What is a key characteristic of flowers that prevent self-pollination?

The key characteristic of flowers that prevents self-pollination is the presence of separate male and female parts. Flowers that have distinct male (stamens) and female (pistils) reproductive organs can promote cross-pollination, which is essential for genetic diversity and the overall health of the plant population. By having separate male and female structures, these flowers can encourage pollen transfer from the male part of one flower to the female part of a different flower, rather than allowing pollen to fertilize its own ovules.

This mechanism is beneficial, as it increases genetic variation within a species, making them more resilient to diseases and environmental changes. In contrast, flowers that have only male parts or only female parts cannot achieve self-pollination, as they lack the necessary structures to do so. Brightly colored flowers and excessive pollen can attract pollinators, but they don't inherently prevent self-pollination; rather, they may facilitate it if the flower structure allows.