Xenia Includes Metaxenia

This discussion deals with the terminology and biology of direct, or immediate, pollen effects on seeds and fruits. Such effects are called "xenia" and include differences in the size, shape, color, developmental timing, and chemical composition of seeds and fruits found as a result of fertilization by different pollens. The term "xenia" was originally coined to describe such pollen effects only on maternal plant tissues, that is, on seed coats, pericarp, and attending structures, and these effects were contrasted with those associated with hybridization as expressed in the embryo. Xenia was applied to direct pollen effects on endosperm at a time when this tissue was considered to be purely maternal, that is, before the discovery of double fertilization in the angiosperms. The term continued to be applied to such effects on endosperm after double fertilization was invoked to explain the phenomenon and later was extended to apply to direct pollen effects in embryos in species with exalbuminous seeds. Later still, the term "metaxenia" was mistakenly coined to describe direct pollen effects on maternal plant tissues. This discussion suggests that double fertilization per se does very little to explain xenic effects on the embryo and endosperm. Also, little or no scientific evidence now exists for maintenance of a strict distinction between xenic effects on syngamous tissues ("gamoxenia") and those on maternal tissues ("metaxenia"). Such a distinction appears to be purely hypothetical. Indeed, differences in size, shape, color, developmental timing, and chemical composition may be simultaneously found in embryo, endosperm, and maternal tissues in some species. The term "xenia," therefore, covers all direct pollen effects in seeds and fruits, whether discerned in embryo, endosperm, or maternal tissues, in the period from fertilization to germination. Whereas there has been adequate description of xenic effects, explanation of the effects by way of focused scientific research has been lacking in most species, and the

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