Summary

The flowers of hermaphrodite plants have both male and female parts. Hermaphrodite plants can change their allocation to both sexual functions in various ways, such as by changing the production ratios of pollen grains to ovules within flowers and of flowers to fruits. We discuss the problems involved in measuring sex allocation, trade-offs and fitness gain curves and present a simple model for the evolutionary stable allocation to fruits and flowers. The model provides an explanation for the low fruit-to-flower ratio found in many species and for the increasing allocation to female function with increasing selfing rate. Theoretical models predict that evolutionary stable sex allocation depends on plant size and this prediction is supported by literature data on monocarpic hermaphrodites and on monoecious species.

Introduction

By far the most common mode of plant reproduction is through hermaphrodite flowers. Although such flowers serve both male and female functions, this does not mean that hermaphrodites are invariant in their sexual behaviour. Substantial variation in intraspecific sex allocation has been found and related to environmental conditions or plant size. A large body of theoretical literature is now accumulating that predicts how allocation to male and female reproduction should vary with a variety of factors such as pollination type, resource status, selfing rate, selective abortion, population structure, dispersal mechanisms, etc. Unfortunately empirical evidence lags far behind, mostly because the required measurements are notoriously difficult to collect and the methods full of pitfalls.