Abstract
The present study of Achillea ptarmica, a perennial in which the basic attraction units are inflorescences (heads) rather
than single flowers, examines whether the production of attracting organs (rays) entails
a cost in terms of fruit and flower production. To test for such tradeoffs, I subjected
plants to ray removal and compared the reproductive success of these plants with the
reproductive success of unmanipulated controls. Hand-pollinated plants on which all
rays were removed at the bud stage had a significantly higher percent fruit set than
hand-pollinated plants in the control group, suggesting that the investment in attractive
structures is made at the expense of fruit maturation. Ray removal did not significantly
affect the number of heads initiated. Given these observations and the lack of tradeoffs
between fruit set and other yield components, I hypothesize that the optimal ray size
is a compromise between pollinator-mediated selection for larger displays and tradeoffs
with fruit production.
Key words
Achillea ptarmica
- floral evolution - pollination - tradeoffs - selection
