The Motor Mechanism of Plants

movement of sap on stimulation ? From the ordinarily accepted point of view, nothing could be more unlikely ; for under the special conditions of the experiment, the accession and removal of water at the two ends of the organ on which the movement of sap is supposed to depend are com¬ pletely eliminated. The question was put to experimental test, by subjecting one or the other end oi the organ to various modes of stimulation, electric, chemical, or thermal.

1 describe first the method of electric stimulation by tetanising indue cion-sliocks of moderate intensity, applied (i) below the contact, (2) above the contact, and (3) both below and above the contact simultaneously. Two platinum pins were thrust, 5 mm. apart, into the stem for the passage Fig. 236. Application of electric stimulation §E b Tow contact, a is th~ apical, and b the basal end of the stem. The direction of arrow indicates the direction of responsive propulsion of sap.

of the shock, the stem being kept sealed and dry. Electric stimulation has this advantage, that its intensity can be maintained constant in successive experiments. The experimental method is diagrammaticaUy repre¬ sented in fig. 236. The sealed stem is represented by A B, of which A is the apical, and B the basal end. The direction of the peristaltic wave causing the normal ascent in the intact plant is upwards from B to A. The specimens which give the best res ills are young stem? 1 in which the rate of ascent ic most rapid, such as Cosmos and Antirrhinum in their proper season. It is better to choose stems which a> e in a condition of moderate drought, for over-distension is unfavourable lor the transmission of +he contractile wave. The pokh of

1 In yenng stems the nervous tissue is but feebly developed ; there is therefore no transmission of excitatory impulse, specially when the stimulation is of moderate intensity. application of stimulus is at a distance of about 15 mm. from the sphygmographic contact. Electric stimulation applied on the right, or below the contact, is distinguished by the symbol SK ; stimulation on the left, or above th* contact, is indicated by S'E. Experiment 245. Electric stimulation below contact. — • This and the following experiments were carried out with an identical sealed stem of Cosmos. Stimulation on the right or towards the basal end caused, in the course of 2 minutes, an increase of pressure at the contact-point above, indicated by a deflection of -f- 400 mm. This increase of pressure, as previously explained, is caused by the sap being forced into the region of contact by a peristaltic impulse in the same direction as that of the normal rise of sap. Stimulation tints initiated a movement ''f sap in the quiescent stem, the sap- flow being t directed from the stimulated point to the un stimulated region of the contact.

Experiment 246. Electric stimulation above contact. — Stimulation of the same intensity applied at the same distance above the contact gave rise to an antiperistaltic impulse, which also caused an increase of pressure measured b} a deflection of + 100 mm. It may be asked why stimulation, applied either above or below, should produce in every case an increase of pressure. Again, why should the effect of the peristaltic wrave be greater than that of the antiperistaltic wave ? J will presently explain the cause of the difference.

Photographic records 01 the effects o* electric stimula¬ tions of the same intensity applied successively below and’ above the contact are given in fig. 237. In the first case the transmission is peristaltic, in the second antiperistaltic* The characteristic difference in the effecf s transmitted in the two directions is quite evident, the peristaltic reaction being abo”* a times more intense. Experiment 247. Effect 0} simultaneous electric stimula¬ tion below and above the contact. — Stimulations below and

Fig. 237. Record of transmitted effect of electric stimulation sE above the contact have been shown to produce similar effects. Their simultaneous effect is clearly additive, since the result actually obtained was a response of larger amplitude. In the experiments described above the responsive flow of s.ip was from the stimulated end, basal or apical, towards the unstimulated point of contact. The flow of sap followed the stimulation-gradient from the point activated by stimula¬ tion to the contact- point which had not been so activated , lhe results obtaineu with a sealed stem with its activity at standstill are thus exactly parallel to those obtained with the quiescent stomach. In both, peristaltic, or antiperistaitic

waves are generated by stimulation, the direction of propaga¬ tion of which is from the more stimulated towards the less stimulated end. Moreover, the peristaltic wave in both is at least 3 times more intense than the antiperistaltit wave. The facts of peristalsis under electric stimulation are thus identical in the plant and in the animal; it has now to be ascertained whether this is equally true in regard to chemical and thermal stimulation. Lire stimulating action of a drug, camphor for example, was demonstrated in a previous chapter by its enhancing the rate of ascent of sap in a cut stem bearing leaves. In the condition of balance, the pressure at the sphygmographic contact was constant ; application of a chemical stimulant was shown to produce an upsetting of the balance due to an increase of pressure (Experiment 233). The conditions of the experiment with the sealed stem are, however, very different, inasmuch as the propulsive cells are in a state of quiescence to begin with. Any movement of sap that may be induced must therefore be due to the initiation of peristalsis by the chemical stimulation.

The sealed stem was cleared of varnish for a short length to the right and to the left of the contact, and the epidermis slightly abraded for +he application of a small quantity of the stimulant with a camel-hair brush. Inasmuch as the stem was sealed, there was no suction exerted by transpiration, so that the quantity of solution absorbed at the abraded point was minute and practically negligible. The induced movement ot sap observed wTas therefoie due, not to the absorption of the minute quantity of the chemical solution but to its stimulating action. That the absorption of the solution itself had nothing to do with the directive move¬ ment of sap is demonstrated by the action of a depres¬ sant solution, which caused a reversal in the direction ot propulsion (Experiment °so).

Experiment 248. Chemical stimulation below contact . — - The effect of stimulation by camphor was a positive deflec¬ tion indicating an increase of pressure at the sphygmo- graphic contact. The peristaltic wave thus initiated had caused a movement of the sap, forcing it into the region of contact. Experiment 249. Chemical stimulation above contact . — The effect was the same as in the last case, namely, an increase of pressure at the sphygmograpnic contact. The impulse in this case was antiperistaltic, the sap being forced down¬ wards into the contact region, against the direction of the normal ascent (upper diagram, fig. 238).

Fig. 2 38. Diagrammatic representation of effects of chemical Chemical stimulation sg below, induced peristaltic movement of sap towards contact c shown by arrow. Chemical stimula¬ tion 'i*c above contact, induced antiperistaltic movement of sap towards c (upper figure). Chemical depressant applied either dc below or d'c above, induced movement of sap away from contact c (lower figure). * J he effect of a depressant was precisely opposite to that of a stimulant.

Experiment 230. A solution of a chemical depressant, bromide of potassium, was applied successively below and above the sphygmographic contact. In both cases a diminu¬ tion of pressure was recorded, due to removal or withdrawal of sap from the contact-region. The effect of the application of a depressant in initiating movement of sap away from the contact is represented in the V^er diagram of fig. Experiment 251. Effect of alternate application of stimu¬ lant and depressant. — Chemical stimulation, whether above or below the contact, has been shown to produce an increase of pressure there, the effects being concordant. Simula taneous application of a stimulant above and below the sphygmographic contact was therefore found to produce an additive effect of increased pressure. Similarly, simul¬ taneous application of a depressant above and below induced

Fig. 239. Increase of pressure and movement of sap towards contact induced by simultaneous application of camphor Cam below and above contact. Simultaneous application of bromide br both below and above reversed the direction of movement of sap away from contact. Effects are repeated in second series. a great diminution of pressure. The alternating increase and diminution of pressure are exhibited in the record (fig. 239), in which simultaneous application of solution of camphor above and below produced a great increase of pressure, shown by the up-curve ; simultaneous application of potassium bromide above and below not only arrested the impulsSf but. reversed the response into one of marked diminution oi pressure. The alternating positive and negative responses were obtained many times in succession (fig. 239).

In both these cases the movement of sap is from the stimulated to the less stimulated or depressed region. Chemical stimulation above or below caused sap-movement towards the unstinu dated point of contact. Chemical depres sants applied above or below made the sap flow away from the* contact, which was relatively speaking the more active region. The following tabular statement clearly presents the different results : Table XXV. — Effects of Chemical Stimulant and Depressant Applied at Two Ends of the Organ.

Antiperistaltic wave ; sap-movement to¬ wards contact Antiperistaltic wave ; sap-movement away from contact Peristaltic wave ; sro- movement away from contact Movement of sap transporting chemical substances :n leafless trees in spring. — The young spring buds on the leafless trees require a supply of chemical substances for their development. These substances, stored at a distance, can only be transported in solution to the growing-points by the movement of the sap. The foregoing experimental results suggest the satisfactory explanation of the necessary directive movement of sc p, that it is attributable to differ¬ ential chemical stimulation.

I next describe the effects of thermal change in initiating tne movement of sap. Application of heat has been shown to enhance the rhythmic propulsive activity, whereas cold caused depression (Experiment 197). The directive move¬ ment of sap under thermal Stimulation or depression has been fully demonstrated by means of .the Potograph (Chapter XXV), the movement induced being indicated by the positive or negative excursion of the water-index. The direction of propulsion was found to be' always from the more to the less stimulated or depressed end (Experiments

In the present case the conditions of the experiment, as well as the method of detection of the movement of sap, are very different. 'I he stem, in the first place, is completely sealed ; and the initiation of sap-movement is detected by the Sphygmograph. 'the results obtained by methods so widely different are, however, in absolute agreement with each other. As one oi the objects of the investigation was the demon¬ stration of the identical reaction of plant and animal, I at

Fig. 240. Method of thermal stimulation by loop of wire heated by passage of an electric current. first used the well-known means that has been employed for local stimulation of different regions cf the heart, by a loop of electrically heated wiie. Thermal stimulation of the \entricle was thus found to enhance its a tivity, as shown by an increased amplitude of pulsation. A precisely simi¬ lar method was employed for local stimulation of the stem, either below or above the cont vet ; or both below arid above the contact simultaneously.

Thermal stimulation can also be effected more simply by applying, by means of a pipette, a sms 11 quantity of warm water on a particular point in the stem, round which is wrapped a piece of tinfoil. Stimulation by variation of temperature can thus be produced without the possibility 01 introduction of water into the tissue, ^he characteristic results were found to be in every way similar Whether the simulation was produced by a loop of electrically heated

wire, or by application of warm water. I describe typical results obtained, first with the sealed stem of Cosmos , thermal stimulation was successively applied below and above the sphygmographic contact at a distance of 15 mm. ; the resulting variation was observed alter an interval of 30 seconds. Experiment 252. Thermal stimulation below. — this in¬ duced sap-moverrent upwards, the wave being peristaltic. The time of transmission of the impulse to the sphygmo¬ graphic contact was found to be 4 seconds. The increase of pressure produced a deflection of -f- 1200 mm. the next experiment was continued with the same plant.

Experiment 253. Thermal stimulation above. — ibis caused a movement of sap downwards against the direction of normal ascert. The time of the antiperistaltic trans¬ mission was 11 seconds, that is, nearly 3 times longer than that of the peristaltic wave. The response by pressure-variation was of the same sign as in the last case, the deflection being -f 400 mm. The notable fact is that thermal stimulation whether above or below the contact induces the same effect, namelv, an increase of sap -pressure at that point. The induced move¬ ment of sap, as in the case of other modes of stimulation, is from the more to the less stimulated region.

The above results wer i obtained with Cosmos at its best season, at the beginning of February, By the end of March the plants Tiad grown too old, resulting in a considerable decline of sensitivity. I was therefore obligea to continue tne investigation with other species of plants, which, though less sensitive, still gave consistent results, as summarised below. It is very significant that, as in the stomach so also in tiie stem, the eifect of the antiperistaltic wave is about a third of that of the peristaltic wave. This was found to be of

general occurrence in young internodes of plants. On account of lignification in the older internodes, the sensi¬ tivity falls rapidly downwards. i able XXV I. Effect of Thermal Siimulation applied Below and Above the Sphygmographic Contact. Point of application of stimulation in relation to the sphygmographic contact Experiment 254. Summated effect of simultaneous thermal stimulation below and above contact. — As the sign of the two responses is the same, the effect of simultaneous stimulation below and above the contact is found to be additive.

Experiment 255. Abolition of response after death.— This was demonstrated by experiments carried out with a stem the cut end of which was placed in a toxic solution of KCN. Ihe whole length of tl 3 stem became gradually killed from below upwards by the slow absorption of the poison, in the course of half an hour or so. \ sealed specimen of the poisoned stem showed no sap-movement under thermal stimulation. Experiment 256. Effect of stimulation of stem killed through half its length. — A very interesting result is obtained wnen the specimen is not kept too long with its lower end in the solution of poison. Under such circumstances it is only the lower half of the stem that is killeT the poison not

having yet reached the upper half. Thus a specimen of Sunflower that had been kept in a poisonous solution for 20 minutes, gave no response to thermal stimulation applied below the ''sphygmographic contact. Stimulation of the unkilled portion above the contact gave/ however, an anti- peristaltic response of increase of pressure, the deflection being 250 mm. Similarly, stimulation of the killed lower half of the stem of Cosmos geve rise to no response, while stimulation of the livirg upper half evoked a responsive antiperistaltic movement of the sap, giving a deflection of + 150 mm.

Application of cold induces the reaction of depression. The following experiments were carried out with Cosmos. Experiment 257. Application of cold below the sphy bio¬ graphic contact. — This resulted in a diminution of pressure at the contact, measured by a deflection of — 10c mm. This diminution, as previously explained, is due to movement of sap away from the contact. Experiment 258. Application of cold above the contact .— A diminution of pressure occurred in a different specimen on application of cold at a point above the contact. This diminution was indicated by a deflection of — 150 mm.

These results are now easily explained on the general principle or Law of Peristaltic Movement of Sap already established, that the res7 onsive flow is from the more stimulated to the less Simulated or depressed region. In the case of depression of activity induced by cold, the sphygmographi 3 point of contact is relatively the more active. Hence the sap flows away from it to the depressed lower or upper ends, resulting in contraction or diminution of pressure at the contact.

Experiment 259. Effect of alternate thermal stimulation and depression above and below.— 1 give a series of results obtamed with an identical specimen of Cosmos under th° following sequence of operation. The general results are shown diagrammatically in fig. 241. Heat sx applied below and s'x above the contact. Responsive flow of sap towards the nnstimulated contact-point c (upper figure). Cold dt and d't applied below and above the contact. Respon¬ sive flow of sap from the relatively more active c to the depressed regions dx> d't (lower figure).

Table XXVTI. — Effects of Four Cycles of Thermal Stimulation and Depression (Cosmos!. The demonstrations given above offer full explanation of , apparently capricious results described in Experiments 242 arid 243, in which local application of heat above the sphyg- mographic contact gave rise to an increase of pressure, while application of cold above caused a diminution of pressure. In both these cases the observed variation of pressure illustrates the law that the flow of sap is from the more active to the less active region. In the first case the increase of pressure was due to the movement and forcing in of sap to tiie less active contact-region. In the second case the diminution of -pressure was due to the flowing away of sap from the relatively active contact-region to the depressed region above.

Experiment 200. Effect of simultaneous stimulation or de¬ pression at the two end s. — Thermal stimulation below or above induces in each case an increase of pressure at the sphygmo- graphic contact ; depression caused by application of cold below and above induces a dimin ution of pressure. The effects of alternate simultaneous stimulation and depression both below and above the contact are shown in the photographic record given in fig. 242. These alternating directions of flow of sap induced in response to stimulation and depression can be recorded many times in succession.