Introduction

Although many aspects of rarity have been studied, the relationship of plant reproductive biology and rarity is poorly understood. In this chapter, I will address this relationship, which is relevant to recent efforts to conserve rare plant species ex situ and to reintroduce rare species to habitats within their former ranges.

Reproductive systems in flowering plants are extraordinarily diverse. Unlike the majority of terrestrial animal species, which have separate female and male sexes, most flowering plants are hermaphroditic, producing male and female gametes in the same flower or in separate flowers on the same plant. Despite the potential for self-fertilization, diverse mechanisms have evolved that promote outcrossing, presumably to avoid the loss of fitness that often results from self-fertilization. Some of these mechanisms are spatial or temporal separation of pollen and stigmas; self-incompatibility, which prevents self-fertilization and matings among individuals possessing the same incompatibility type; and dioecy, or the separation of sexual function on different individuals (Table 4.1).

Outcrossing plant species depend on animals, wind, or water as pollen vectors. The degree of specialization of the pollen vector and the nature of the morphological and physiological adaptations of the plant species determine how effectively pollen is transferred among individuals of the same species. Some plant species have evolved adaptations for pollination by a very limited array of pollinators, although such specialization may be uncommon.

Many characteristics of plant reproductive systems may be associated with rarity.