“The benefit which heterostyled dimorphic plants derive from the existence of the two forms is sufficiently obvious, namely, the intercrossing of distinct plants being thus ensured. Nothing can be better adapted for this end than the relative positions of the anthers and stigmas in the two forms…” —Darwin, 1877 (p. 30)
Heterostyly is defined as the occurrence in a species of two or more floral morphs that exhibit reciprocal herkogamy (cf. Barrett and Shore, 2008) (“Herkogamy is the spatial separation of pollen presentation and pollen receipt within or between blossoms of an individual plant.” [Webb and Lloyd, 1986]). Heterostyly is often associated with other physiological and structural characters, such as self- and intramorph-incompatibility (SI) and pollen and stigma polymorphisms (Richards, 1997; Barrett et al., 2000a; Barrett and Shore, 2008). Indeed, Darwin’s definition of heterostyly (1877) specified the presence of self- and intramorph-incompatibility in addition to the morphological differences. Furthermore, the features of the heterostylous syndrome only rarely disassociate in the progeny of heterostylous species (Darwin, 1877;Ernst, 1928, 1936a, 1936b). Researchers have demonstrated that in some taxa this tight linkage is due to the presence of a supergene, which they hypothesize to include the genes involved in the development and function of heterostyly (e.g., Dowrick, 1956; Barrett and Shore, 2008), although in other taxa, the tight linkage between the morphology and SI may have other explanations (see Genetics, Physiology, and Molecular Biology of Heterostyly later).
In distylous taxa, one morph—the short-style (SS) morph—is reverse herkogamous, with anther(s) positioned above the stigma(s), and the other—the long-style (LS) morph—is approach herkogamous, with the stigma(s) situated above the anther(s). The anther(s) of each morph are at the same height as the stigma(s) of the other morph (fig. 1). These morphs have often been referred to as thrum and pin, respectively. Darwin referred to the SS morph as thrum (or thrum-eyed [Darwin, 1862, 1877]) because the anthers at the apex of the corolla tube of this morph resemble “the ends of weavers’ threads” (Darwin, 1877, p. 14), and the LS morph is called pin (or pin-eyed or pin-headed [Darwin, 1862, 1877]) because of the similarity of the capitate stigma(s) of some taxa, situated at or near the apex of the corolla tube, to the head of a pin. Darwin used this terminology because his initial studies of heterostyly focused on the species of Primula L. (Primulaceae), and the SS morph of these species actually produce flowers that resemble the ends of weavers’ threads. He even stated, “I suppose that some weaver who cultivated the polyanthus invented this name [for the SS morph], from being struck with some degree of resemblance between the cluster of anthers in the mouth of the corolla and the ends of his threads” (Darwin, 1877, p. 14). However, the flowers of other heterostylous species take different forms, and even though Darwin initially used the names pin and thrum, he abandoned these terms in the majority of his discussions of heterostyly (Ornduff, 1992). However, some botanists continue to use “pin” and “thrum” to refer to the two distylous morphs (e.g., Wong et al., 1994; Yasui et al., 2004; Matsui et al., 2004).