The flowers of some plants are unisexual. The male and female flowers may be borne on different plants (in which case the plants are called dioecious) or on the same plant (in which case they are called monoecious).
Other flowers are hermaphroditic, meaning that each flower has male (stamens) and female (pistil) parts. This is the case for most flowering plants.
Fertilization may occur between different flowers (we call this cross-pollination). Pollen is carried from one flower to another by wind, insects or other means.
Lastly, there is self-pollination, when a flower’s own pollen fertilizes its ovule.
Some general seed-saving principles apply to all vegetable species
You should isolate different cultivars, to prevent them from being fertilized by “undesirable” pollen, by planting them a specific distance apart. The appropriate spacing depends on the species in question.
You can also isolate them by planting cultivars that do not flower at the same time.
Finally, you can use a mechanical barrier, by covering the flowers with light-coloured cloth before they open.
For all cultivated vegetables, remove the seeds from a fruit that has the features of a particular cultivar. Consider its shape, size and colour, as well as any other specific features you wish to reproduce.
It is best to take seeds from several plants rather than just one, so as not to interfere with your harvest, and to have seeds from a variety of sources. Seeds must always be taken from insect- and disease-free plants.
Based on an article by Nathalie Leuenberger in Quatre-Temps magazine, Vol. 23, No.4