Annals of Botany 90: 119-126, 2002
© 2002 Annals of Botany Company
Allocation to Floral Structures in Thalictrum pubescens (Ranunculaceae), a Cryptically Dioecious Species
1 Department of Biology, University of Louisiana at Monroe, Monroe, LA 71209, USA
* For correspondence. Fax 001 318 342 3312, e-mail bidavis{at}ulm.edu
Received: 14 January 2002; Returned for revision: 4 March 2002; Accepted: 2 April 2002
Females of Thalictrum pubescens produce stamens that contain sterile pollen, whereas males are both functionally and morphologically unisexual. This study examines the investment in stamen production by females of T. pubescens by comparing the female structures with those of their fully functional male counterparts. Stamens from females had the same biomass and contained the same amount of nitrogen and phosphorus as stamens from males. Anther size was the same in males and females, but filaments were longer in stamens from males. Females produced more pollen per anther than males, and pollen size was the same in both sexes. Within flowers, there was a positive correlation between the amount of pollen per anther and the length of anthers in males, but not in females. This would be expected if males growing in better environmental conditions or with greater vigour invested more resources in pollen production, thereby increasing fitness. Females, who receive no fitness benefits from increased pollen production, did not show this pattern. There was also evidence of a trade-off within female flowers between the number of stamens and the number of pistils. This trade-off was noted in conditions when variance among plants was reduced, namely in the field during a year when flower size was particularly small and in a previous glasshouse study. Therefore, it appears that when environmental variance is low, stamens are produced at the expense of producing more pistils, and hence seeds. In conclusion, stamen production does not appear to be inconsequential to females of Thalictrum pubescens.
Key words: Thalictrum pubescens, tall meadow rue, sex allocation, trade-offs, cryptic dioecy, floral traits, sexual dimorphism.