Collected Physical Papers/General Summary
The investigations described in the present volume are on: A, the optical properties of electric waves; B, the characteristics of metallic-contact receivers which respond to electric radiation, either by a diminution or an increase of resistance; C, different types of molecular receivers which respond to diverse modes of stimulation; D, the similarity of response of inorganic and living substances; E, the physiological response of plants; and F, the identity of physiological mechanism in animal and plant.
A. Optical Properties of Electric Waves
Electric waves of great length curl round corners; accurate angular measurements are thus rendered impossible. The difficulty has been obviated by securing a narrow beam of electric radiation of relatively short length.
1. Complete Electric Wave Apparatus
The Radiator.—Electric radiation is produced by a single spark between two hemispheres and an interposed small sphere of platinum, the wave-length being reduced to about 5 mm. The sparking coil and battery are enclosed in a small double-walled metallic box with a tube for the passage of the electric beam. Magnetic disturbance due to the make and break of the sparking coil, was found to affect the receiver; the magnetic disturbance was eliminated by making the inner box of soft iron which acted as a magnetic screen.
The Receiver.—A satisfactory form of receiver was constructed of steel or of nickel; a single point contact receiver made of steel, nickel, aluminium or magnesium is also found to be highly sensitive and reliable. The sensitiveness can be exalted to any degree desirable by adjustment of electromotive force acting on the circuit. Angular measurements are made with the spectrometer circle (p. 87).
2. Selective absorption
The transparency or opacity of various substances, brick, wood, pitch and others as well as strata of different liquids and solutions can easily be demonstrated by interposing them on the path of electric beam. Water is opaque on account of absorption of radiation; liquid air is, on the other hand, quite transparent. Sheets of metal do not transmit, but reflect the beam according to laws of reflection. A long trough filled with irregularly shaped pieces of pitch is opaque on account of multiplicity of reflection and refraction. It becomes more or less transparent when partial homogeneity is restored by pouring in kerosene (p. 89).
3. Determination of Index of Refraction
The prism method is quite unsuitable for exact determination of the index for the electric ray. Accurate results have, however, been obtained by the method of total reflection. The rotation of two semi-cylinders separated by parallel air-space, produce a sudden extinction of the image at the critical angle. The index of refraction of glass for the electric ray is found to be 2.04, which is much higher than the index 1.53 for the sodium light. The index for sulphur is 1.73 (pp. 21, 31).
4. Influence of Thickness of Air-space on Total Reflection
On account of extreme shortness of wave-length of the visible rays, the thinnest air-film produces total reflection at the critical angle. The case is, however, different with comparatively greater wave-length of electric radiation. The critical thickness of air-space is modified by the angle of incidence and by the wave-length. When a cube of glass is interposed between the radiator and receiver placed opposite to each other, the radiation striking one face perpendicularly is transmitted across the opposite without deviation and causes response of the receiver. On cutting the cube across a diagonal, two right-angled isosceles prisms are obtained. When the two prisms are separated slightly, keeping the two hypoteneuses parallel, thus securing parallel air-space, the incident radiation is found divided into two complementary portions, of which one is transmitted and the other reflected by the air-film at right angles to the incident ray. When the thickness of air-space is reduced to about 0.3 mm., there is a complete transmission and no reflection. The two prisms in spite of the breach due to the air-space, are electro-optically continuous. A greater separation of the two prisms increases the reflected, at the expense of the transmitted portion; and at a certain critical thickness, there is no transmission but total reflection. If a thin piece of cardboard or any other refracting substance be next interposed, a portion of the radiation becomes transmitted, necessitating further separation of the prisms to reduce the transmitted portion to zero. This offers a means of determination of the index of refraction of plates of different substances (p. 44).
5. Double-refraction and Polarisation
The shortness of wage-length of electric radiation has enabled investigations on the polarising action of even small crystals. The polariser and analyser are made of fine wire gratings; they can also be made of a number of crystals such as Nemalite, Chrysotile, Serpentine and Epidote. The most perfect polariser and analyser are constructed of jute-fibres or of a book, both of which produce complete polarisation of the electric ray. Crystals of tetragonal, orthorhombic, hexagonal, monoclinic and triclinic systems produce double refraction and polarisation. Locks of human hair and vegetable fibres also produce polarisation (p. 96).
6. Double refraction by strained dielectric
Various stratified rocks exhibit very strong double refraction when the plane of stratification producing polarisation, is inclined at 45° to the crossed polariser and analyser, there being no effect when this plane is parallel to either the polariser or the analyser. Unannealed glass and ebonite also produce double refraction and polarisation of the electric ray (p. 95).
7. Double absorption and double conductivity
Certain crystals like Nemalite, Chrysotile and Epidote exhibit selective transparency to polarised radiation; these are transparent in one direction but opaque in a direction,which is perpendicular to the first. Complete polarisation is due to their property of double absorption. Investigations showed that they also exhibit unequal electric conductivity in the two directions and that the direction of maximum conductivity is also the direction of maximum absorption. Electric vibrations perpendicular to the direction of maximum conductivity are thus alone transmitted, the emergent beam being thus completely polarised (p. 75).
8. The Production of a "Dark Cross"
The interposition of a circular structure like that of a disc of rolled paper on the path of polarised electric radiation is found to project a dark cross into space, the outline of which can be definitely found by means of the exploring detector. The production of a dark cross can also be demonstrated by woody stems with concentric rings as also by ringed concretion of flint round a central nodule. Stalactite also gives similar results (p. 115).
9. Rotation of Plane of Polarisation
Right handed and left handed rotations of the plane of polarisation are produced by oppositely twisted jute elements. The electro-optic analogues of two varieties of sugar, dextrose and levulose, were thus obtained. An apparently inactive variety was produced by a mixture of equal numbers of positive and negative elements (p. 109).
B. The Characteristics of Metallic-contact Receivers
The systematic study of effect of electric waves on contact-resistance of various metals and metalloids showed certain characteristic differences.
1. Positive and negative classes of substance
In regard to the variation of resistance induced by electric waves, it is found that substances could be divided into two classes positive and negative. The positive represented by Iron, Magnesium, and others, characteristically exhibits a diminution of resistance; the negative (Potassium, Arsenic, etc.) exhibits on the other hand, an increase of resistance. An intermediate class is found in Silver which under different molecular modifications, exhibits either a diminution or an increase of resistance. Some of these substances, moreover, exhibit an automatic recovery on the cessation of electric radiation (p. 125). The increase of resistance induced by electric radiation and automatic recovery prove that the theory of 'coherence' is inadequate in explanation of the phenomenon. The induced variation of resistance positive or negative, increases with the intensity of radiation. Under continued radiation a maximum effect is produced; there is now a balance between conductivity distortion and force of restoration, recovery becoming complete on the cessation of radiation. These and other facts prove that radiation induces a molecular change of an allotropic character, and that the variation of conductivity is an expression of the induced molecular change (p. 161).
2. Molecular Recovery
When the range of electric elasticity is not narrow, the substance exhibits an automatic recovery. In other substances the recovery is more or less prolonged. Even in such cases the recovery is hastened by molecular vibration produced by a mechanical tap, or by heat. These restore the sensitive substances from the state of induced molecular strain (attended by change of conductivity) to their original condition and original conductivity. In the positive class, a tap or application of heat restores the substance to original condition by an increase of resistance; in the negative class the restoration is by a diminution of resistance to the normal (p. 158).
3. Characteristic Cyclic curves under E. M. Variation
The conductivity change is produced not only under very rapid electromotive variation by electric waves, but also by a comparatively slow electric variation. Electric conduction in metallic particles sensitive to electric radiation does not obey Ohm's Law. The conductivity is not constant and independent of E. M. F. but varies with it. The two classes of substances, positive and negative can be discriminated from each other by their characteristic curves under cyclic electromotive variation.
The characteristic curve is obtained by recording the variation of the electric current produced under increasing E. M. F. For surface contact of positive class of substance like iron, the curve is concave to the axis of current. Under cyclic E. M. F. variation, the forward and return curves do not coincide; there is a hysteresis. The greater is the range of E. M. F. the greater is the area enclosed. The residual effect can be dissipated by mechanical vibration.
In the negative class of substance like potassium, the curve is of an opposite character being convex to the axis of the current. Increase of E. M. F. produces an actual diminution of current or an increase of resistance. This class exhibits an increase of resistance under electric radiation (p. 251).
C. Different Types of Molecular Receivers
External disturbances or stimuli impinging on a substance induce a molecular upset, the effectiveness of the stimulus depending on the rapidity of the onset of the disturbance. The molecular upset is attended by changes induced in the properties of the substance. The most sensitive means for its detection is electrical: by the method of conductivity or of electromotive variation (p. 170).
Stimulation of the substance can be produced by electric radiation, by visible light, and by mechanical vibration.
1. Continuity of Effect of Electric Radiation and Light
Method of conductivity variation.—This is specially well suited for studying the effect of electric radiation on discontinuous particles; for since the action of radiation is one of surface layer, the larger the area, the more pronounced is the effect, and in loose particles the effective surface is greatly enlarged. The effective total resistance of the particles, moreover, is due to the resistances of surface contacts; any change in the property of the surface layer therefore causes a great variation of the total resistance. There are two classes of substance of which the positive responds by a diminution and the negative by an increase of resistance (p. 135). The effect of radiation is annulled by mechanical vibration. The continuity of effect of electric radiation and light is seen in the galena receiver which responds to both visible and invisible lights by a diminution of resistance (p. 269).
Method of electromotive variation.—A difference of electromotive force is generated between two areas of the same substance, when one is exposed and the other shielded from electric or visible radiation. The characteristics of response to electric radiation and light under diverse conditions are found to be essentially similar (p. 187).
2. Response to Mechanical Vibration
Rapid mechanical vibration, also induces an electromotive variation between the stimulated and unstimulated portions of the same wire (p. 195). The response to mechanical stimulation is found to be of opposite sign to that under light as demonstrated by alternately subjecting one of the two wires of a strain cell to the two modes of stimulation. It is moreover possible to exactly balance the effect of one by that of the other, a slight increase or decrease of either producing an immediate upset of the balance in one direction or its opposite (p. 206).
3. Molecular Response common to Electric Radiation, Light and Mechanical Vibration
1. The molecular effect induced by stimulus can be detected in all cases by the methods of conductivity or of electromotive variation.
2. Substances exhibit quick recovery after moderate stimulation; the recovery becomes protracted in consequence of overstrain caused by intense stimulation.
3. Response is modified by the previous history of the substance and by changes of the environment. Slight rise of temperature and annealing ensures an increased sensitiveness and quick recovery.
4. The ascending portion of the curve of response is abrupt, whereas the fall during recovery is at first rapid and then comparatively slow; the curve of recovery is thus convex to the abscissa which represents time.
5. Under increasing intensity of stimulation, the amplitude of response undergoes an increase which reaches a limit.
6. Sub-minimal stimulus induces a response of opposite sign to that under moderate stimulation.
7. Under rapidly succeeding stimuli, the individual responses become fused; the curve rises to a maximum, when the force of restitution balances the force of distortion due to stimulation.
8. Under prolonged stimulation, the response tends to become reversed; during this process the reversal may become recurrent.
4. The Strain Theory of Photographic Action
The photographic effect of light is detected in a few cases only, when the induced change happens to be visible or is rendered visible on development; the image often disappears on account of self-recovery during darkness. The changes induced can, however, be followed in all its phases in the resulting curve of electric response. In this curve the period of overcoming molecular inertia corresponds to the induction period of photographic action; the automatic recovery explains relapse of the impressed image.
This impression can be rendered more permanent by molecular overstrain under strong and long-continued action of light, the recovery from overstrain being thus greatly prolonged. The fact that molecular strain induced by light is universal, is shown by images produced not merely on photographic plates, but also on sheets of ordinary metals (p. 213). Molecular impression produce in other ways than by light is seen in the inductoscripts and in the development of pressure marks.
Recurrent photographic reversals are produced under continued action of light similar to those under continuous mechanical stimulation (p. 215).
Owing to tendency towards self-recovery, the resulting effect does not solely depend on the total quantity of incident light, but also on the time-rate of illumination. Hence for the same duration of exposure, the photographic effects of intermittent and continuous illuminations are not the same (p. 217).
D. The Similarity of Response of Inorganic and Living Substances
A molecular upset is produced in inorganic substances by the impact of stimuli, electrical or mechanical. The response is recorded by methods of conductivity and electromotive variations.
In inorganic receivers for electric radiation, continuous stimulation induces fatigue which is removed after a period of rest. Prolonged rest, however, makes the receiver inert, and the lost responsivity is then only restored after a period of stimulation (p. 353). These characteristics are similar to those in the response of living substance.
Successive stimuli of equal intensity give rise to uniform inorganic responses. Superposition of stimuli causes incomplete or complete "tetanus" according to slow or quick frequency of stimulation (p. 256).
Certain chemical substances produce a great enhancement in the amplitude of response; these act as stimulants in inducing an increase of excitability (p. 274). Others produce a depression. The variation of excitability induced by various chemical substances, or different doses of the same substance can be detected by the Electric Comparator (p. 282). Slight differences of physico-chemical change in the same piece of metal are detected and recorded by the Electra-molecular Explorer (p. 287).
The effect of an identical chemical agent is modified by the dose of application, a minute dose often producing a reaction opposite to that of a large dose (p. 301). Poisons like oxalic acid cause a molecular arrest and 'kill' the response of metals (p. 303).
E. The Physiological Response of Plant
Among the manifestations in the life of the plant may be mentioned its irritability for response to external stimulation, its growth and its power of storing energy supplied by the environment.
1. Electrical Response of ordinary Plants
The power of response in plants has generally been regarded as confined to the sensitives like Mimosa pudica. Experimental investigations are described which prove that all plants and their different organs are fully sensitive, and that the characteristic electric response by induced galvanometric negativity given by them is in every way similar to the electric response of animal tissues (p. 309).
Successive equal stimulations give rise to uniform responses. Shortening of the intervening period of stimulation brings about fatigue, which is removed after a suitable period of rest. Stimulus singly ineffective becomes effective by summation of several. The amplitude of response increases with the intensity of stimulus till a limit is reached. Anæsthetics induce a depression of excitability. A large dose of chloroform and poisonous substances produce a permanent abolition of response with the death of the plant (p. 315). The response is also abolished when the plant is scalded to death (p. 314). The fatal temperature for the plant is about 60° C.
2. Response of Plant to the Stimulus of Light
The plant responds to the stimulus of light by contraction on account of which the directly stimulated side of a stem becomes concave seen in the positive heliotropic curvature of stems. The normal electromotive response under light is by an induced change of galvanometric negativity. I succeeded in devising another method by which the excitatory change of the plant-tissue is detected by a diminution of its electrical resistance. The most sensitive device for this purpose is the Quadrant Method which records response to light of so excessively short a duration as that emitted by a single spark (p. 320).
The amplitude of response, by resistivity variation, increases with the intensity light. Very dilute vapour of ether increases the excitability as demonstrated by the enhanced amplitude of response. Strong dose of chloroform, on the other hand, causes depression and death as indicated by gradual diminution and final abolition of response.
3. The Vegetable Photo-electric Cell
A very sensitive method of record of the action of light by the induced electromotive variation of galvanometric negativity is afforded by the photo-electric cell in which two similar leaves, immersed in an electrolyte, form the voltaic elements. There is no current in the cell when it is kept in the dark, but exposure of one of the two leaves gives rise to a responsive current. The amplitude of response increases with the intensity and duration of exposure to light. An abnormal positive response occurs when the physiological vigour of the specimen is feeble and below the normal.
In the green leaves two opposite reactions are simultaneously induced under the action of light; of these the A-reaction is associated with the building up process and storage of energy by assimilation; the D-reaction of break-down and expenditure of energy occurs under excitatory action of stimulus. The A-effect is often masked by the predominant D-reaction; the existence of A is demonstrated by the positive electric response exhibited by actively assimilating plants like Hydrilla; it is also shown as an after-effect on the cessation of stimulus (p. 330).
4. Automatic Record of Response of the Photosynthetic Organ
The living plant is in a state of unceasing activity, absorbing and storing energy from without, setting free and dissipating it from within. The expenditure of energy may be manifested in movement, or it may not be externally perceptible, being employed in working the internal mechanism of the body—such for instance, as the distribution of water, which as I have shown elsewhere involves a considerable expenditure of energy. The fundamental importance of photosynthesis is, that it is the process by which the plant absorbs the energy it requires, the radiant energy of sunlight, and stores it in the form of latent or potential energy in the process. The energy so stored can readily be set free again and become kinetic by the chemical decomposition of the organic substances, manifesting itself in heat, electric current or movement.
All these changes are effected by the living protoplasm and are the expressions of its physico-chemical reactions. This is made clear by the observation that all the various manifestations of them that have been made accessible to investigation are affected in a similar manner by a given stimulus or change in internal or external conditions.
Automatic Recorder for Photosynthesis.—The estimation of the activity of photosynthesis with water plants from the rate of evolution of oxygen is direct and requires no prolonged chemical analysis. The automatic record by the Electro-magnetic Writer of successive bubbles representing equal volumes of pure oxygen eliminates all personal error of observation. The methbd is so sensitive that records may be obtained from which it is possible to estimate the formation of quantities of carbohydrate as small as the millionth of a gramme (p. 333).
The automatic method of record that has been described, can also be utilised in physico-chemical investigations, such as the determination of the rate of evolution of a gas under controlled conditions of temperature, of concentration, of intensity of light, of catalytic agents and others, either separately or in combination.
5. Hourly variation of photosynthetic activity
In the median range of the photosynthetic curve the increase of activity is proportional to the intensity of light. The latter is determined by the Self-recording Radiograph (p. 338). As regards the effect of rise of temperature, the increase of photosynthetic activity is uniform in the median range of temperature variation, there being an abrupt decline beyond the optimum. The combined effects of the factors of variation of light and temperature, explain the hourly variation of photosynthetic activity which is at its maximum at about 1 p.m. (p. 334).
6.Effect of infinitesimal traces of chemical substances
Investigations on the effect of infinitesimal traces of certain chemical substances show that they produce a very great increase in the activity of assimilation. The demonstration of this is of special interest since it enables us to understand the effect of infinitesimal quantities of vitamin on general assimilation and of hormones on physiological reaction.
The effect of minute quantity of formaldehyde in enhancing the activity is of special significance in regard to the possible formation of formaldehyde as the first product of photosynthesis. This substance is toxic only in a strong dose; before there could be any great accumulation of this substance in the cells it would have become polymerised into carbohydrate.
7. Efficiency of photosynthetic storage of energy
The estimates hitherto obtained indicate generally a very low efficiency; the experimental methods employed in this determination have been defective from absence of means for the exact measurement of the energy absorbed, and the energy stored in photosynthesis. The difficulties have been obviated by the new methods devised for the purpose. The energy absorbed is found by the Calorimetric method. The accuracy of the calorimetric determination was tested independently by results obtained with the highly sensitive Magnetic Radiometer (p. 360). The energy stored was simultaneously found from the volume of oxygen given out by the plant, the carbohydrate factor of which had been very carefully determined. The photosynthetic efficiency of the leaves of Hydrilla is fairly high, being about 7.4 per cent. (p. 336).
8. The activity of growth
The essential difficulty of the investigation arises from the extraordinary slowness of growth, the average rate of which is about 1 inch per second, a length which is half that of a single wave of sodium light. Even with the magnifying growth recorders hitherto employed, it takes a very long time to detect and measure its rate. For accurate investigations on the effect of a given agent on growth, it is necessary to keep all other variable conditions, such as light and warmth, strictly constant during the whole period of the experiment. We can keep these conditions absolutely constant for only a few minutes at a time. Experiments which require several hours for their completion are, therefore, subject to serious errors which vitiate the results.
The only satisfactory method is one that reduces the period of the experiment to a few minutes; that, however, necessitates the devising of an apparatus for exceptionally high magnification, and for the automatic record of the magnified rate of growth.
The High Magnification Crescograph gives automatic record of growth under a magnification of ten thousand times (p. 348). With this it is possible to obtain growth-record in a time shorter than a second and determine its absolute rate, which in S. Kysoor is 0.95 μ per second, where μ represents micron or 0.001 mm.
Effect of variation of temperature.—The effect of rise of temperature in acceleration of growth can be determined in a few minutes (p. 349). It is thus possible to make accurate determinations of the optimum temperature for maximum growth and the minimum temperature for the arrest of activity.
Effect of Chemical agents.—The effect of manures, anæsthetics, drugs and poisons can be similarly determined in a few minutes and with unprecedented accuracy. The effect of a chemical agent is found to be modified by the dose of application (p. 351).
The Balanced Crescograph.—A still higher sensitiveness in recording the slightest change of growth was secured by the method of balance in which by a clockwork device, the plant is made to descend exactly at the same rate at which the growing tip of the plant was rising (p. 352). The rate of growth is thus accurately compensated and the recorder dots a horizontal line of balanced growth. The minutest change induced in the rate of growth by the environment is at once indicated by the upset of the balance shown by the up and down movements of the curve. The method is so extremely sensitive that it detects and records variations in the rate of growth so excessively minute as 1 millionth of an inch per second.
Wireless waves and growth.—The results obtained by the Balanced Crescograph prove that electric waves used in signalling through space are effective in modifying the rate of growth. The perceptive range of the plant is far greater than ours; it not only perceives but also responds to different rays of the vast ethereal spectrum.
Opposite reactions under feeble and strong stimulation.—This appears to be a universal phenomenon, characteristic of response of both inorganic and living substances under diverse modes of stimulation. This is seen in the responses of inorganic matter to electric radiation. In the positive class, e.g. Osmium in which the response to moderate stimulation is by a diminution of resistance, a subminimal stimulus induces a response by an increase of resistance (p. 137). Similar opposite responses under moderate and feeble stimulation are also exhibited by negative class of substance represented by Arsenic (p. 137). The electromotive response of metallic wires under mechanical stimulation also exhibits this characteristic (p. 204). A short-lived negative twitch is often observed preceding the normal response designated as positive. This is due to the fact that it takes a short time before the responding substance can absorb the whole amount of incident stimulus, the first moiety absorbed being sub-minimal (p. 181). This characteristic effect is exhibited not only by electromagnetic receivers (p. 189) but also by photo-electric cells responding to light (p. 190) and by strain cells under mechanical stimulation (p. 204).
In plants also similar reactions are observed. This is seen in the opposite effects of feeble and strong intensity of electric waves in enhancement and retardation of growth respectively (p. 355).
Parallel effects are also observed in regard to chemical stimulation of both inorganic and on living matter. The same reagent which In large doses causes a depression of excitability and abolition of response in metals, induces, on the other hand, a great enhancement of response when the dose is sufficiently minute (p. 301).
In the living plant, poisons produce death and permanent abolition of response. In minute doses, however, they induce a great enhancement of vital activity thus acting as a highly efficient stimulant in promotion of growth (p. 351).
The Magnetic Crescograph.—The magnification is very greatly increased by the Magnetic Crescograph which produces a magnification of about 50 million times; this order of magnification would lengthen a single wave of sodium light to 2,500 cm. (p. 357).
The Magnetic Radiometer.—This enables comparison of energy of every ray in the solar spectrum (p. 362).
F. Identity of Physiological Mechanism in Plant and Animal
A continuity of response having been established between the responses of inorganic matter on the one hand and living plants on the other, inquiry was continued to find out whether the fundamental physiological mechanisms were similar in plant and in animal life. The plant world affords an unique opportunity for studying the changes by which a simple and primitive organ becomes gradually transformed into one of greater complexity. The evolutionary process has been active not only in morphological differentiation, that is, in the development of new forms, but also in physiological differentiation, that is, in the development of specialised mechanisms for the performance of the various vital functions. There still exists a long prevalent idea that physiological mechanisms of animals and plants are fundamentally different. The evidence afforded by results of experimental investigations show that this idea is unfounded.
The most important characteristics of certain animal tissues are (1) contractility on account of which rapid movement is produced by muscular organ; (2) conductivity or power of transmitting excitation to a distance and (3) rhythmicity or so called spontaneous movements.
Muscular organ in plants.—The functional similarity between the two contractile organs, pulvinus and muscle, is not confined to the manifestation of outward movement, but can be traced to the ultimate protoplasmic mechanism. There are three types of contractile organs distinguished by their rapidity of reaction. The wing-muscle of a bird of prey like the falcon is very active; that of the goose is less active, while the muscle of the domestic fowl is almost inactive, its power of flight having been practically lost. The activity of animal muscle is found to be dependent on the presence and relative distribution of an active substance.
In the leaves of plants there are similarly three types of motor organ—active, semi-active and inactive. The first is exemplified by Mimosa pudica, the second by Neptunia oleracea; and the third by the pulvinus of Phaseolus in which the movement is very feeble and extremely sluggish. By means of selective staining I succeeded in producing a sharp differentiation of the actively contractile from other inactive cells. In Mimosa the active substance is present in great abundance; in Neptunia the particular substance is quantitatively less and scattered. In the inactive Phaseolus the active substance is altogether absent. The pulvinus of Mimosa may thus be regarded as functionally equivalent to an active animal muscle.
Nervous tissue in plants.—Some of the crucial tests for discrimination of nervous impulse are (1) that excitation is initiated for transmission by the characteristic polar action of a constant electric current; (2) that the velocity of transmission is increased within limits, by a rise of temperature; (3) that the transmission can be arrested temporarily or permanently by various physiological blocks. The conducting power is temporarily arrested during the passage of an electric current in a portion of the conducting tissue through which the impulse is being transmitted, the block being removed on the stoppage of the current; (4) that the conducting power is permanently abolished by poisonous solutions. The results of application of these crucial tests offer conclusive proof that the conduction in the plant is a phenomenon of protoplasmic excitation as in the nerve of the animal.
The velocity of transmission in plants is accurately determined by the Resonant Recorder by which time intervals as short as 1 sec. can be recorded. The velocity of transmission is sometimes as high as 400 mm. per second. It is slower than the nervous impulse in higher, but quicker than in lower animals (p. 369).
Rhythmic tissue.—I have demonstrated elsewhere the remarkable similarity of rhythmic mechanism in animal and plant. In both, lowering of temperature slows down the pulsation culminating in an arrest. Rise of temperature to an optimum, on the other hand, enhances the frequency. Diminution of internal pressure causes a similar arrest in both. The rhythmic tissues in animal and plant have a long refractory period. In both, application of external stimulus has no effect during systolic phase of contraction, whereas an extra-pulsation is produced by stimulus during the diastolic phase of expansion. (p. 372).
Records of rhythmic cardiac pulsation were obtained with Resonant Cardiograph which gives the most accurate record of the different rates of movement at different phases of the cardiac cycle (p. 371).
The effects of certain drugs are found to be remarkably similar on rhythmic tissues in animal and plant. Certain drugs thus cause in both a great depression of activity; subsequent application of a particular drug then causes a marked revival of activity.
A considerable portion of this volume deals with the optical properties of electric waves, the study of which has been facilitated by devices for the production of short waves and reliable means for their detection. Results have been obtained which show that electric radiation produces allotropic modification in matter analogous to those by visible light. The most unexpected results are those which demonstrate a continuity of response in the Living and Non-living.
The intricate mechanism of life can only be elucidated by extension of our power of investigation, often in the realm of the invisible. It is only from facts so ascertained that a fully satisfactory theory can be established in regard to diverse activities of life; it is not improbable that these will some day be ultimately traced to physico-chemical reactions.
Physics in a larger sense includes investigations on the reactions of matter, both inorganic and living. The methods of investigation are identical in two cases. The various appliances described, the very high magnification and record of imperceptible movements, the automatic record of extremely short intervals of time and the rate of reaction, the measurement of energy of different rays of the solar spectrum and of the transformation and storage of the energy by the photosynthetic organ—these and others will also be found of value in the advancement of purely physical and physico-chemical investigations.
- Physiology of the Ascent of Sap, 1922.
- Plant Autographs and their Revelations, 1927.
- Irritability of Plants, 1913.