Plants normally have only bisexual flowers. Some species, however, also produce male flowers to make the dispersal of pollen more efficient.
SOME PLANTS produce two types of
flowers - starninate (males) and
bisexual. Scientists thought the role
of starninate flowers was more to
attract agents of pollination than to
donate pollen. But new research
shows that while larger floral displays attract more pollinators, staminate flowers are more effective in the
dispersal of pollen than bisexual
ones. Experiments conducted by
Robert D. Podolsky of the University
of Florida in Gainesville showed the
receipt of pollen by flowers increased
when starninate flowers were used to
enlarge floral displays (Science, Vol
258, No 5083).
Hummingbird-pollinated Besleria
triflora plants produce both staminate and bisexual flowers. The staminate flowers disperse substantially
less pollen than the bisexual ones
and also position their stigma and
anthers away from pollinators formuch of their life. This was what had
led scientists to believe that staminate flowers are produced mainly for
pollinator attraction than for pollen
donation.
Podolsky set up three displays in
a field experiment: one with five
bisexual flowers, the second with 10
bisexual flowers and the third with
five bisexual flowers and five stami
nate flowers. The two displays with
10 flowers were visited by hum
mingbirds more often than the dis play with five flowers. And, pollen
receipt was greater in the mixed
flower display than in either of the
So bisexual displays.
In the laboratory, Podolsky tried
another experiment to check the differences in pollen receipt between
the displays. Using temporarily captive hummingbirds, he compared the
effects of their intervening visits to
staminate flowers as against their visits to bisexual ones on pollen receipt
by subsequent flowers. Pollen delivery was greater when the intervening
flowers were staminate. He also
found that intervening visits by pollinators to staminate flowers did not
dilute the amount of pollen transfer
to flowers.
Because flowers on a plant have
limited pollen supply, plants displaying both types of flowers would
benefit from more frequent pollen
delivery and increased amounts
of pollen per flower. If only
bisexual flowers were produced,
it would increase the number
of flowers but lower the amounts
of pollen received by each
flower.
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