Collected Physical Papers/The Photosynthetic Recorder
XXIV
THE PHOTOSYNTHETIC RECORDER
The incessant activities of life require expenditure of energy that has been previously stored by the organism. Taking, for example, the rise of sap, the ceaseless pumping activity of certain propulsive tissue raises enormous quantities of water to a considerable height. The energy of doing this work resides in the breakdown of complex chemical substances in internal combustion or respiration. The loss of energy must be restored by absorption and storage of energy from outside.
This is secured in green plants by photosynthesis, carbon being fixed for nutrition of the plant with the help of sunlight.
The carbon-assimilation of plants is of great theoretical interest as an example of the simplest type of assimilation. The plant absorbs the carbonic acid, CO2, and the rate of its intake therefore measures the rate of assimilation. The measurement of assimilation from the intake of CO2 necessitates a complicated chemical analysis, which is therefore a very prolonged and laborious process. It is neither a very sensitive nor a highly accurate method. The following automatic method was therefore devised for recording normal rate of photosynthesis and for quickly indicating any change induced in that rate.
Automatic Recorder of Assimilation
Water-plants obtain their carbon from the carbonic acid dissolved in the water. When sunlight falls on these plants, carbonic acid gas is broken up, the carbon becomes fixed in the form of carbohydrates, and oxygen is evolved which rises as a stream of bubbles from the plant. The rate of evolution of oxygen thus measures the rate of assimilation.
Numerous difficulties were encountered in making this method practical; they have been completely removed 
Fig. 103. The Plant-Vessel and the Bubbler.
S, stop-cock; B, Bubbler; O, mercury valve.by the Automatic Recorder. A piece of water plant, e.g., Hydrilla verticillata, is placed in a bottle completely filled with tank-water containing sufficient CO2 in solution, the open end of which is closed by a
special Bubbler apparatus, for measuring the oxygen evolved. The Bubbler consists of a U-tube, the further end of which is closed by a drop of mercury acting as a valve. The oxygen evolved by the plant, entering the U-tube, produces an increasing pressure, which eventually lifts the mercury valve and allows the escape of a bubble of the gas. The valve then immediately closes until it is lifted once more for escape of another equal volume of gas (fig. 103). The movement of the mercury completes an electric circuit, which either rings a bell or makes an electro-magnetic writer inscribe successive dots on a revolving drum (fig. 104). The
Fig. 104. The Automatic Recorder for Photosynthesis.
automatic method eliminates all personal errors of observation; it is so extremely sensitive that it is possible to indirectly measure by its means, a deposit of carbohydrate as minute as a millionth of a gram.
The following example illustrates the practical working of the apparatus. The plant with the apparatus is so placed as to face the northern light; the bell rings each time it has absorbed a certain amount of CO2. If a person now stands obstructing the light, the assimilation is slowed down and the bell now strikes at longer intervals. When strong sunlight is thrown on the plant, the successive strokes on the bell become greatly quickened. The plant is such a sensitive detector of light that it may be employed as a photometer for indicating the slightest variations in the intensity of skylight.
The Hourly Variation of Assimilation
For this determination I took successive records from 7-30 a.m. until 5 p.m. for five minutes at a time. When the sun rose at 6-45 a.m. the light was too feeble to be effective. At 7-30 a.m. assimilation began, and the plant evolved four bubbles of oxygen in the course of five minutes; with the progress of the day it became
Fig. 105. Automatic Records of successive Bubblings for five minutes during different periods of the day.
Note the slow rate at 7-30 a.m. and 4-30 p.m., and the quick rate at mid-day.
more and more active until at 1 p.m., the activity increased four times that in the morning. The real reason for increased assimilation at 1 p.m. is the favourable condition of light and temperature. The activity declined in the afternoon and became arrested with the onset of darkness (fig. 105).
Effect of Infinitesimal Trade of Chemical Substance
One of the most unexpected results, was the, discovery of the extraordinary increase in the power of assimilation by inconceivably small traces of certain chemical substances. It was found that when these were diluted, one part in a billion, they produced an increase in the power of assimilation of more than a hundred per cent. The immediate and concrete demonstration of minute traces of chemicals on assimilation is of special interest, since it enables us to understand the effects of infinitesimal quantities of vitamin on general assimilation and of hormones on physiological reaction.
Formaldehyde, which in large doses acts as a poison, is found in a solution of one part in a billion, to produce an increase of activity of 80 per cent. The stimulating effect of traces of formaldehyde has a special significance in regard to the 'first product' of photosynthesis. There is reason to believe that this first product is formaldehyde, which by polymerisation, becomes converted into carbohydrate. The poisonous nature of formaldehyde stood in the way of acceptance of this view, but the experiment just described shows that traces of this substance, instead of being poisonous, actually enhance the photosynthetic activity.
Efficiency of the Photosynthetic Organ in Storage of Solar Energy
The results hitherto obtained indicate, in general, a very low order of efficiency in storage of energy. The methods employed have been defective from absence of means of exact measurement of the incident energy, from the indeterminate loss of the energy received and from the difficulty in the exact determination of the energy stored. The efficiency is found from the ratio of the energy stored Es, to the energy absorbed Ea.
Efficiency = EsEa
I have been successful in obviating the various difficulties in the accurate determination of the energy absorbed, and of the energy stored. The energy absorbed is found by two different methods, the Calorimetric and the Radiometric, these two independent determinations following each other in quick succession. While the energy absorbed is being determined, simultaneous measurement is made of the energy stored by the production of carbohydrate. This is calculated from the volume of oxygen evolved.
The efficiency of the photosynthetic organ is found to be much higher than had been usually supposed, being half that of an ordinary steam engine. In vigorous Hydrilla plant it is as high as 7.4 per cent.
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. It is obvious how it can be applied in various ways for measuring chemical reactivity or velocity of chemical reactions, e.g., as in the study of Vant Hoff's Law, of the effect of light on chemical change and of other allied phenomena.
(Physiology of Photosynthesis, 1925.)