1. E. J. Underwood, “Trace Elements in Human and Animal Nutrition,” 4th Ed., Academic Press, New York, 1977, pp. 243–254.
2. M. R. S. Fox, “Trace Elements in Human Health and Disease,” Vol. II, ed. by A. S. Prasad, Academic Press, New York, 1976, pp. 401–416.
3. G. F. Leedale, “Biology of Euglena,” Vol. I, ed. by D. E. Buetow, Academic Press, New York, 1968, p. 216.