1. Gaffron, H., Photosynthesis, in Plant Physiology: a Treatise, edit. by Steward, F. C. (New York, 1960).
2. Planck, M., The Theory of Heat Radiation (1914).
3. Spanner, D. C., Physiol. Plant., 7, 475 (1954).
4. Wohl, K. (1935), quoted in ref. 1 (1960).