Encyclopedia of Evolution.pdf - Online Reading Center

own cones, from which the seeds are released. In birches,
dangling catkins of male flowers release pollen into the air
from branch tips, while further back on the branches spikes
of female flowers contain ovules, from which seeds eventually
fall. In these monoecious plants (monoecy), crossbreeding
usually occurs, since the pollen is more likely to land on
the female parts of a different plant than on the female parts
of the same plant.
Most flowering plant species have male structures (stamens)
and female structures (pistils) in the same flowers. In
these hermaphroditic flowers, it would be very easy for the
pollen from the stamens to come in contact with the pistils of
the same flower, which would be a failure of crossbreeding.
However, many flowers have adaptations that prevent this
from happening:
• In many flowers, the stamens and the pistils are not active
at the same time. In protandrous flowers (Greek for “male
first”), the stamens release pollen, then wither away; only
afterward does the pistil become receptive to pollen. In
protogynous flowers (Greek for “female first”), the pistils
receive pollen, then after the seeds are fertilized the
stamens open and release pollen. In the first example, the
flower acts as a father first, then as a mother; in the second
example, the flower is a mother first, then a father. A flower
cannot pollinate itself, because it does not release and
receive pollen at the same time.
• In many flowers, the pollen cannot germinate unless it
receives the correct chemical signal. If the pollen lands on
the pistil of the same plant, it does not receive the correct
signal, therefore it never germinates. These self-incom-
reproductive systems
patible flowers cannot fertilize themselves. In some cases,
flowers may be self-incompatible when they first open, but
if they do not get pollinated, they become self-compatible.
This allows them to produce some seeds rather than none,
in the event that they do not cross-pollinate.
• In some plant species, the flowers contain both male and
female parts but the flowers of different individuals have
different structures. In some of the flowers the stamens are
short and the pistils are long. In others, the pistils are short
and the stamens are long. If a pollinating insect visits the
first flower, it gets pollen on its head, not on its back. If the
insect visits another flower on the same plant, the pollen on
its head does not come in contact with the pistil. But if the
insect visits another plant, with the other kind of flower, the
pollen on its head rubs off on the short pistil; and the long
stamens put pollen on the insect’s back. If the insect then
visits the first kind of flower again, pollen rubs off from its
back onto the long pistil. The insects can visit whichever
flowers they wish, but pollen is transferred successfully only
between flowers with different structures. In this case, the
plant cannot pollinate itself, even if the insect visits flower
after flower on the same plant (see figure below).
Ancestral flowering plants appear to have been hermaphroditic.
What caused dioecy to evolve in flowering plants?
Dioecy promotes crossbreeding among individual plants in a
population, but this does not prove that the enhancement of
crossbreeding was the reason that dioecy evolved in the first
place. In order to address this question, it is important to find
a species of plants in which some individuals or populations
are hermaphroditic and some dioecious. Botanist Summer
In many species of angiosperms, the flowers come in more than one form. In this example, the pollinator can carry pollen only from one form of flower to
another form. This prevents self-pollination.