Ovules within each ovary of self-fertile Lotus species develop uniformly while those within each ovary of self-sterile species vary considerably in rate of development. All ovules in self-fertile species may be fertilized while less than one-half of those in self-sterile species are generally fertilized. Time of pollination is determined by the time when the stigmatic membrane ruptures in self-fertile species, by the time of rupturing by pollinating insects in self-sterile species. Higher numbers of ovules per ovary compensate for the proportion not fertilized in self-sterile species. The relationships of self-sterility to self-incompatibility in this genus, the evolution of this mechanism, and evidence that similar mechanisms are found in other legumes are discussed.

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Studies using pubescence and cyanogenesis as genetic markers indicate that a high proportion of self-seed is set when selfing is followed by pollinations with mixtures of self- and cross-pollen. Also, considerable selective fertilization occurs when pollination is with mixtures of compatible pollen. It is suggested that the incompatibility mechanism may be used for commercial production of "double-cross hybrids" of trefoil.