The period of seed development on the parent plant can be one of the most hazardous phases in a plant's life cycle. In many plants, only a very small proportion of the ovules eventually mature into viable seeds. This is because many flowers fail to produce fruits, and many of the ovules in fruits fail to produce seeds. Studies on a wide range of species have recorded huge variations in fruit set and seed set (Wiens, 1984; Sutherland, 1986). Seed losses in the pre-dispersal stage may be due to pollination failure, genetic defects, lack of resources for development or seed predation. This chapter considers both the proximate causes of mortality and the evolutionary consequences of mortality in seeds before they have been shed by the parent plant.

Fruit and seed set

Fruit set is characteristically very low in certain species. In Yucca elata, only 6.6% of the flowers were recorded as producing mature fruits under field conditions (James et al., 1994). In Aesculus californica, fruit set in nature was shown to be about 10% (Newell, 1991); in Cornus sanguinea, it was shown to be between 8 and 22% among different populations (Guitián et al., 1996). The Proteaceae as a family are notable for their low fruit set (Charlesworth, 1989; Ayer & Whelan, 1989; Wiens et al., 1989). In a survey of 18 species growing under natural conditions, Collins & Rebelo (1987) recorded fruit set values that ranged from only 0.1 to 7.2%.