SINCE the discovery of heterothallism, or separate sexes, in Mucors by Blakeslee some twenty-five years ago, the conception has been widely extended in the fungi and has recently been applied to the green algae. But in recent years the facts regarding multiple sexes in fungi have become so complex as to strain credulity, and new views, such as Dame Helen Gwynne-Vaughan's conception of nutritive heterothallism, have been coming in to relieve the tension on the earlier rigid hypothesis of fixed + and − strains corresponding respectively to the female and the male sex.