Plant sex: Size matters

Posted: Sunday, June 08, 2008

When spring comes, the birds gear up for the nesting season.

Male songbirds sing to defend their territories and attract females, and this makes our trail-walking extra-pleasurable in spring. Then the birds begin their courtship displays. Male rufous hummingbirds do great swooping flight displays in front of females. Male dippers stand tall and strut on rocks near a prospective mate. Other birds may dance, flutter, show off fancy plumage, or demonstrate their agility - all to capture the attentions of a "lady fair."

Plants cannot dance or sing or strut or flutter alluringly. However, they certainly can do their utmost to attract good mates. They must have an intermediary in the form of a pollinator (an animal or the wind), so the "courtship" is rather indirect. Flowering plants attract pollinators by means of their floral display. This may involve the number of flowers, their size or color, the amount of nectar, aroma, or the way the flowers are arranged.

My interest in plant courtship was stimulated by observations of common milkweed plants back in the Midwest. Milkweeds make great balls of complex pink flowers, often several hundred each, with several of these balls per stem. Each flower is both male (pollen-producing) and female (pollen-receiving and potentially seed-producing). But although many flowers receive pollen, each ball of flowers produces only a few seed pods.

So the question in my mind was what are all those other flowers for? Why make all those "extra" flowers? Because so many flowers had received pollen, it did not seem likely that the extra flowers served merely to bring in still more pollen-carrying bees or flies or butterflies. Instead, it seemed reasonable that the extra flowers served a male function - attracting insects that would remove pollen from this plant and carry it to others.

This led to the idea that (in many plants, at least), the male-functioning portion of the plant is competing with other males to donate pollen to female receivers. How might plants compete (as males) for mates?

Plants are different from animals in that most of them have both male and female parts and can function in both sexual roles. Sometimes male and female parts of a flower mature at different times, and then it often happens that more scent or more nectar is produced during the male phase than the female phase. If male and female parts are in separate flowers on the same plant, the male flowers are frequently slightly larger than the females, presumably attracting more pollinators. Some plants produce hundreds of male flowers but only one or two female flowers. In short, such plants are allocating more resources to male than female flowers.

These differences in sexual allocation only make sense if the plants are competing as males to become fathers of the next generation. The pattern also implies that, in these plants, female reproductive success (seed production) is not limited by the amount of pollen received - if it were, the female flowers would be more aromatic, nectar-laden, or numerous.

Instead, female reproductive success is limited by resources, such as light or nutrients. In contrast, male reproductive success is limited by the number of other flowers that receive that male's pollen.

Charles Darwin wrote a long book about a process he called sexual selection, which involves competition among males for mates. Darwin was thinking about animals, trying to account for the evolution of differences between males and females in size or color.

Sexual selection is the process that has led to male animals of many species to have larger antlers, brighter colors, showier ornaments, or larger size than females, and the males commonly spend a lot of effort in showing off these features. I have argued that the process is essentially the same in plants, even though the two sexes are not always in different individuals and the form of the competition differs.

Many traditional botanists have hated this perspective on plant reproduction. They think of plant reproduction in terms of seed production, i.e., the female side of the game. But this is clearly only half the story, because most plants have two sexes and the male side of the story is as important as the female side. By changing the way of looking at plant reproduction, I was thus in the ironic position of arguing for "male liberation" in plants at the same time as I was establishing myself in an academic world that was still largely dominated by men.

There is more to this story, but no space to tell it. However, the difference in perspective is important. Just thinking about plants in this way really made them come fully alive for me. I think they can do more than they are usually given credit for. That's something to contemplate as you enjoy the lovely floral displays along the trails in summer!

All those flowers are not "just another pretty face!"

• Mary F. Willson is a retired professor of ecology and a Trail Mix board member.



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