Stigma form and surface in relation to self‐incompatibility in the Onagraceae

The Onagraceae is unique among families of comparable size in the heterogeneity of stigma form and physiology. Studies of the receptive surfaces of stigmas from representatives of all genera of the family have shown that a range of types exists from papillate to non‐papillate, and from “wet”, bearing a free‐running secretion, to “dry”, lacking such a secretion, most genera falling in the former category. Self‐incompatibility is wholly restricted to wet‐stigma species, and direct and circumstantial evidence indicates that it is always likely to be of the gametophytic type, as in certain Oenothera species that have been the subjects of detailed genetical and physiological research. Whilst Oenothera and most other self‐incompatible species in the family appear to reject incompatible pollen mainly in the stigma surface layers, the precise site is subject to both genetical and environmentally‐determined variation. Intraspecific polymorphy in sex function is known in the genus Fuchsia , most species of which have wet stigmas. Female sterility in the functionally male flowers of certain species of this genus appears simply to be due to a failure to produce adequate stigma secretion. All species of the dry‐stigma genera in the family are self‐compatible, but not all are inbreeding because of the existence of other mechanisms enforcing outcrossing. The stigmas of some possess certain unique features, apparently related to water conservation. Possible evolutionary trends in the family and in the breeding and pollination systems are briefly discussed in the light of what is now known of pollen‐stigma relationships.