In many pine species (Family Pinaceae), ovulate cones structurallyresemble a turbine, which has been widely interpreted as an adaptation for improving pollination by producing complex aerodynamic effects. We tested the turbine interpretation by quantifying patterns of pollen accumulation on ovulate cones in a wind tunnel and by using simulation models based on computational fluid dynamics. We used computer-aided design and computed tomography to create computational fluid dynamics model cones.
We studied three species: Pinus radiata, Pinus sylvestris, and Cedrus libani. Irrespective of the approach or species studied, we found no evidence that turbine-like aerodynamics made a significant contribution to pollen accumulation, which instead occurred primarily by simple impaction. Consequently, we suggest alternative adaptive interpretations for the structure of ovulate cones.
Seed plants depend on their ability to cross-pollinate for sexual mating. In consequence, natural selection shapes their female reproductive organs to be effective pollen receptors. For animal-pollinated plants, adaptation in receptor form can be demonstrated empirically. For example, manipulative surgery can prove that changing a flower’s form reduces pollination (1–3). For wind-pollinated plants, however, theoretical arguments based on the principles of fluid dynamics are customarily used to predict an ideal receptor form (4). Stokes’ law of particle motion dictates that the proportion of oncoming suspended particles that impact with a receptor decreases with its diameter(5), all else being equal. Optimization of an impact collector under Stokes’ law explains the features of many anemophilous flowers (i.e., those suited to wind pollination); typically, their petals are minute, and the stigma is a protuberant thin filament (4). However, the ovulate cones (megastrobili) of conifers such as pines (Pinus spp.) defy similar interpretation because they are robust structures with diameters of 4–10 mm at the time of pollination (6). Individual mature pine trees produce prodigious amounts of pollen (7), but pollination can limit seed set nevertheless(8, 9). Therefore, the size of ovulate cones appears counteradaptive for wind pollination unless the cones accumulate pollen by a mechanism other than simple impaction.
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