvini contain no trace of the active substance. The plasma of active cells exhibits a marked granular character.

The energy for rapid movement must be ultimately derived from a catabolic process of combustion, using that term in its wider sense. The active substance is shown to be highly oxidisable, being an unsaturated compound with double or triple bond-combinations. The question lias already been raised whether or not the contractile ieaction of the plant is fundamentally similar to that of the animal. The prevailing belief has been that the two phenomena are very different. It must have been the want of proper means for the accurate analysis of plant - response and its induced variations that led to this wrong conception. New devices and instruments have now removed all experimental difficulties. The results already given show that the phenomenon of contractile response in the plant is essentially similar to that in the animal. Both exhibit, under parallel circumstances, a similar staircase increase of response on stimulation when in the condition of subtonicity, and a similar decrease of response under fatigue. The. effects of drugs, of narcotics, and of poisons will presently be shown to be very similar in the two cases. Finally, the power of quick reaction is found in both to be due to the presence of some active substance. The physio¬ logical mechanism in the two cases would thus appear to be fundamentally similar. The simpler reaction of the plant may therefore be expected to throw light on the more complex reaction of the animal.

There is a very special advantage offered by the plant which arises from the relative slowness of its reactions, bor example, the latent period of the pulvinus of Mimosa is about forty times longer than that of frog’s muscle ; more¬ over, the phase of contraction of the muscle lasts Rlcr of a second, in contrast to the longer period of i*i secund of the pulvinus. It thus happens that, in consequence of instrumental inertia, the course of events in a rapid muscular twitch is very inaccurately reproduced in the record. When under certain circumstances there are two opposite responses rapidly following each other — a short-lived and feebL positive followed by a stronger negative — then the record of the former will be completely obliterated by the latter. It is through the fortunate circumstance of the relatively slow reaction in the plant that it has been possible to demonstrate the important fact that stimulation gives rise to a double response, a positive and a negative.

I proceed to describe the effects of variation of light, of turgor, of temperature, and also of the action of drugs on the responsive movement. A series of normal responses is first obtained under uniform stimulation ; then the effect of any given agent is observed in the variation induced in the amplitude of response. The experiments were carried out in an open veranda under uniform intensity of light. Experiment 33. — Mimosa is extremely sensitive to varia¬ tions of light. Even a passing cloud induces a marked

I* ic. 38. Depressing effect ot a passing cloud on the response change in the excitability of the motor organ. This is clearly shown in the record (fig. 38), in which the first four are the normal uniform responses ; the next three show the depression induced by slight darkening due to the passage of a cloud across the sky. The sky cleared and the subse¬ quent responses indicate restoration of the normal moto- excitability. Various sensitive plants, growing in the open, are found to lose their moto-excitability on rainy days. This loss of