SOME PLANTS produce two types offlowers - starninate (males) andbisexual. Scientists thought the roleof starninate flowers was more toattract agents of pollination than todonate pollen. But new researchshows that while larger floral displays attract more pollinators, staminate flowers are more effective in thedispersal of pollen than bisexualones. Experiments conducted byRobert D. Podolsky of the Universityof Florida in Gainesville showed thereceipt of pollen by flowers increasedwhen starninate flowers were used toenlarge floral displays (Science, Vol258, No 5083).
Hummingbird-pollinated Besleriatriflora plants produce both staminate and bisexual flowers. The staminate flowers disperse substantiallyless pollen than the bisexual onesand also position their stigma andanthers away from pollinators formuch of their life. This was what hadled scientists to believe that staminate flowers are produced mainly forpollinator attraction than for pollendonation.
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 triedanother experiment to check the differences in pollen receipt betweenthe displays. Using temporarily captive hummingbirds, he compared theeffects of their intervening visits tostaminate flowers as against their visits to bisexual ones on pollen receiptby subsequent flowers. Pollen delivery was greater when the interveningflowers were staminate. He alsofound that intervening visits by pollinators to staminate flowers did notdilute the amount of pollen transferto flowers.
Because flowers on a plant havelimited pollen supply, plants displaying both types of flowers wouldbenefit from more frequent pollendelivery and increased amountsof pollen per flower. If onlybisexual flowers were produced,it would increase the numberof flowers but lower the amountsof pollen received by eachflower.