1. Downs, R. J., Hendricks, S. B., and Borthwick, H. A., Bot. Gaz., 118, 199 (1957).
2. Downs, R. J., Plant Physiol., 30, 468 (1955). Borthwick, H. A., Hendricks, S. B., Parker, M. W., Toole, E. H., and Toole, V. K., Proc. U.S. Nat. Acad. Sci., 38, 662 (1952).
3. Flint, L. H., and McAlister, E. D., Smithson. Misc. Coll., 94, No. 5 (1935).
4. Borthwick, H. A., Hendricks, S. B., Toole, E. H., and Toole, V. K., Bot. Gaz., 115, 205 (1954).
5. Fortanier, E. J., Meded. LandbHoogesch. Wageningen, 54, 103 (1954). Åberg, B., Symb. Bot. Upsal., 8, 1 (1943).