Collected Physical Papers/The High Magnification Crescograph
THE HIGH MAGNIFICATION CRESCOGRAPH
The difficulty of investigations on growth arises from its extreme slowness, which is two thousand times slower than the movement of the snail. The auxanometers usually employed magnifies growth to about 20 times; under this magnification several hours must elapse before growth becomes perceptible. During this long period the external conditions, such as light and warmth, undergo change thereby confusing and vitiating the result. The external conditions can be kept constant for a few minutes only; hence the effect of variation of an individual factor can only be found by increasing the magnification to about ten thousand times and thus reducing the period of the experiment. The apparatus devised for this purpose not only produces this enormous magnification but also automatically records the rate of growth and its induced variation in the course of time as short as a minute or so.
The recorder consists of a compound system of two levers; the first magnifies a hundred times, and the second enlarges the first a hundred-fold, the total magnification being thus ten thousand times. The difficulty introduced by the weight of levers was surmounted by the employment of navaldum, an alloy of aluminium, which combines great rigidity with exceptional lightness. The friction at the points of support was removed by the employment of jewel bearings. The record is taken on a smoked glass plate kept oscillating to and from by means of a crank K and eccentric R, actuated by a clockwork C (fig. 109). Successive dots are produced at definite intervals which could be made to vary from 1 to 10 seconds.
Two different methods are employed for obtaining the record. In the first, the records are taken on a stationary plate, the first series under normal condition, and the second under a given variation. The increase or diminution of intervals between successive dots in the two series, demonstrates the stimulating or depressing nature of the changed condition, In the second method, the record is taken on a plate moved at a 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 rate of growth at any part of the curve. As long as growth is uniform, so long the slope of the curve remains constant. Enhancement of the rate of growth by a stimulating agent causes an upward flexure of the curve; a depressing agent, on the other hand; lessens the slope of the curve.
Determination of the Absolute Rate of Growth
The record of growth was taken with a vigorous specimen of S. Kysoor on a stationary plate.
For securing uniformity of growth, it is advisable that, the plant be kept in darkness or uniform diffused light. So sensitive is the recorder that it shows a change of growth-rate due to slight increase of illumination by the opening of an additional window. The oscillation frequency of the recording plate was once in a second, the magnification being 10,000 times. After taking the first series of record, the second series was obtained after an interval of 15 minutes (fig. 110A). The magnified growth elongation is 9.5 mm. per second; since it is quite easy to measure 0.5 mm., it is possible to record growth for a period as short as 1/20th of a second. It is further seen that the Crescograph enables us to magnify and record a length of 0.0005 mm. which is a fraction of a single wave length of light.
Employing one second, as the unit of time and μ or micron as the unit of length (1/1000 mm.), the absolute rate of growth of S. Kysoor is found as follows:—
If m be the magnifying power, l the average distance between successive dots in mm. at intervals of t seconds,
|the rate of growth||=||l × 103 per second|
|=||9.5 × 103 per second.|
|=||0.95 μ per second.|
Effect of Variation of Temperature.
The effect of variation of temperature on growth is shown in fig. 110a. The middle series N was taken at the temperature of the room; the next C was obtained when the temperature was lowered by a few degrees. Finally H was taken when the plant chamber was warmed. The spaces between successive dots are shortened under cooling which induces a diminished rate of growth. The enhancement of the rate of growth by rise of temperature is exhibited by the widening of intervals between successive dots. The effect of lowering of temperature in retardation of growth is also exhibited by record (fig. 110b) taken on a moving plate, in which the diminished slope of the curve demonstrates the depressed rate of growth.
Effect of Chemical Agents
The effects of manures, anæsthetics, drugs and poisons, can be similarly determined in the course of a few minutes, and with unprecedented accuracy. A few agents only have hitherto been employed in stimulating growth, whereas there are numerous others of whose actions we have been profoundly ignorant. The crude method hitherto employed in the application of a few chemical agents and of electricity has not been uniformly successful. The cause of the anomaly is found in the discovery of an important factor, namely, the dose of application, which has hitherto not been taken into account. It was thus found that while a particular intensity of electrical current accelerated growth, any excess above a critical point retarded it. The same is true of chemical stimulants; a striking result was obtained with certain poisons which in normal doses killed the plant, but in doses sufficiently minute acted as a highly efficient stimulant in promotion of growth.
The Balanced Crescograph
The growth of plants is affected by changes of environment which are even below human perception. It now became necessary to devise a new method of still greater sensitiveness which would instantly show by the up- or down-movement of an indicator, the stimulating or depressing nature of an agent on growth. The desideratum is to compensate the up-movement of growth by some regulating device, by which the plant is made to descend exactly at the same rate at which the growing tip of the plant was rising, whatever that rate may be. The special difficulty encountered was in obtaining exact balance for widely varying rates of growth in different plants, and even in the same plant under different conditions. In the Balanced Crescograph, (fig. 111) a train of revolving clockwork actuated
by the fall of a weight, lowers the plant at the same rate at which it is growing. The exact adjustment is obtained by the gradual turning of a screw to the right or to the left, by which the rate of compensating fall is retarded or accelerated. In this way the rate of growth becomes exactly compensated, and the recorder now dots a horizontal line instead of the former curve of ascent. The turning of the adjusting screw of the Balanced Crescograph also moves an index against a circular scale (not shown in the figure) so graduated that its reading at once gives the rate at which the plant is growing at that instant. When balanced, the recording apparatus is extraordinarily sensitive. Any change, however slight, in the environment is at once indicated by the upset of the balance with up- or down-movement of the curve. This method is so extremely sensitive that it is possible to detect variation of rate of growth so excessively minute as 1/1500 millionth of an inch per second.
As an illustration of the delicacy of this method, a
record is given of the effect of carbonic acid gas on growth (fig. 112). A jar is filled with this gas, and emptied over the plant; the invisible gas, on account of its heavier weight, falls in a stream and surrounds the plant. The record shows that this gave rise to an immediate acceleration of growth, which continued for two and-a-half minutes; this preliminary acceleration was followed by retardation of growth as shown by the down curve. The Balanced Crescograph thus not only exhibits the beneficial effect of an agent, but also indicates the dose which prolongs the beneficial effect.
Effect of Wireless Stimulation on Growth
Growth is modified by the action of visible light; two different effects are produced depending on the intensity. Strong stimulus of light induces a diminution while feeble stimulus induces an acceleration of the rate of growth. The effectiveness of light in modifying growth depends moreover on the quality of light; the effect is very strong in the ultra-violet region of the spectrum with its extremely short wave-length of light; it declines almost to zero as we move towards the less refrangible rays, the yellow and the red, with their comparatively long wave-length. As we proceed further to the infra-red region we come across the vast range of electric radiation, the wave-lengths of which vary from the shortest wave I have been able to produce (0.6 cm.) to others which may be miles in length. There thus arises the important question whether plants perceive and respond to the long ether-waves, including those employed in signalling through space.
At first sight this would appear to be very unlikely, for the most effective rays are in the ultra-violet region with wave-length as short as 20 × 10−6 cm.; but electric waves used in wireless signalling are 50,000,000 times as long. The perceptive power of our retina is confined within the very narrow range of a single octave, the wave-lengths of which lie between 70 × 10−6 cm. and 35 × 10−6 cm. It is difficult to imagine that plants could perceive radiations so widely separated from each other as the visible light and the invisible electric waves.
The results obtained prove, however, that electric waves are effective in modifying the rate of growth. The experiment was carried out with the help of a portable electric Radiator, the intensity of radiation being capable of variation. The Radiator was placed at a distance of 200 metres from the growing plant which was suitably mounted on the Balanced Crescograph.
Effect of feeble Stimulation.—The response was found to be an acceleration of growth as seen in fig. 113 (a). This is analogous to the stimulating action of light of sub-minimal intensity.
Effect of strong Stimulation.—Even more striking is the effect of stronger stimulation; the balance wasimmediately upset, indicating a retardation of the rateof growth (fig. 113 b). The latent period, i.e., the interval between the incident wave and the response, was only a few seconds. The record given in the figure was obtained with the moderate magnification of 2,000 times.
Under an intensity of stimulus slightly above the sub-minimal, the response exhibited retardation of growth followed by quick recovery, as seen in the series of records given in figure 113 (c). The perceptive
range of the plant is inconceivably greater than ours; it not only perceives, but also responds to different rays of the vast ethereal spectrum.
(Proc. Roy. Soc. Oct. 1917 and Transactions Bose Institute, 1919.)