The Motor Mechanism of Plants

The identical character of the rhythmic mechanism in the plant and animal has been demonstrated in the foregoing chapters. The peristaltic action by which the contents of the heart and of the stomach are propelled forward has been discussed. The Jaw determining the direction of pro¬ pagation of the peristaltic wave has also been established. Turning now to the propulsion of sap in plants, the hypothesis of physiological action received a severe blow from the inconclusive experiments of Strasburger on the action of poison. He found that when the cut end ot a tree-trunk was kept in a bath of poison, the ascent of sap still persisted, the leaves exhibiting no sign of drooping for many days. The inference drawn from the experiment was that living elements could not in any way co-operate in the ascent of sap, since application of poison must necessarily kill ail living tissues. My investigations, to be presently described (Experiment 193b lead to a conclusion diametrically opposite to Strasburger’ s, proving that the movement of sap is brought about by the activity of living cells. It should be stated that Strasburger himself was fully aware ol the inadequacy of a purely physical theory.1

1 1 he character of the forces which cause the ascent of the trans¬ piration current is ./till unexplained ; whilst transpiration makes a place for the inflowing water, it does not furnish the force which is necessary to rapidly convey a iarge volume of water for a considerable distance through the wood. Every operation by which work is accomplished implies a corresponding expenditure of force; and the force capable of raising grea+ masses of water to the tops of a tall Poplar, or of a Eucalyptus 150 m. high, must be considerable. B at, as yet all efforts to determine the nature of this force ha\ e been fruitless. Osmotic forces act too slowly to be of any value, and, moreover, there is no fixed distribution of osmotic substances that would account for such a current.’ — Strasburger Text-book oj Botany (English translation), p. 187.

Since the activity of living cells is precluded according to the erroneous inference drawn from Strasburger's results, it has been supposed that the ascent of the sap is mainly due to the activity of the terminal organs, of which the root exerts an upward pressure from below and the transpiring leaves a suction from above. 1 here is no specific activity in the stem itself for effecting propulsion. The removal of water from the leaves by transpiration is supposed to exert a pull along cohering columns of water in the vessels present in the wood. But since the water- columns in the vessels are not continuous, being interrupted by air-bubbles, it is obviously impossible for these strands of water to withstand tension, particularly if they are inter¬ rupted here and there. So much for the puil from above. A push from below is supposed to be exerted by root-pressure. But in the Palm there is no detectable root-pressure, yet the sap rises to a height of more than a hundred feet. Again, when the need of the tree is greatest during active trans¬ piration, the root-pressure, instead oi being positive, is actually negative. The fact that the presence of the terminal organs is by no means essential lor the process will he demonstrated later by evidence that the sap-movement persists even after complete removal of the root and the leaves.

The principal difficulty in the investigation of the pro¬ pulsion of sap has been the absence of any adequate method for its detection. This has been removed by a number of independent devices and instruments of very great sen¬ sitiveness. The methods are as follows : The details of each of these methods will be given later. The important point to be noted is tha. though the methods of experimentation are so widely different, yet they all bear concurrent testimony to the correctness of the results that have been secured.

The results of numerous experiments performed on these methods led to the conclusion that the propulsion of sap in the plant is effected by a living rhythmic mechanism. The important results are as follows : modified by all conditions which enhance or depress other pulsatory activities of animals or plants. tissue is demonstrated by the concomitant electric pulsations ; the active layer is localised b}' means of the Electric Probe. sap is exhibited in the sphygmographic record of pressure-variation, analogous to that of the varia¬ tion of blood-pressure in the arter}' of an animal.

of blood can be reversed. Similar reversal is de¬ monstrable in the case of the plant. In regard to the action of poison, Strasburger was led to believe (1) that the leaves of the poisoned tree did not show any sign of drooping, and (2) that the rate of ascent was not affected by the poisoning. The following experiments demonstrate, however, that the leaves do exhibit drooping and chat the ascent of sap does become abolished under the action of poison.

Effect of poison on the attitude of the leaf. — The physio¬ logical activity underlying the ascent is strikingly demon¬ strated by the contrasted effects of a stimulant and a poison on the leaf. There are various herbaceous plants in which the rate of ascent is exceptionally high. When water is withheld from such a plant, the stem doubles over : the leaves become dry, crumpled up, and to all appearance, dead. When water is applied to the freshly cut end of the drooping stem, a remarkable change takes place. The original turgor is restored : the bent stem straightens up and the withered leaves spread out in their original vigour. In an Indian plant, Swertia Chi rata , the transformation from apparent

Fig. 191. Left: Erect shoot placed id poisonous solution p, and drooping shoot in stimulating solution s. Right : The poisoned shoot droops and dies, while the drooping shoot becomes erected. death to a fully erect and outspread attitude is attained in a period as short as 4 minutes. An even quicker revival takes place if the water contains traces of a stimulant. The effect of a poisonous solution on an erect and vigorous specimen is precisely the opposite; the leaves, from beiow upwards, collapse in succession ; the stem also undergoes collapse, and the plant soon becomes a huddled mass of dying tissue.

Experiment 193. — An erect shoot of Centaurea was placed in a vessel P containing a poisonous solution of Potassium Cyanide, while a drooping specimen of the plant was placed in a vessel S which contained a stimulant (the photograph on the left, tig. 191). The opposite effects of poisonous and of stimulating solutions are strikingly shown in the photograph on the right, taken after 20 minutes. The erect specimen in the poison shows complete collapse, while the drooping shoot in the stimulant exhibits a vigorous recover*7 (hg. 191).

1 will next show that under the action of poison the rate of movement of sap undergoes a diminution which culminates in permanent arrest. Experiment 194.— The cut end of a shoot of Helianthus annum was placed in a poisonous solution of arsenic. Ob¬ servation was taken every 5 minutes of the changing rate oi suction b}.7 the Potograph fully described in a subsequent chapter {see hg. 216). The responsive movement of the leaf was also taken simultaneously.

Ihe normal rate of suction associated with the ascent of sap was 3 * t cubic mm. per minute. 1 his fell to 2 ^4 cubic mm. 3 minutes after the application of the poison, the subse¬ quent diminution of the rate was very rapid, suction being abolished alter 45 minutes. Ihe diminishing rate of ascent of sap is shown inde¬ pendently by the responsive fall of the leaf, which was determined by .he following simple method. A glass fibre was attached to a leaf to serve as an index, the responsive movement being measured against a circular scale. The leaf, borne on the stem with its cut end in water, was m the normal outspread position about 20° above the horizon or at an angle of 90° + 20° = iio° measured from the vertical stem below. Substitution of poison for the

water produced an almost immediate movement of fall, which increased continuously with the duration of applica¬ tion. The total angular fall in 45 minutes was 85°. The fall, which occurred immediately, was not due to the action of poison on the leaf itself, for it takes a considerable length of time for the poison to ascend through the intervening distance. The fall of the leaf indicated an insufficient supply of water, which is clearly attributable to a diminution in the rate of ascent of sap in the stem caused by the action of poison. The poison reached the leaf later, and killed it. The following table gives the successive rates of suction every 5 minutes, and the increasing angular fall of the leaf.

Table XVII. — Diminishing Rates of Suction and the Angular Fall of the Leaf under Poison (Helianthus annuus). The curves given in fig. 192 show that the effect of ; poison was a simultaneous diminution of the rate of suction and an increasing fall of the leaf. The movement of sap takes place in even small pieces of cut stem ; the activity * underlying the propulsion of sap is therefore not confined , to any particular region of the plant, but exists throughout 4 its length, in the experiment described, the diminishing * rate of ascent and its final abolition indicated that the .rise of poison put successive active zones of the living stem out of action.

clusively prove that the ascent of sap is essentially due to the activity of living cells. The question whether this activity is of a rhythmic character can be settled by some of the tests previously employed in the determination of the pulsatory character of a tissue. It has been shown that stimulation causes an enhancement of the activity of the I'lG. 192. Effect of poisonous solution in diminishing the rate of suction (continuous curve) : the responsive fall’ of the leaf (dotted curve).

lieait when in a subtomc condition ; a similar reaction was observed in the case of the pulsating leadet of Desmodium. Their rhythmic activity was enhanced by a rise and depi essed by a fall of temperature, and underwent characteristic modifications under the action of stimulating and depressing drugs. In carrying out the Various tests in this chapter the leaf served as the indicator of the movement of sap. idle accompanying records are curves traced by a leaf of the shoot under experiment.

The principle of this method depends on utilising the leaf as an indicator of the movement of sap. When the activity of the propulsion of sap undergoes a diminution, the leaf exhibits a movement of fall ; enhanced activity,- on the other hand, causes a rapid erection. These move¬ ments are, however, too slight in themselves for direct ob¬ servation ; they are rendered conspicuous under high magni¬ fication, as in the Phvtograph already described (see fig. 7).

The response obtained with the leaf of a cut shoot is practically the same as that of an intact plant with roots ; a cut shoot obviously offers greater facilities for manipula¬ tion. The cut end of the shoot Fig. 193. The Applicator for applying different solutions at cut end of the svem. water or b} chemical de¬ pressants or stimulants. The Applicator has been devised for this purpose . it consists of a nose-piece carrying two small test-tubes containing

different solutions, which can be raised or lowered by means of the key K, while the handle H brings one or the other test-tube under the cut end of the stem (fig. 193)- 1 For securing good results, it is necessary to choose young and vigorous specimens. It may be said in general that the power of response is dependent on the physiological activity of the specimen, which is modified by age, season, and climatic conditions. Old specimens exhibit feeble

activity ; the excitability of the plant is greatly depressed during stormy weather, but on succeeding sunny days the excitability is found to be above the normal. It is probable that this is due to the favourable after-effect of mechanical stimulation caused by the swaying movements of the plant during storm. Temperature has a very marked influence on the activity of the ascent. The temperature at midday m summer in the plains is often as high as 38° C., which is above the optimum of the plant. Satisfactory records can therefore be obtained only in the morning, when the temperature is near 32 0 C. In my Mayapuri Research Station in the hills at Darjiling, at a height of 7000 ft., the temperature in the morning inside the greenhouse was as low as 13 °, and good results were obtainable only at midday when the temperature had risen to 25 0 C.

The following is a list of plants arranged according to their activity in the transport of sap : '1 hough Impatiens is not very active, I have used it for a large number of my experiments, since it is easily available in all parts of the world, and practicallv through all seasons of the year. Effect of Stimulation in Reviving Propulsive Activity in Subtonic Shoots Experiment 195. Effect of photic stimulation. — In order to induce the condition of subtonicity, the specimen was kept in the dark for 24 horns within a glass covered with black

cloth. The feeble rate of ascent of sap in the depressed con¬ dition of the plant was indicated by a slow erectile response of the indicating leaf after irrigation. The removal of the black cloth, exposing the plant to the stimulus of strong diffuse light of the sky, caused increased activity, as shown Fig. 194. Effect of stimulus of light applied at l in enhancing Fig. 195. The arrested activity of ascent revived by electric in the very erect curve of respcnse (fig. 194). It might be thought that the enhanced rate of ascent was not due to stimulation as such, but to the indirect effect of light in opening the stomata of the leaves and causing greater tran¬ spiration. But the greater loss of water by transpiration would have produced, not the erectile movement which actually too place, but the fall of the leaf. (f

Experiment 196. Effect of elect ; ic stimulation. — In order to eliminate the complications possible in the case of light, I next employed stimulation by tetarnsing induction- shocks. To begin with, the specimen was so subtonic that there was no active ascent, the record given by the leaf being horizontal. On subjecting the length of ihe stem to electric stimulation, the activity of ascent was revived, as shown by the up-curve traced by the leaf (fig. 195).

Experiment 197. ---The plant was in a condition of in¬ cipient drought and the first part of the record indicates a ri(-. [96. Effect of alternate application of cold c in depressing (down-curve) and of heat h in enhancing (up-curve) rate of ascent of sap, as traced oy the indicating leaf. fall of the leaf. Water at ordinary temperature was applied at the vertical line. This caused, after a short latent period, an erectile movement of the leaf. Cold water was next substituted at C ; this resulted in an arrest of the erectile movement, due evidently to induced depression in the rate of ascent. Warm wa^er applied at H caused a greater

amplitude of the erectile response. Cold and warm water were subsequently applied in succession, the duration of applica¬ tion being also increased. The increasing responsive fall and erection of the leaf prove conclusively the depression of ascent of sap under cold and acceleration under heat In another experiment, the effect of irrigation with water at different temperatures was studied. The rate of ascent induced by water at 340 wa^ found to be two and a half times that by water at 20 0 C.

Experiment 198. — The leaf, in a condition of balanced turgor, was in a horizontal position. Application of potas- Fig. 197. Alternate arrest and enhancement of ascent of sap by chemical depressant KBr and stimulaiV Camphor. iium bromide produced so great a depression that the leaf exhibited a rapid fall. The application of a small dose of ;amphor, which is a stimulant, produced arrest of the fall and the subsequent erection of the leaf (fig. 197). These results indicate the characteristic stimulation and depression of propulsive activity induced by drugs.

Experiment 199. — The abolition of activity of ascent under poison has been previously demonstrated (c/. fig. 192). The following experiment shows the irreversible effect of Abolition of ascent of sap by poison caused fall of leaf. Sub¬ stitution oi fresh water at horizontal arrow induced no revival. poison in arresting ascent of sap as indicated by the responsive fail of the leaf. As the plant was in a condition of balanced turgor the initial -ecord gi-en by the leaf was horizontal. Application of very strong poisonous solution of KCN caused the fall of the leaf within a short time. The irreversible action of the poison is shown by the fact that application of fresh water did not arrest the fall [fig. 198) : clearly, the ascent oi sap had been permanently abolished.

It has been explained how change induced in the activity of ascent of sap can be detected (i) by the Potograph, which measures the rate of suction, and (2) by the responsive move¬ ment of a leaf which is dependent upon the supply of water from the stem. Enhanced activity of ascent of sap in the stem causes an erectile movement, while depressed activity produces a fall of the leaf. The responsive movements of the leaf, an erection or a fall, are respectively due to the induced increase or diminution of turgor of the leaf-joint ; this is not a passive but an active process. The turgor of a tissue can be increased only when sap i: pumped into it ; diminution of turgor, on the other hand, can occur only when sap is removed from it.

Poisonous solutions cause retardation and final abolition of the ascent of sap. This is simultaneously indicated by the continuous • diminution of the rate of suction, and by the increasing fall of the indicating leaf. The activity under¬ lying the propulsion of sap is not confined to any particular region, but exists throughout the length of the plant. After the application of poison, successive zones of the living stem are put out of action, resulting in a diminishing rate of ascent and its filial abolition. »

Physiological changes which enhance the pulsatory activity of the heart and of the Desmodium leaflet, also cause an enhancement of the activity of the ascent of sap ; agents which depress the activity of these organs also The activity of the ascent Q.f sap undergoes diminu¬ tion or arrest in the condition of subtonieity. External stimulation revives the activity, renewing it or enhancing the rate of ascent. Irrigation with cold water lowers the rate of ascent of sap, while application of warm water enhances it.

Solution of potassium bromide induces its characteristic depression of activity, and thus diminirhes the rate of ascent. A stimulating agent, like a small dose of camphor, revives the activity and enhances the rate. Poisonous solutions produce permanent abolition of the ascent of sap. The foregoing results of experimental investigation lead to the conclusion that the movement of sap is dependent upon the action of a physiological, not a physical, force, the source of which is to be sought in the living tissue of the stem, and that the propulsion of sap is maintained by a pulsating mechanism similar to that of the cardiac tissue of the animal and of the pulsating tissue of the Desmodium leaflet.