Plant Autographs and Their Revelations

favourable outside influences and ephemeral activity, or of those of Desmodium, with its characteristic of patient and long-enduring accumulation of forces, which afterwards find uninterrupted and persistent expression. A continuity of response has thus been traced, both in the plant and animal, from simple, through multiple, to autonomous response. A feeble or moderate stimulation evokes a single response, whether this be mechanical or physical. Strong stimulation, on the other hand, gives rise to multiple response in various forms. Excess of energy- derived from the absorption of various environmental stim¬ uli, bubbles over, as it were, and shows itself in spontaneous rhythmic response. This is variously manifested, now in the pulsation of the Desmodium leaflets, again in the beating of the heart, and elsewhere in echoing sensation and thought.

A living organism may, from one point of view, be regarded as simply a machine. But in order to keep this living machine at work, in all those complex and wonderful ways of which it is capable — from mechanical movement, through throbbing sensation to spontaneous thought — some¬ thing more than mechanical perfection is necessary. It can only throb with life when placed in constant communion with all the forces of the universe about it. The phenomenon of growth furnishes us with another example of automatic activity. Can any similarity be dis¬ covered between this and other modes of spontaneous activity? Do variations of external conditions modify all automatic activities in a similar manner?

The question of growth and its modifications under external change is a matter of great practical importance, for the world’s supply of food depends on the growth of plants. It is, therefore, of the highest importance to be able to discover all those conditions which are favourable to growth. In what way do external agencies modify it? The essential difficulty of the investigation arises from the extraordinary slowness of growth. Even the proverbial slow-moving snail moves two thousand times faster than the tip of a growing shoot. The average rate of growth is about i/iooooo 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 investigation of the effect of any given agent on growth, it is necessary to keep all other variable conditions, such as light and warmth, strictly con¬ stant 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 a very high magnification, and for the automatic record of the magni¬ fied rate of growth. I have succeeded in devising an apparatus by which growth becomes instantly visualised. This produces a magnification of ten thousand times, which would increase the rate of a snail’s pace to that of an express train.

P, plant; C, clockwork oscillating device bv which the smoked glass plate is made to move to and fro. The High Magnification Crescograph consists of a system of two levers. The first magnifies a hundred times, and the second enlarges the first a hundredfold, the total magnifica¬ tion being thus 10,000 times (fig. 47). The record is taken on a smoked glass plate kept moving to and fro oy an oscil¬ lating device, at regular intervals of time varying from one to ten seconds. The recording plate is kept moving laterally at a uniform rate. A curve of growth is thus obtained. A stimulating agent which enhances the rate of growth, pro-

Fig. 48. Record of Pulsation of Growth taken with a High Magnification Crescograph. duces a flexure of the curve upwards ; a depressing agent, on the other hand, lessens the slope of the curve. The method of high magnification has revealed many inter¬ esting characteristics of growth hitherto unsuspected. It has also established the essential similarity of growth to other modes of automatic activity. Like other modes of automatic activity, growth is found to be rhythmic or pulsatory. This is seen in the record given

in fig. 48, which shows a sudden uplift followed by a slower recoil, the amount of recession being about one-fourth of what had been gained. The difference between the two rep¬ resents the permanent growth. The growth process is, there¬ fore, not steady, but like the wavelets of a rising tide. In the Desmodium leaflet the pulsating activity is arrested when the internal pressure is reduced, as under conditions of drought ; the pul¬ sations are, how¬ ever, revived after irrigation.

Similarly, growth, under conditions of drought, is brought to a standstill. It i s interesting t o find that the appli¬ cation of a few drops of water to the root now gives rise to a few pulses of growth, which, however, soon come to an end. More growth. I have already stated that increased internal pres¬ sure, an enhanced state of turgor, accelerates growth. Tur¬ gor of the tissue is maintained by the pumping activity which supplies sap to the growing region. It may be stated in a general way, that any condition which increases the rate of sap-ascent also increases the turgor and the rate of growth. Conversely, conditions which retard the rate of assent or cause a diminution of turgor also inhibit growth.

Fig. 49. Effect of electric stimulation of in- creas'ng intensities of .5, i, and 3 units on growth elongation. The last record exhibits the actual shortening of the growing organ under stimulation. I explained in a previous chapter that stimulation produces in a pulvinated organ a diminution of turgor and contraction (see p. 36). The effect on a growing organ is essentially similar. I reproduce records of growth under increasing intensity of stimulation. Application of electric stimulus of 0.5 unit produced a slight down-flexure in the curve, indi¬ cating a moderate retardation of growth. An intensity of I unit induced a greater retardation, whereas a stimulus of 3 units induced an actual contraction, as shown by the reversal of the curve (fig. 49). It will be seen that stimula¬ tion induces a reaction which is opposite to that of normal growth-elongation. This may conveniently be described as ‘incipient’ contraction, for under increased intensity of stimu¬ lus it culminates in an actual contraction.

A striking result discovered in the course of this investi¬ gation is that while a stimulus of moderate intensity retards growth, a feeble stimulus accelerates it. Two diametrically opposite effects are thus produced under strong and feeble stimulation. We shall find a wider application of this prin¬ ciple in the action of drugs which may be regarded as chem¬ ical stimulants. I have shown that under a sufficiently strong stimulus, growth-elongation is reversed into an actual contraction. After a period of rest, growth is renewed. The record of response and recovery shown in fig. 50 is essentially similar to the response and recovery of a pulvinated organ. The difference is merely one of degree.

There is a prevalent wrong assumption that different modes of stimulation induce dissimilar reactions. The results of my investigations prove that this is not the case. All modes of effective stimulation of a certain intensity, by contact, by electric shock, or by light, produce the same responsive contraction. The growing organ subjected to diffuse stimulation under¬ goes, as we have seen, an equal contraction all round, on account of which it exhibits a shortening. But if one side

of the organ be subjected to lo¬ cal stimulation, then that side alone will contract, producing a bending of the organ to one side. This phenomenon will be described in greater detail in a later chapter. Change of temperature has a marked effect on the rate of growth. It is depressed by a fall and becomes arrested at a critical low temperature. Conversely, warmth produces an astonishingly rapid increase of growth up to an optimum temperature, beyond which it becomes once more retarded ; at 6o° C. a death-spasm occurs, nent abolition of growth.

Fig. 50. Contractile response of growing organ under electric shock. Vertical lines below indicate intervals of one minute (mag¬ nification 1,000 times). a\ small does of a mild amesthetic like ether stimulates growth. The first part of the curve (fig. 51) indicates by its slope the normal rate of growth ; diluted vapour of ether produces a sudden erection of the curve, indicating an enhancement of the rate of growth. Under application of chloroform, the quantity of vapour absorbed at the begin¬ ning being slight, the effect of a small dose is seen to be an acceleration of growth. As more choloroform is absorbed, the result is a retardation instead of acceleration, leading ultimately to a spasmodic contraction which causes a

reversal. The apex of the curve demarcates life from death : after this reversal, spots of discoloration appear in the plant ; these spread very rapidly and the specimen becomes wilted as a consequence of death. The experiment on the action of ether shows the possi¬ bility of reviving dormant growth by a mild stimulant. (a) Enhancement under small dose of ether; (b) prelimi¬ nary enhancement followed by spasmodic death-con¬ traction under the action of chloroform.

Any marked advance in scientific agriculture is only pos¬ sible if we succeed in discovering agents which greatly enhance the activity of growth. Only a few stimulating agents have been used for this purpose, whereas there are numerous others of the action of which we have been pro¬ foundly ignorant. The rule-of-thumb method hitherto employed in the application of a few chemical stimulants and of electricity has, moreover, not been uniformly suc¬ cessful. The cause of this anomaly is found in the dis-

covery of an important factor, namely, that of the amount of the dose, which has hitherto not been taken into account. Thus we find that while a particular intensity of electrical current accelerates growth, any excess above a critical point retards it. The same is true of chemical stimulants. A striking result was obtained with certain poisons, which in normal doses kill the plant, but in excessively minute doses acted as an extraordinarily efficient stimulant in promoting growth. In fact, plants behave in this respect like human beings. Investigations on plants thus open out fresh lines of research in Pharmacology and Medicine. The High Magnification Crescograph, moreover, enables espeedy test¬ ing of the action of manurial agents, which takes only a few minutes instead of a whole season, thus avoiding the errors arising from changing conditions of protracted experimentation.

Not only is growth modified by the intensity of the stimu¬ lation, physical or chemical, but also by the point of its application and by the tonic condition of the plant. The effect of stimulation is modified not only by the intensity but also by its point of application. Growth and all modifications associated with it take place in a more or less restricted region, a little behind the tip of the growing organ. When a stimulus is applied directly at this growing- region, the result is a retardation of growth, which under strong stimulation culminates in actual contraction and shortening of the organ. But a totally unexpected result occurs when the stimulus is applied at a distance from the responding region of growth. Under this indirect stimula¬ tion, the rate of growth becomes actually enhanced. This is illustrated by the records given in fig. 52. The normal growth is represented by the upward slope of the first part of the curve. Indirect stimulation applied at the point marked by the arrow is seen to produce a sudden erection of the curve, proving the acceleration of the rate of growth. Stimulation was next applied directly at the point marked

by the cross, with the result that the growth was not only arrested, but the organ exhibited a marked contraction^ shown by the reversed curve (see also hg. 62). The Law of the Effects of Direct and Indirect Stimula¬ tion is : Direct Stimulation induces Contraction: Indirect Stimu¬ lation induces the opposite effect of Expansion. In medical practice the anomaly is frequently met with, that the same drug induces diametrically opposite effects in Fig. 52. Effects of indirect and direct stimulation on growth. Indirect stimulation at dotted arrow enhances the rate. Direct stimulation at cross induces a reversed response by contraction.

different individuals. The cause of the anomaly lies in the fact that the tonic level or constitution of different indi¬ viduals is not the same. For rational treatment, it is essen¬ tial to take into account the constitution of the patient, for the reaction of a given drug is greatly modified by his tonic condition. In experimenting with plants, I artificially modified the tone or constitution of different batches of similar seed¬ lings. One was kept normal for reference, another depressed to a sub-tonic conclition, and the third raised to an optimum state of exceptional vigour. A measured dose of dilute

poison was applied to the three batches of plants. The nor¬ mal plants survived after a period of struggle. The weaker specimens succumbed at once without any struggle. But the reaction of the vigorous specimens was quite different ; the toxic agent not only failed in its illegitimate work, but actually exalted the growth of its intended victims ! It will be noted how numerous are the factors which modify growth. It is only by studying the separate effects of these individual factors that we can unravel the com¬ plications which at first sight appear hopeless. The reduc¬ tion in the duration of experiments, rendered possible by a ten thousandfold magnification of growth-movement, has enabled us to discover characteristics of growth hitherto unsuspected. I hoped to find other subtle reactions indi¬ cated by the minutest variations in the rate of growth ; but such effects could onlv be discovered bv increasing still further the sensitiveness of the method of detection.

The method described in the last chapter showed how changes produced in the rate of growth can be detected by the resulting flexures in the recorded curve. But it is con¬ ceivable that minute variations in the rate of growth may be attended by changes in the slope of the curve too slight to be detected by that method. It was therefore necessary to devise a new method which would instantly show, by the up or down movement of an indicator, the accelerating or retarding effect of an agent on growth. I was successful in carrying out this idea by devis¬ ing the Method of Balance. The plant was made to descend at the exact rate at which its growing tip was rising. A regulating device had to be introduced, analogous to the compensating movement of an astronomical telescope, which neutralises the effect of the earth’s movement round her axis once in twenty-four hours. The problem was, however, more difficult; for instead of compensating a definite rate, adjustments had to be made for balancing widely varying rates of growth in different plants and even in the same plant under different conditions.

In the Balanced Crescograph, a train of revolving clock- wheels, actuated by the fall of a weight, lowers the plant exactly at the same rate at which it is growing. By the gradual turning of a screw S to the right or to the left, the rate of compensating fall can be continuously increased or decreased. The rate of growth is thus accurately compen¬ sated, so that the growing tip of the plant remains exactly at the same level (fig. 53). The tip is attached, in the usual manner, to the High Magnification Crescograph, and the Recorder now dots a horizontal line instead of an ascend¬ ing curve as in the previous method. The apparatus thus balanced is rendered extraordinarily sensitive. The minut¬ est change induced in the rate of growth by the environment

Compensation of growth-movement produced by equal subsidence of the holder containing the plant (P). Adjusting screw (S) regu¬ lates the speed of the governor (G). W, heavy weight actuating clockwork. is at once indicated by the upset of the balance shown by the up and down movement of the curve. The method is so extremely sensitive that it detects and records variations in the rate of growth so excessively minute as i /iooo mil¬ lionth of an inch per second.

the effect of carbonic acid gas on growth (fig. 54). A jar filled with this gas was emptied over the plant; the gas, on account of its weight, fell in a stream and surrounded the plant. The record shows that this gave rise to an immediate acceleration of growth which continued for 2^ minutes ; under the continued action of the gas, the preliminary accel¬ eration was fol¬ lowed by retarda¬ tion of growth, as shown by the down- curve. The Bal¬ anced Crescograph not only exhibits the beneficial effect of an agent, but also indicates the proper dose of ap¬ plication.

Plants are re- garded as ex¬ tremely sluggish in their perception of light, a continuous exposure to light for more than five minutes being re¬ garded as minimally effective. I undertook investigations to find out whether the plant responded to light-exposure of excessively short duration. It is impossible to conceive of anything more fleeting than a single flash of lightning. I subjected the growing plant balanced on the Crescograph to an artificial flash of lightning, that is to say, to the light emitted by a single electrical spark between two metallic balls. The plant perceived and responded to this light of

Fig. 54. Record showing the effect of car¬ bonic acid gas on growth. Horizontal line at the beginning indicates balanced growth. Application of carbonic acid gas induces temporary enhancement of growth, shown here by up-curve. Suc¬ cessive dots at intervals of 10 seconds. incredibly short duration, as was manifest from the upset of the balance and the resulting automatic script made by the plant. The plant has thus been shown to perceive not only a feeble stimulation, but also stimulation of excessively short duration. The next subject of investigation was the range

Fig. 55. Record of response of plant to wireless stimulation. (a) Response to feeble stimulation by acceleration of growth; (b) re¬ sponse to strong stimulation by retardation of growth; (c) response to medium stimulation — retardation followed by recovery. Down- curve represents acceleration, and up-curve retardation of growth. of perception of the plant when exposed to the various visible and invisible rays. The plant responds to ultra¬ violet light, with its extremely short wave-length, by slowing the rate of growth. The retarding effect of light on growth declines towards the less refrangible rays, the yellow and red. As we proceed further into the infra-red region, we

come across the vast range of electric radiation, the wave¬ lengths of which vary from the shortest wave that I have been able to produce (0.6 cm.) to others which may be miles in length. There thus arises the very interesting question whether plants perceive and respond to the long aether- waves, including those employed in signalling through space. The Balanced Crescograph was employed to record the possible change induced in growth by the action of wireless waves. The results of my investigations show that wireless waves produced characteristic variations in growth, depend¬ ing on the intensity of stimulus. Feeble waves produced an acceleration in the rate of growth. Very strong waves produced a retardation, the effect persisting for a long time after the cessation of stimulation. But with waves of medium intensitv the induced retardation was followed by a quick recovery (fig. 55). The perceptive range of the plant is inconceivably greater than ours ; it not only per¬ ceives but also responds to the different rays of the vast aetherial spectrum.

Perhaps it is as well that our senses are limited in their range. For life would otherwise be intolerable under the constant irritation of these ceaseless waves of space-sig¬ nalling to wEich brick walls are quite transparent. Her¬ metically-sealed metal chambers would then have afforded us the only protection. The High Magnification Crescograph magnifying 10,000 times, in conjunction with the Method of Balance, gave results beyond all expectation. This should have satisfied one, but man is never satisfied, and it is his insatiable long¬ ing to surpass his previous achievements that is the real impetus for all progress. I therefore set about inventing a super-crescograph ; if by using two levers I had succeeded in raising the magnifying power from a hundred to ten thousand times, a third lever might raise it proportionately higher. But the attempt failed completely. The weight and therefore friction at the points of contact which linked