On a New Application of Photography to the Demonstration of Certain Physiological Processes in Plants

Abstract

MR. WALTER GARDINER, Lecturer on Botany in the University of Cambridge, who delivered the evening address at Newcastle on “How Plants maintairt themselves in the Struggle for Existence,” has discovered a new method of printing photographic negatives, employing living leaves in place of sensitive paper. Mr. Gardiner read a paper on the subject before the British Association. Before dealing with the immediate subject of his paper, the author described how prints may be obtained from Protococci, or the free-swimming swarm-spores of many green Algæ. It is possible to take advantage of their sensitiveness to light. Into one end of a watertight box, a thin glass plate is securely fitted. The negative to be printed is then placed next the glass, film side nearest. The box is filled with water containing a fairly large quantity of swarm-spores. The lid is shut down, and the whole is exposed to diffused light. In the case of a strong and well-developed negative, the swarm-spores swim towards the most highly-illuminated parts, and there in the greatest numbers come to rest, and settle upon the glass, so that, after some four or six hours, on pouring out the water and removing the negative, a print in green swarm-spores can be obtained. The print may be dried, fixed with albumen, stained, and varnished. The author then dwelt upon the well-known fact that the whole of the animal life upon the globe depends directly or indirectly upon the wonderful synthetic formation of proteid and protoplasm which takes place in the living tissue of plants containing chlorophyll, i.e. green plants, or, to be more exact, in the green chlorophyll corpuscles. He stated that, whatever is the exact chemical nature of the process, this is at least clear, that the first visible product of the assimilatory activity is starch, which, moreover, is found in the chlorophyll grains. The presence of this starch can be made manifest by treating a decolorized leaf with a water solution of iodine dissolved in potassic iodide. This formation of starch only takes place under the influence of light; the radiant energy of the sun providing the means of executing the profound synthetic chemical change, and building up proteid from the carbonic acid of the air which is taken up by the leaves and the salts and water of the soil absorbed by the roots. If a plant (and preferably a plant with thin leaves) be placed in the dark over-night, and then brought out into the light next morning, the desired leaves being covered with a sharp and well-developed negative, starch is formed when light is transmitted, and in greatest quantity in the brightest areas. Thus a positive in starch is produced which can be developed by suitable treatment with iodine. [A leaf was then developed, and handed round to the audience for inspection.] The author showed that it might be possible to obtain a permanent print by suitable washing and treatment with a soluble silver salt, silver iodide being formed. The author regards this discovery as a most striking illustration of the way in which plants are working for themselves, and so for all living things, and points out that the extraordinary manner in which the green parts of plants (so to speak) catch the radiant energy of the sun, and employ it for analytical and synthetical chemical processes, may be easily and clearly demonstrated.