About half of the more important food crops are naturally cross-pollinated, and their
reproductive systems include various clever mechanisms to encourage cross-pollination
and discourage self-pollination, including:
- pollinated species, where the pollen is shed before the ovules are mature, eg carrot
eg Arum lilies and many wind-pollinated plants such as grasses; some species such
as avocados have both protogynous and protandrous varieties, often grown together
to promote cross-fertilisation.- Heterostyly, a structural mechanism in which the plants have variable length styles
(neck of the pistil), eg common primrose, wood sorrel and flax. - Dioecy, where the stamens and pistils are borne on different (dioecious) plants, eg
asparagus, date palm, most papaya varieties and hops; and - Genetically determined self-incompatibility, which is activated by chemicals
produced by the plant, disables the pollen from growing on the stigma of the same
plant, eg cabbage, white clover and many other species.
Cross-pollination has major advantages over self-pollination, since it gives rise to
offspring that have much greater genetic variability, providing the potential for the
Many of the major food crop species are in fact predominantly self-pollinating;
examples include wheat, barley, oats, rice, beans, peas and tomatoes. There are
relatively few mechanisms that promote self-pollination, the most positive of which is
known as cleistogamy, where the flowers fail to open, as in certain violets.
Self-pollination is also ensured in various processes known collectively as apomixis,
which includes parthenocarpy (parthenogenesis) and apogamy where the ovule
develops into a seed without any kind of fertilisation. This development of seedless
In barley, lettuce and wheat the pollen is shed before or just as the flowers open.
With tomatoes, pollination follows opening of the flower, but the stamens form a cone
around the stigma. There is always the risk of unwanted cross-pollination occurring with
these species.
A cross-pollinated plant, which has two parents, each of which is likely to differ in
many genes, produces a diverse population of plants that are hybrid (heterozygous) for
many characteristics.
A self-pollinated plant, which has only one parent, produces a more uniform
population of plants that are pure breeding (homozygous) for many characteristics.
Thus, in contrast to outbreeders, inbreeders (“self-breeders”) are likely to be highly
homozygous, and will therefore breed true for certain characteristics.
3
Dichogamy, in two forms: Protandry, the more usual form, often found in insect-and walnut; and Protogyny, where the ovules mature before the pollen is released,species to adapt. Self-pollination (“selfing”) may even lead to the eventual extinction
of certain species that are not able to adapt to new circumstances. On the other hand
self-pollination may have certain other evolutionary advantages, for example where
animal pollinators are temporarily absent, or when individual plants are widely scat-
tered. Self pollination also successfully maintains genotypes that would otherwise be
lost in the gene shuffle of sexual reproduction.
GROWING FOOD – THE FOOD PRODUCTION HANDBOOK
fruit is discussed in Section 2F. “Fruits”, pages 243–266.