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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