Being a flower

it isn’t easy!

The first day the tram was running, a friend and I went up to try the trail on Gold Ridge. We didn’t get very far, being unwilling to cope with lots of trail-covering snow, and settled instead into a sheltered spot, out of the wind and in the sunshine, for a little picnic.

The false hellebore (= corn lily) was just poking up above the ground in snow-free areas, and fox sparrows and robins were singing. Especially enjoyable was the cheerful sight of yellow-flowered Cooley’s buttercup (now known as Cooley’s false buttercup), which some fastidious taxonomist has decided is no longer a ‘true’ buttercup (genus Ranunculus) but rather a poor cousin–a ‘false buttercup’—in a different genus.

Cooley’s false buttercup. Photo by Kerry Howard

I doubt that the wide-open flower of this (former) buttercup has a special kind of pollinator. It is probably visited by several types of insect, from bumblebees to flies and beetles, some or all of which could be effective pollinators.

Looking at those flowers, however, gave rise to a train of thought in my messy brain, starting with the question of what a flower does and the varied problems a flower has to solve.

The most fundamental function of a flower is sexual advertisement, sometimes flagrant, sometimes coy. The advertisement is commonly visual—colored petals (visible and sometimes UV wavelengths) or petal-look-alikes, pollen-bearing stamens (the male parts, often displayed conspicuously), multiple flowers bunched together to enlarge the floral display. Less accessible to humans but often an essential component of advertisement is aroma, appealing to the scent organs of various animals—in our region, chiefly insects. Nectar, and pollen itself, serve as pollinator rewards for visiting a flower.

Traditional botanists measured the reproductive success of plants by seed-set—the female function of flowers. That view of course neglects the (in most cases) necessary role of males in pollination and seed production, and the fundamental biological fact that plants can pass on their genes through both male and female functions. Passing on genes is all that counts in the evolutionary game! (Finally recognizing the importance of male function is, in effect, male liberation!)

Most flowers contain both male and female parts, although sometimes the sexes are in separate flowers on the same plant or on different plants. When males and females are separate, we sometimes see that the male flowers are bigger or more numerous than females, suggesting that more or bigger petals are very important for male function (i.e., sending pollen to the female flowers) in those species. This observation led to the idea that male flowers are competing with each other for the privilege of siring offspring, as often happens among male animals. It also became clear that showy petals could serve one gender-function more than the other, even in flowers that contain both, but this takes some clever experimentation to figure out.

There are several further complications to understanding why a floral display looks (or smells) the way it does. One factor is the availability of resources. In the case of our local fern-leaf goldthread, any given individual can switch from being entirely male, with only stamens, to being both male and female (hermaphrodite). If an individual contains female parts and produces fruit, that is a costly process, and that individual is not likely to produce female parts or fruits the next year; instead, it will produce only stamens and function only as a male. In this case, individuals are flexible, and can switch from one role to the other. In other cases, the response to resource limitation is genetically fixed, and whole populations exhibit resource-saving traits. For example, making a flower costs a lot of water (to expand the bud into a full flower), and in drier regions, populations of certain species conserve water by making smaller flowers.

Herbivores can exert tremendous pressure on flowers, sometimes in conflict with pollinator attraction. In wild strawberries, for instance, pollinators prefer large flowers, but so do destructive weevils, leaving the plant in a tight spot: make big flowers to attract pollinators or small ones to avoid the weevils? In certain species, flowers that are colored by anthocyanins (reds, purples, pinks) are better at deterring hungry insects, and where herbivorous insects are very common, whole populations of such species have anthocyanin-colored flowers instead of pale ones, which are found where the herbivores are scarce. Just how this works is unclear: There might be some direct protection by the pigments, or there may be an association between the genes for flower color and the ability of the plant to defend itself, or perhaps the same chemical pathways are involved in both. More to be learned!

In some cases, flower color is associated with the competitive ability of the plant. Certain colors are expressed where competition for space is intense, and others where competition from other plants is weaker. Apparently the same genes, or at least associated genes, control both aspects of the plant.

That is just a sample of the complex set of factors that can contribute to determining the floral display of a plant. I don’t mean to imply that all of them are operating in every case, but they (and others) need to be considered in understanding floral biology. These complexities are greatly understudied.