1. Oenothera rubrinervis is a half mutant, produced by the copulation of a mutated gamete with a normal velutina gamete of O. Lamarckiana. 2. In consequence, it produces about one-fourth empty grains, a mass mutation of about one-fourth pure or double mutants, and one-half specimens of O. rubrinervis, which will repeat the splitting. 3. The pure or double mutant is called O. mut. deserens. It is very similar to O. rubrinervis, but the leaves of its young rosettes and the bracts of its flower spike are broader and more even. 4. O. mut. deserens is constant from seed. It has no hereditary empty grains. 5. The formula for the self-fertilization of O. rubrinervis is therefore O. (deserens + velutina) = des. x des. + velu. x velu. + des. x velu. The first combination gives the mass mutation, the second the empty grains, the third the normal plants of O. rubrinervis. 6. In crossing with other species the two kinds of gametes will produce twin hybrids, as, for example, laeta and velutina. This assertion has been...