Life Movements in Plants

be greatly ■ reduced ; or a constant tension may be exerted; by means of a weight T (Pig. 56). In .my'later type Fis. 5S. —Compound lever. P, plant attached to short arm of lever L; T, weight exerting tension; C, connecting link; L/ second lever with bent tip for record; B, B, balancing counterpoise. Fork F, carries at its side two conical" agate cups, on which lever rests by two pin-points. (From a photograph.)" of the apparatus the plant connection is made- to the right, instead of the left side of the first fulcrum. This gives certain practical advantages. The second lever is then made practically to balance the first, only a very slight weight being necessary for exact counterpoise. The reduction of total weight thus secured reduces materially the friction at the fulcrum with great enhancement of efficiency of the apparatus.

The second * or the recording lever has a normal excur¬ sion through 8 cm. on the recording surface, which is a very thin sheet of glass 8x8 cm. coated with a layer of smoke. As the recording lever is about 40 cm. in length, the curvature in the record is slight, and practically negli¬ gible in the middle portion of 4 cm. The. dimensions given allow a magnification of ten thousand times. A far more compact apparatus is made with 15 cm. length of levers. This gives a magnification of a thousand times.

Another great difficulty in obtaining an accurate record of the curve of growth arises from the friction of contact of the bent tip of the writing lever against the recording surface-. This I was able to overcome by an oscillating device by which the contact, instead of being continuous, was made intermittent. The smoked glass plate, G, rs made to oscillate, to and fro, at regular intervals of time, say one second. The bent tip of the recording lever comes periodically in contact with the glass plate during its ex¬ treme forward oscillation. The record would thus con¬ sist of a series of dots, the distance between successive dots representing magnified growth during a second.

The drawback in connection with the obtaining of record on the oscillating plate lies in the fact that if the plate approaches the recording point with anything like suddenness, then the stroke on the flexible lever causes an after-oscillation; the multiple dots, thus produced, spoil the record. In order to overcome this, a special contrivance is necessary, by which the speed of approach of the plate should be gradually reduced to zero at contact with the '-■recording point. The rate of recession should, on the other hand, continuously increase from zero to maximum. The recording point will in this manner be gently pressed against the glass plate, marking the dot, and then gradu¬ ally set free. It was only after strict observance of these

conditions that the disturbing effect of the lever could he obviated. T his particuiar contrivance consists of an eccentric rod actuated by a rotating wheel. A cylindrical rod is SUn ported eccentrically, so that semi-rotation of the eccentric causing a pull on the crank K (Fig. 57) pushes the plate Fig. 57.—Eccentric for oscillation glass plate G. A, adjusting screws tor lateral movement of the plate. is is released periodically by clockwork at intervals of W °’ ^ teD< ° r fifteen sec °nds respectively, according

to the requirements of the experiment. The complete apparatus is shown in figure 58. Fitr. 58. Complete apparatus. P, plant: S, micrometer screw for raising or owenng the plant; C } clockwork for periodic oscillation of plate; W, Rotating wheel. V, cylindrical plant-chamber. (From a photograph.) ‘ the .fact that the magnification actually obtained was sometimes very different from the calculated value. This unreliability I was able to trace to the defects inherent in thread connections, employed at first to attach the plant

to the first lever, and the first lever to the second. These flexible connections were found to undergo a variable amount of elastic yield. Hence it became necessary to use nothing but rigid connections. The plant attachment, A, of triangular shape is made of a piece of navaldum ; its knife-edge rests on a notch at the short arm of the lever, L. There are several notches at various distances from' the fulcrum. It will be understood how the magni¬ fication can be modified by moving A, nearer or further from the fulcrum. The lower end of the attachment is bent in the form of a hook. The end of the leaf of the plant P, is doubled on itself and tied. The loop thus j formed is then slipped over the hooked end of A.

The link, C, connecting L and IP consists of a pin pointed at both ends, which rests on two conical agate cups fixed respectively to the upper and lower surfaces of the levers L and L’. This mode of frictionless linking is rigid and allows at the same time perfectly free movement of the levers. The fulcrum .—The most serious difficulty was in connec¬ tion with frictionless support of the axes of the two levers. The horizontal axis was at first supported on jewel bearings, with fine screw adjustment for securing lateral support. Any slight variation from absolute adjustment made the bearing either too loose or too tight, preventing free play of the lever. When perfect adjustment was secured by any chance, the movement of the levers became jerky after ‘a few days. This I afterwards discovered was due to the deposit of invisible particles of dust on the bearings. These difficulties forced me to work out a very perfect and at the same time a much simpler device. The lever now carries two vertical pin-points which are supported on conical agate cups. The axis of the lever passes through the points of support. The friction of support is thus reduced to a minimum. The levers are kept in place under the constant

pressure of their own weight. The excursion of the end of the recording ■ lever, which represents magnified movement of growth, was now found to be Without jerk and quite uniform. The soil in a flower pot is liable to be disturbed by irrigation, and the record thus vitiated by physical disturb¬ ance. This is obviated by wrapping a piece of cloth round the roo f imbedded in a small quantity of soil. The lower end of the plant is held securely by a clamp. In order to subject the plant to the action of gases and vapours, or to variation of temperature it is enclosed in a glass cylinder (V) with an'inlet and an outlet pipe (Fig. 58). The chamber is maintained in a humid condition by means of a sponge soaked in water. Different gases, warm or cold water vapours, may thus be introduced into the plant chamber.

Any quick growing organ of a plant will b© found suitable for experiment. In order to avoid all possible disturbing action of circumnutation, it is preferable *o em¬ ploy either radial organs, such as flower peduncles and buds .of certain flowers, or the limp leaves of various species of grasses, and the pistils of flowers. It is also advisable to select specimens in which the growth is uniform. I ap¬ pend a representative list of various specimens in which, under favourable conditions of season and temperature, ■ the rates of growth may be as high as those given below:—

The specimen employed for experiment may be an intact plant, rooted in a flower pot. It is, however, more convenient to employ cut specimens, the exposed end being wrapped in moist cloth. The shock-effect of section passes off after several hour3, and the isolated organ renews its' growth in a normal manner. Among various specimens I find S. Kysoor to be very suitable for experiments on growth. The leaves are much stronger, than those of wheat and different grasses, and can bear a consider¬

able amount of pull without harm. Its rate of growth under favourable condition of season is Considerable. I shall now proceed to describe certain typical experi¬ ments which will show: (1) the extreme sensibility of the Crescograph ; (2) its wide applicability in different investi¬ gations; and (3) its capability in determining with great, precision the time-relations of responsive changes in the rate of growth. In describing these typical cases, I shall give detailed account of the experimental methods employed, and thus avoid repetition in accounts of subset quent experiments.

Determination of the absolute rate of growth: Experi¬ ment SL —For the determination of the absolute rate, I 'Shall' interpret the results of a record of growth obtain¬ ed with a vigorous specimen & Kysoor on a stationary plate. ■ The oscillation frequency of the plate was once in a second, and the magnification employed was ten thousand 'times. The magnified-growth-movement was so rapid that the record consists of a series of short dashes instead of

Fig. 6».-Crescographio record? : (A) successive records of growth at inter¬ vals of one second (magnification 10,000 times), (a) Effect of temperature on a stationary plate; N, normal rate of growth; C, retarded rate under cold- H enhanced rate under warmth: (6) record on moving plate, where diminished slope of curve denotes retarded rate under cold. (Magnification 2,000 times.) growth, it is advisable that the plant should be kept in uni¬ form darkness or in uniformly diffused light. So sensitive •is the recorder that it shows a change of growth-rate due to the slight increase of illumination by the opening of an additional window. One-sided light, moreover, gives’rise to disturbing phototropic curvature. With the precautions described the growth-rate in vigorous specimens is found to be very uniform.

After the completion of the first vertical series, the recording plate was moved 1 cm. to the left; the tip of the recorder was brought once more to the top by the micrometer screw, S, (Fig. 58), and the record taken once tfiore after an interval of 15 minutes. The magnified growth for 4 seconds is 38 mm. in the first record; it is precisely the same in the record taken fifteen minutes after. The successive growth elongations at intervals of 1 second is practically the same throughout, being 9-5 mm. This uniformity in the spaeings demonstrates not only the

regularity of growth under constant conditions, but also, the precision of the apparatus. It also shows that by keeping the external condition constant, the normal growth-rate could be maintained uniform for at least fifteen minutes. The magnified rate of growth is nearly 1 cm. per second, and since it is quite easy to measure 0*5 mm., the Crescograph enables us to magnify and record a length of 0*0005 mm., that is to say, the sixteenth part of a wave of red light. The absolute rate of growth, moreover, can be determined in a period as short as 0*05 of a second. These facts will give some idea of the great possibilities of the Cresco¬ graph for future investigations.

As the period of experiment is very greatly shortened by the method of high magnification, I shall, in the determination of the absolute rate of growth, adopt a second as the unit of time, and /*, or micron, as the unit of length,—the micron, being a millionth part of a metre or a thousandth part of a millimeter. If m be the magnifying power of the compound lever and Z, the average distance between successive dots in mm. at intervals of t seconds then :—

Having demonstrated tbe extreme sensitiveness and reliability of the apparatus, in quantitative determination, I shall next proceed to show its wide applicability for various researches relating to the influence of external agencies in modification of growth. For this two different methods are employed. In the first of these methods, the records are taken on a stationary plate : of these the record is at first taken under normal condition, the subse¬ quent series being obtained under the ’ given changed

condition; the increase or diminution of intervals between successive dots, in the two series, at once demonstrates the stimulating or depressing nature of the changed condition. In the second method, the record is taken on a plate moving at an uniform rate by clockwork. A curve is thus obtained, the ordinate representing growth elongation and the abscissa the time. The increment of length divided by the increment of time gives the absolute rate of growth at any part of the curve. As long as the growth is uniform, so long the slope of the curve remains con¬ stant. If a stimulating agency enhances the rate of growth, there is an immediate upward flexure in the curve; a depressing agent, on the other hand, lessens the slope of the curve.

I shall now give a few typical examples of the employment of the Crescograph for investigations on growth: the first example I shall take is the demonstra¬ tion of the influence of variation of temperature. Stationary method; Experiment 52 .—The records, given in Fig. 59 a, were taken on a stationary plate. The specimen was S. Kysoor; the Grescographic magnifica¬ tion was two thousand times, and the successive dots at intervals of 5 seconds. The middle series, N, was at the temperature of: the room. The ne c xt, C, was obtained with the temperature lowered by a few degrees. Finally H was taken when the plant-chamber was warmed. It will be seen how under cooling the spaces between successive dots have become shortened, showing the diminished rate of growth. Warming, on the other hand, caused a widen¬ ing of intervals between successive dots, thus demonstrating an enbhncement of the rate of growth.

Calculating from ■ the data obtained from the figure we find :—- Moping plate method: Experiment 54. —This was carried out with a different specimen of S. Kysoor, the record being taken on a moving plate (Fig. 595). The first part of the curve here represents the normal rate o„f growth. The plant was then subjected to moderate cooling, the sub¬ sequent curve with its diminished slope denotes the depres¬ sion of growth. The question of influence of temperature will be treated in a subsequent Paper of the preset) f series in much greater, detail.

h Precaution against physical disturbance: Experiment 54 There may be some misgiving about the employment of such high magnification : it may be thought that the. accuracy of the record might be vitiated by physical disturbance, such as vibration. In physical experimenta¬ tion far greater difficulties have, however, been overcome, and-the problem of securing freedom from vibration is not at all., formidable. The whole apparatus need .only, be placed on a heavy bracket screwed on the wall to ensure against mechnical disturbance. The extent to which this has. been realized will be found from the inspection of the first part of the record in. figure 60, taken on .a moving plate.. A thin dead twig was substituted for the 'growing plant, and the perfectly horizontal record not only demon¬ strated. the absence of , growth movement but also of all disturbance. There is an element of physical change, against which precautions have to be taken in-experiments on varia¬ tion of therate of growth .at different ■ temperatures.. In order to determine its character and extent, a record was taken with the dead twig, of the effect of raising the temper¬ ature of the plant-chamber through ten degrees. The record

that there is an expansion dur¬ ing the rise of temperature,' and that the variable period lasted for a minute, after' which there was a cessation of physical movement, the record becoming once more horizontal. The obvious pre¬ cautions to be taken in such a case, is to wait for several minutes for the attainment of steady temperature. The movement caused by physical change abates in a shoyt time whereas the change of rate of growth brought about by phy¬ siological reaction is persistent.

Experiment 55, —In the determination of time-relations of, responsive change in growth under external stimulus^ I shall take the typical case of the effect of electric shock from a secondary coil of one second’s duration. Two electrodes were applied, one above and the other below the growing region of a bud of Crinum. The record was taken on a moving plate, the magnification employed being two thousand times, and successive dots made at intervals of two seconds. It was a matter of surprise to me to find that the growth of the plant was affected by an intensity of stimulus far below the limit of our own per¬ ception. As regards the relative sensitiveness of plant and animal, some of my experiments show that the leaf of Mimosa pudica in a favourable condition responds to an electric stimulus which is one-tenth the minimum intensity

Fig. 60.—Horizontal record shows absence ct growth in a deed branch ; physical expansion on application of warmth at arrow followed by hori¬ zontal record on attainment of steady temperature. (Magnification 2,000 times.) that causes perception in a human being. For convenience I shall designate the intensity of electric shock that is barely perceptible to us as the unit shock. When an intensity of A 25 unit was applied to the growing organ, it responded >,o it by a retardation of growth. Inspection of Fig. 61 shows

that there is a flexure induced in the curve in response to stimulus, cue flattening *o£ the curve denoting retardation of growth. The latent period, in this case, is 6 seconds. The normal rate was restored after 5 minutes. The intensity of shoes was* next raised® from. 0*25 unit to one unit. The second record shows that the latent period is reduced to 4 seconds, and a relatively greater retardation of growth was induced by the action of the stronger stimulus. The recovery of the normal rate was effected after the longer period of 10 minutes. I took one more record, ihe intensity being three units. The latent period was now reduced to 1 second, and the induced retardation was so great as to effect a temporary arrest of growth.

TABLE X.—TIME-RELATIONS OF RESPONSIVE CRoWTH-VA IUATION UNDER ELECTRIC SHOCK (Critium). Fig. 61 . -'limo-relations of response oi growing organ to electric stimulus of increasing intensities applied at the short horizontal lines. Successive dots at intervals of 2 seconds. It is thus found that growth in plants is affected by an intensity of stimulus which is below human perception that with increasing stimulus the latent period is diminished and the period of recovery increased ; and that the induced retardation of growth increases continuously with the stimu¬ lus till at a critical intensity there is a temporary arrest of growth. I shall speak later of the effect induced by stimulus above this critical point.

Experiment 56 .—As a further example of the capabi¬ lity of the (Jreseograph, i shall give the record of a single pulse of growth obtained with the peduncle of Zephyran- ihes Sulphured (Fig. 62). The magnification employed was 10,000 times, the successive dots benig at intervals of one secofkl. It will be seen that the growth pulse com¬ mences with a sudden elongation, the maximum rate being 0*4 p per sec. The pulse exhausts itself in 15 seconds, after which there is a partial recovery in the course of lb seconds* The period of the complete

Fis. 62. Record of a single growth-pulse resultant growth in each pulse is therefore the differ¬ ence between elongation and recovery. Had a very highly magnifying arrangement not been used, the resulting rate would have appeared continuous. In other specimens, owin^ probably to greater frequency of pulsation and co-operation of numerous elements in growth, the rate appears to be practically .uniform* Advantages of the Crescograph .—There is no existing method which enables us to detect and measure such infinitesi¬ mal movements and their time-relations. The only attempt made in measuring minute growth has been by observing the movement of a mark on a growing plant through a microscope. The magnification available in practice is about 250 times. The observation of the movement would itself be sufficiently fatiguing. But a simultaneous estimate of the time-relations of rapidly fluctuating changes would prove so bewildering, that accurate results from this method would be altogether impossible. A vV* objective gives a linear en¬ largement of about 1,200 times. But the employment of this objective is impracticable in the measurement of growth elongation of an ordinary plant. With the Crescograph, on the other hand, we obtain a magnification ’which far sur¬ passes the highest powers of a microscope, and it can be used for all plants. It does not merely detect growth but automatically records the rate of growth and its slightest fluctuation. The extreme shortness of time required for an experiment renders the study of the influence of a single factor at a time possible, the other conditions being kept constant. The Crescograph thus opens out a very extensive field of inquiry into the physiology of growth ; and the dis¬ covery of several important phenomena mentioned in this Paper is to be ascribed to the extreme sensitiveness of the apparatus, and the accuracy of the method employed.

The magnification obtained with two levers was, as stated before, 10,000 times. It may be thought that further magnification is possible by a compound system of three levers. There is, however, a limit to the number of levers that may be employed with advantage, for the slight over¬ weight <of the last lever becomes multiplied and exerts very great tension on the plant, which interferes with the normal rate of its growth. The friction at the hearings also becomes added up by an increase in the number of levers, and this interferes with the uniformity of the movement ot the last recording lever. For securing further magnification, additional material contact has, therefore, to be abandoned.