The Motor Mechanism of Plants

Simultaneous record of the diametric expansion or con¬ traction of the stem and of the responsive up- and down-move¬ ment oi the leal due to the passage ol the came hydraulic wave shows that the phytogram is a replica of the sphygmogram. This proves that the cellular mechanism is identical in the stem and in the leaf, the characteristic responses of expan¬ sion and contraction being due to absorption or expulsion of sap from the cortical tissue in both. Continuity of con¬ tractile cells through the length of the plant has also been demonstrated. * *

The sap moves not only in an upward direction, but can be made to flow in the reverse or downward direction. The transference of sap in tile normal upward direction is about 6 times more rapid than that in the unusual downward direction. By means of tht^ Sphygmograph it is possible to record the diurnal variation of sap-pressure in the stem. In a leafy tree, under normal conditions, the sap-pressure is maximum at thermal dawn, at about 6 in the morning ; it is minimum at thermal noon. In a leafless tree the above relation is reversed ; the maximum pressure is attained at thermal noon and the minimum at thermal dawn.

These characteristic results confirm my previous ipano- metric results on the variation of sap- pressure. The experiments described in the previous chapters prove that there is an active tissue somewhere in the interior of the plant, the pulsation of which effects the propulsion of the sap in the plant, just as the pulsation of the heart maintains the circulation of the blood in the animal. The question arises whether it is possible to localise this pulsating layer in the plant, and to obtain an actual record of cellular pulsation bv which the movement of sap is main¬ tained. The problem Was solved by the employment of an electric method the principle of which will be understood from the following experimental demonstration.

It has been shown in a previous chapter that when an electric contact is made with a resting tissue of the plant, the galvanometer in circuit remains quiescent. But when the tissue undergoes a sudden contraction under stimulation, an electric change of galvanometric negativity occurs, shown by a deflection of galvanometer in one direction. Expansion of the tissue during recovery produces a deflection in the opposite direction. The pulvinule of the leaflet of Desniodium gyranz exhibits automatic contraction and expansion of its cortical cells, as shewn by the pulsatory up- and down-

movements, i have shown in earlier publications that these movements are accompanied by corresponding electric pulsations. Experiment 208. — Two electric connexions were made, the first with the more excitable lower half of the pulvinule, and the second with a distant indifferent point on the stem, a sensitive galvanometer being interposed in the circuit (fig. 208). The records of mechanical and electric Fig. 208. Method for record of electric pulsation of pulvinule of

Electric connexions made one contact with the pulsating pulvinule, the other contact with an indifferent point on the pulsations taken simultaneously show that the electric record is practically a replica of the mechanical (fig. — 09). The fact that the electric pulsation is due to an underlying cellular pulsation, and not to the movement of the leaflet, was demonstrated by holding it in a fixed position ; 1 he electric pulsation was nevertheless found to persist. The experiment described proves that :

(1) A resting non-rhythmic tissue exhibits no electric pulsation. Tig. 209. Simultaneous record of mechanical pulsation (upper figure) and corresponding electric pulsation (lower figure) of pulvinule of Desmodium. I have obtained simitar results, oy the same method, with tne propmsive tissue of the stem. Experiment 209.-111 order to localise the tissue active in the propulsion of sap, I introduced the Electric Probe (insulated except at the tip) transversely into the stem of

Impatiens by successive steps of *05 mm. (fig. 210). No pulsation was observed in the epidermis or in the sub¬ epidermis ; but when the probe reached the internal cortex abutting on the vascular tissue, evidence of pronounced rhythmic activity ot the tissue was given by electric pulsa¬ tions (fig. 211) ; the pulsations disappeared when the probe came in contact with the xylem, which does not, therefore, take any active part in the propulsion of sap.1

F ig. 2 10. The Electric Probe for the localisation of the pulsating The point of the Probe enters the stein at a, the second electric contact being made with a. distant' leaf. The figure to the right is an enlarged view with the micrometric screw for the gradual introduction of the Probe into the tissue of the plant. These and other results to be presently described offer conclusive proof that the ascent of sap is brought about by the pulsating activity of a propulsive layer situated ir. the inner cortex surrounding the vascular tissue. It should, j however, be borne in mind that under exceptional circurn- || stances all lining cells are capable of being thrown into

1 ‘ The inactive xylem vessels are situated very near the cortex whhm a fraction of a mm. or so ; hence it is easy for the active cortex to fence the sap laterally into the xylem during the phase of contraction. - xylem may therefore be regarded as a reservoir, water being pumped into or withdrawn from it according to the different circumstances. Physiology of the Ascent of Cap (iqc j), p. 3S. rhythmic activity ; certain layers of cells are, however, naturally more active than others, and it is by the exception ¬ ally high automaticity of such cells in the inner cortex that the rapid propulsion of sap is maintained.

In an ordinary non-rhythmic cell the state of turgor is normally constant, and its electric potential therefore Fig. 21 i. Record showing the amplitude of electric pulsation in different layers of stem of Impatiens. Note the abrupt enhancement at a distance of 0-3 mm. from th<: surface, the particular layer being in the inner cortex ; a portion of the record has gone out of the plate. remains unchanged. But in a pulsating cell there is a series of changes of turgor with alternate expansion and con¬ traction, indicated in the record by up-strokes end down- strokes. The phase of increased turgor is accompanied by a positive, and of diminished turgor b}^ a negative, electric variation. The galvanometer spot of light thus reveals, by its alternate swings to the right or to the left, the invisible pulsations of the active cells in the interior of the plant.

The electric pulsations in the cortex can be recorded without much difficulty. One of the conditions for obtain¬ ing a satisfactory record is the electromotivity of the tissue, Fig. 212. Electric connexions for record of electric pulsation of a, connected in all cases •with the Electric Probe. The second connexion, b, with the indifferent point is made a floral contact in a, a physiologically isolated contact in b, and a laminar contact in c. by which is meant the intensity of the electric variation induced by cellular expansion or contraction. In this respect, certain plants give better xesults than others. 1 he second and most important condition is that while one of the two electric contacts is on the cortex of the stern, the other should be made with an indifferent point which is

unaffected by changes in the rate of ascent of sap in the stem. The indifferent point may be (x) a floral contact, (2) a physiologically isolated contact, or (3) a laminaf contact. Floral Contact. — The distant indifferent contact B is made with a petal of a flower in which there is little venation (fig. 212, a). Isolated Contact. — A distant indifferent point is better secured by physiological interruption between the two con¬ tacts. Two separate pieces of stem (one shown vertical and the other horizontal) are electrically connected by a silver wire C. It is obvious that changes of turgor in A cannot produce any change in B (fig. 212, b).

Laminar Contact. — A suitable speci¬ men for this is the leaf of Brassica ; contact A is made on the midrib in which conduction of sap takes place ; contact B is made on the lamina, to which the supply of sap is interrupted by cutting across a number of the lateral veins (fig. 212, c). ' Employing one or other of the three methods, the gaivanometric record of the electric pulsations was taken on a moving photographic plate. Under conditions of balance and uniform external conditions, the pulsations are practically uniform, the up-stroke of gaivanometric positivity indicating cellular expansion being equal to the down-stroke of gaivano¬ metric negativity indicating contraction (fig. 213).

It has been shown in previous chapters that all conditions which stimulate the pulsatory activity of the Desmodium leaflet or of the animal heart also enhance the activity of the ascent of sap ; other conditions wThich depress the pulsatory activity of those organs induce depression or arrest of the ascent. Having discovered that the inner cortex of the stem is the actively pulsating layer, it can be shown that Fig. 213. Record or uniform electric pulsation in the Mango tree.

enhancement and depression of the pulsatory activity of that layer (as manifested by corresponding modification of the electric pulsation) induce corresponding variations in the rate of ascent of sap. Desmodimn pulsation. — The effect of diminished internal pressure under drought in arresting the pulsation of theDes- modium leaflet, and the revival of pulsation after irrigation, Fig. 214. Record of electric pulsation under alternate drought and irrigation (stem of Impatiens).

Up-curve indicates electric positivity ; down-curve, electric negativity. Potted line below represents condition of drought, continuous line indicates irrigation. have already been shown ( see fig. 1 57). Drought and irrigation similarly affect pulsation of the propulsive layer as indicated in the electric record. Experiment 21a. Electric pulsation in stem . — The record of the electric pulsation of the propulsive layer in the stem of Impatiens is given in fig. 214. The first record was taken

in a condition oi drought, the pulsation being extremely feeble ; on irrigation the pulsatory activity is seen to have been greatly enhanced, as shown by the increase in the amplitude o 1 electric pulsation, which was repeatedly de¬ pressed under drought and revived after supply of water ( see also fig. 92). Desmodium pulsation— The pulsation of Desmodium exhibited arrest at a low temperature, and revival when the : temperature rose above the critical thermometric minimum {of fig. 165).

Experiment 21 1. Ulcctvic pulsation in stem, — I took a cut shoo 1 ol Centaurea in a condition of drought. The pulsation was very feeble, but application of water at 20° at the cut end of the stem caused a revival. The pumping activitv was arrested by substitution of cold water at 50 for water at 20°, the electric pulsation showing a great de- pi ession. Irrigation with warm water once more revived the electric pulsation. I Tiie mechanical effect of alternate application of cold in lepressnig and of heat in enhancing the rate of ascent ha^ already been demonstrated (< -/ . fig. 196).

DesmodiMn pulsation.— When the plant is kept in the dark 1 or 24 hours, it becomes so subtonic that its pulsatory act.vny comes to a stop ; stimulation by electric shock or v ig it was found to revive the arrested activity (cf fms E*pe£m®nt 2I2' Metric pulsation in stem.— The electric recon. aifords a clear insight into the phenomenon of pro¬ pulsive pulsation. A cut shoot kept in a dark chamber for 2, hours became so subtonic that the electric pulsation

arfnT “ T alm°St t0 extinction' The throbbing activity was, however, revived on stimulation by light and arrested once more on the cessation of the stimulus (tig. 215). Very interesting characteristics of the cellular mechanism are revealed in this record. The up-stroke of each electric FiG. 215. Effect of stimulus of light enhancing electric pulsation, in a ^ubconic specimen (Impatiens). d, feeble pulsation in dark ; l, enhanced pulsation undei light , and d', depression under renewed darkness.

.1 Hiring stimulation the up stroke of each pulsation is larger than the down-stroke, with resulting displacement of base-ime upwards. pulsation indicates expansion and absorption of sap, while the down-stroke exhib’ts contraction and expulsion of sap ; when these two are equal, the tissue, as previously explained, is in a state of balanced turgor. But under the enhanced activity induced by the stimulation, the up-stroke in¬ dicating absorption of sap is larger than the down-stroke exhibiting expulsion. The residual effect of the enhance¬ ment of the turgor of the tissue is therefoie manifested in the increased electric positivity of the tissue which is indicated by the displacement of the base-line upwards.

I have already described (Experiments 195, 196) the effects of photic and electric stimulation in reviving or enhancing the activity of the ascent of sap in subtonic specimens (c/. figs. 194, 195). The unidirectioned propulsion of sap depends upon a sequence of pulsation from cell to cell. This has been demonstrated by the occurrence of definite electric maxima in the channel of transport, the distance between the maxi¬ mum and the minimum being hall the wave-length.1 The sap expelled during the contraction of any one cell is ab¬ sorbed by a cell higher up during its phase of expansion. A succession of sucli peristaltic waves maintains the con¬ tinuous ascent of sap.

It has been shown that automatic electric pulsations, corresponding to the mechanical pulsations, are exhibited by the pulvinule of the Desmodium leaflet. The electric pulsation persists even when mechanical movement is restrained. Electric pulsation indicates automatic cellular contrac¬ tion and expansion in the tissue. A tissue giving electric pulsations was discovered in the stun by the .Electric Probe ) it was localised in the inner cortex. Conditions which stimulate or depress the electric pulsa¬ tion of this tissue cause parallel modifications in the rale of ascent of sap. Hence it is concluded that this pulsating layer is the propulsive tissue effecting the rapid ascent of

activity finds external expression in both mechanical and electric pulsations ; in change in the rate of the ascent of sap ; and in variation of bulk of the tissue, either ex¬ pansion or contraction. The experiments in the previous chapter prove that the move¬ ment of the sap is Lrought about by the pulsatory activity of living cells which exert a pumping action ; but something still remains to be explained If the cellular pumps in the plant simply underwent periodic contraction and expansion, the sap would merely move forward and backward indefi¬ nitely. But this is not the case : the sap normally ascends. What is the mechanism which maintains the unidirectioned flow ?

As previously stated, the rise of sap has been held to be due to the action of a pull exerted by the transpiring leaves from above and a push from below by the root. The fact that the sap-movement is not essentially dependent on the terminal organs has already been demonstrated qualita¬ tively (Experiment 205). I now describe other experiments which give quantitative results. The rate of movement of the sap and the variations induced in it were ascertained by observation of the varying rate of suction of water at the

cut end of the stem by means of the apparatus described below. i his apparatus consists of a capillary potometric tube with a contrivance for recording the excursions of a water- index and its time-relations. The record may be most simply taken by following the water-index with a recording-pen resting on a revolving drum, round which is wound the paper for the record (fig. 216). It is sufficient for many purposes, to determine the normal late and the change of that rate by noting the number of divisions through which the water-index moves in the course of a minute in a definite part of the capillary tube of the potometer. The index can readily be brought to any particular point of the capillary tube by proper manipulation of the stopcocks S or S' ; slight opening of S makes the index move to the left, that of S' to the right. The absolute rate of suction in cubic mm. is found by multiplying the length of excursion of the index per unit of time by the capillary constant of the tube.

Special care has to be taken to prevent all leakage, by smearing the indiarubber cork holding the plant with cocoa-butter. For ordinary experiments in which the effect of variation of temperature is not important, the temperature of the vessel does not, in practice, /ary from tha* of the room. But in investigating the physiological effect of variation of temperature, complications arise from the gain or loss of heat by the water in the vessel. This is reduced to a minimum by enclosing the plant-vessel in a non-conducbng cover of thick felt. It is also easy to construct a correction- curve for the particu^r apparatus. The error introduced in neglecting this correction is, however, less than 2 pei cent.

Variation in the normal rate of suction can be induced by stimulating either the upper or the lower end of the specimen. The subject will be considered in greater detail in the next chapter. The fact already stated that the ascent of sap takes place in Palms in which there is practically no root-pressure, suggests that the co-operation of the root is not essential. This is definitely proved by the results of the following experiments which show that the rate of ascent, instead of being lowered, is considerably enhanced after the removal of the root.

Experiment 213. — The rate of suction and of ascent of sap in an intact specimen of Helianthus, measured by the movement of the index in the potometer, was 15*8 mm., the quantity of water sucked up for every minute being 22 • 1 cubic mm. The root was then cut off, and the stem replaced in the potometer. After a suitable period of rest, the amount absorbed by suction per minute wTas found to have increased to 40 cubic mm., an enhancement of 1*9 times. The root, on the root-pressure of wdiich the pro¬ pulsion of sap is supposed to depend, so far from increasing the rate cf ascent, had actually impeded the flow of sap. In another experiment, with Impatiens, the rate of suction of the intact plant was more than doubled after removal of the root. The root instead of helping the ascent actually