1. Pfeffer, W., Untersuchungen aus dem Botanischen Institut Tübingen, 1, 363 (1884).
2. Keller, H. U., and Sorkin, E., Experientia, 24, 641 (1968).
3. Adler, J., Science, 153, 708 (1966).
4. McCutcheon, M., Physiol. Rev., 26, 319 (1946).
5. Harris, H., Physiol. Rev., 34, 529 (1954); in Function of the Blood (edit. by MacFarlane, R. G., and Smith, A. G. T.), 463 (Academic Press, New York, 1961).