SIZE-NUMBER TRADE-OFFS AND POLLEN PRODUCTION BY PAPILIONACEOUS LEGUMES

The intra- and interspecific relations between the size and number of pollen grains were examined for 21 species of papilionaceous legumes t o test for the trade-off expected from the subdivision of limited reso urces. We observed a tradeoff between pollen grain diameter and the nu mber of grains produced per flower within 17 of the 21 species examine d. For 12 of these species, the observed partial regression coefficien t for In (pollen grain diameter) equaled -3, as expected. The remainin g five species exhibited more negative partial regression coefficients than expected. Relations between pollen grain size and number that ar e more negative than expected may result from a decelerating relation between pollen grain size and the resource investment per grain. Flowe r production significantly influenced pollen production for only eight species so that pollen production seems to be determined primarily on a per flower basis. Interspecifically, pollen size also varied invers ely with pollen number for the 21 species even after the effect of phy logenetic relatedness was removed. Pollen size varies relatively littl e compared to the number of pollen grains produced per Bower and there fore may be the primary target of natural selection. The presence of a trade-off in pollen production probably constrains the evolutionary o ptions by which flowering plants can maximize male success.