ABSTRACT
Angiosperm species typically produce bisexual fl owers including both male (pollen-bearing) and female (ovule-bearing) organs, the stamens and carpels, respectively. Throughout evolution, numerous biochemical, physiological, and morphological mechanisms have evolved to promote outcrossing, such as self-incompatibility systems where “own” pollen is recognized and prevented from successful fertilization; asynchronous development of eggs and pollen; or physical placement of the stamens, such that self-pollen is unlikely to come in contact with the pistils (e.g., heterostyly) (Ainsworth 2000; Barret 1998). Perhaps the most extreme morphological alteration is the development of unisexual fl owers that bear only male (staminate) or female (pistillate) organs (Fig. 12-1). Unisexuality has been estimated to arise more than 100 times within the plant kingdom, and can take two general forms: monoecy and dioecy (Renner and Ricklefs 1995). In monoecy, separate male and female fl owers are formed on the same plant. Dioecy carries the process further, leading to separate male and female plants, each bearing fl owers of only one sex type.
