Fruit-discharge-type in Geranium (Geraniaceae): its use in classification and its evolutionary implications



Three main types of seed-discharge in Geranium are made the basis of its division into subgenera: Geranium subgenus Geranium, with a ballistic expulsion of the seed from the mericarp, termed ‘seed-ejection’; Geranium subgenus Robertium, with forcible discharge of the mericarp with the seed in it, separately from the awn, termed ‘carpel-projection’, and Geranium subgenus Erodioideae, with the seed-containing mericarp being thrown off with the attached awn, which becomes helically coiled, called the ‘Erodium-type’. Variants of the seed-ejecting type permit the division of Geranium subgenus Geranium into three sections. Other criteria are used to divide Geranium subgenus Robertium into eight sections and Geranium subgenus Erodioideae into two. Species are fully enumerated except for Geranium section Geranium, which comprises the bulk of the genus, and for which some tentative subgroups are given in an Appendix. Diversity of fruit-type in Geranium is greatest in the Mediterranean Region. Characters of the fruit in other genera of Geraniaceae are surveyed. Geographical distributions, chromosome numbers, pollen morphology and phytochemistry are reviewed. It is suggested that Erodium-type fruit discharge, shared with the four remaining genera of the family, is primitive, and that carpel-projection and seed-ejection arose from it separately, the latter probably more than once. The very large, mainly perennial, Geranium subgenus Geranium is contrasted with Geranium subgenus Robertium, half of which is hapaxanthic, and which occupies marginal habitats and shows greater morphological and chromosomal variation despite its being only one tenth the size. Geranium subgenus Erodioideae is smaller still and probably relictual.