Making a flower

an intricate dance of parts

Animal-pollinated plants typically make a flower to attract visiting animals. A straight-forward example is a salmonberry flower, familiar to all of us: there’s a circlet of pink petals surrounding a yellowish center composes of male (stamens) and female (pistils) parts. In other cases, Mother Nature has designed a more complex arrangement of colorful petals, as in lupines or orchids.

Turn the salmonberry flower over and look at its underside, where it attaches to the stem. There is a more-or-less cup-like structure (the calyx), composed of several elements called sepals, which are usually green. The calyx covered the developing flower bud and often remains behind an open flower, providing support.

In some species, however, the sepals have become part of the flower—they are colorful and not hidden behind the petals. The fireweeds are good examples: the four wide, pink petals alternate with narrow, darker pink sepals. Three-leaf goldthread, common in subalpine meadows, has several white sepals surrounding stamens, pistils, and some golden petals, shaped like tiny trumpets, that are serving as nectaries.

Fireweed flowers have narrow, dark pink sepals between the wide, paler-pink petals, possibly making an added attraction. Photo by Kerry Howard

Red columbine is a more complex example. Here, the petals are the five yellow funnel-like structures that have long, reddish nectar spurs projecting from the other end of the flower. The five red, flaring wings are the sepals.

The view of a red columbine flower for an approaching pollinator: five spread-out red sepals and five yellow petals opening into reddish nectaries extended behind the flower. Male and female parts protrude in front of the openings and would be contacted as a pollinator probes for nectar. Photo by Kerry Howard

The wild iris goes still further—the three drooping, purple pieces are the sepals, comprising the main part of the flower. The petals are very small, just visible between the bases of the purple sepals. In domestic, horticultural irises, the petals have become much larger, standing up above the sepals (assuming that the ancestor of domestic irises was similar to our wild iris, horticulturalists presumably have selected for greater petal size over a number of generations).

Some plants have abandoned petals altogether, making a flower of showy sepals: marsh marigold, narcissus anemone, Sitka burnet. So, if you look at the back side of those flowers, you see no sepals.

However, one cannot conclude from an observation of ‘no sepals’ that the sepals are in the flower somewhere. That’s because some species drop their sepals fairly early in floral development; baneberry and some buttercups do so.

Attractive displays sometimes incorporate elements that are not, technically, part of the flower. I’ve previously mentioned dwarf dogwood with its white bracts (modified leaves) around the central flowers. Perhaps the best known plant using bracts to make a conspicuous display are the horticultural poinsettias so commonly sold for winter holidays. The sometimes rather colorful but very small flowers are clustered in the middle of all those gaudy bracts.

Do all those details of botanical anatomy matter? Not to those for whom flowers are just part of the scenery. At a trivial level, attention to such details as a bit like a game, a jigsaw puzzle, accounting for all the pieces and fitting them together. At another level, the various contrivances that make a ‘flower’ suggest questions about the relationships with pollinators (and other possible factors) and thus (eventually) to understanding more about the lives of these plants. Here are a few such questions (and no answers, although they could be addressed by experiments or, in some cases, by close examination of the evolutionary history):

–Are fireweed flowers with colored sepals between the petals more attractive to pollinators than those that lack them?

–Colored bracts or sepals around a flower can make the display larger. How does that affect pollinator behavior?

–Horticultural irises, with their tall petals, are (presumably) more attractive to humans than the wild forms with the tiny petals. Although gardeners may not care, do the pollinating insects such as bees react differently?

–Does converting showy petals to nectaries (albeit colorful ones), as in three-leaf goldthread, allow more nectar to be produced or favor visits by certain kinds of insects?

–In the case of columbines, the elongated petals make a nectar spur that can only be accessed properly by long-billed birds or long-tongued insects, effecting pollination. Many other species in the buttercup family have showy petals bearing small nectaries. The nectary development and specialization in columbines included making the petals less conspicuous, except when the sepals are fully reflexed or when viewed from below the pendant flower. Did that development make it useful for the sepals to be so showy? 

–And why do some plants just drop their sepals, while others have no petals but only sepals in the ‘flower’.

As usual, asking questions leads to still more questions. But if curious naturalists didn’t  keep asking, there are many things we’d never know.

Monkshood flowers

specialties and cheating bees

One warm but very windy day, a friend and I were perched on top of Gold Ridge in a small swale that provided some shelter from the wind. The alpine meadow was dotted with the purple flowers of monkshood. I had forgotten the structure of monkshood flowers, so I spent a few minutes opening one and examining the arrangement of parts inside.

The structure is really quite odd—very different from most other flowers in our area—although there are dozens of other species of monkshood (genus Aconitum) elsewhere in the world, which suggests that this odd structure is one that works well. Monkshood flowers are considered to be pollinated chiefly by bumblebees, although other animals may also visit the flowers occasionally. As I examined my specimen flower, it occurred to me that perhaps it would be interesting to learn how bumblebees visit and exploit the nectar while (potentially) depositing or exporting pollen.

Monkshood flowers are slightly complex, requiring a visiting bee to enter and move in a particular way. Most bumblebee species are generalists, capable of exploiting several kinds of flower. Naïve, inexperienced bees have no trouble figuring out simple, open flowers such as those of buttercups or roses, but they have to learn how to exploit more complicated flowers such as lupine or monkshood, and it may take a number of tries before the bee succeeds in getting to the nectar—indeed, these failed foragers often just give up. One species of bumblebee is considered to be a specialist on monkshood (elsewhere) and this species has a very short learning period, quickly getting to the nectar.

To explain what a bee has to do in a monkshood flower, I first need to describe a typical monkshood flower so an interested reader can then visualize a bee’s activity. In most other flowers, there are colorful petals that are backed by protective green sepals, but in monkshood, the purple exterior of the flower is composed of sepals that have been transformed to function like petals. There are two small sepals at the lower edge of the flower and two large, lateral sepals. Most conspicuously, there is a large, expanded sepal that forms a hood on the top of the flower (botanists perversely call this a ‘helmet’). The hood is reminiscent of the cowl of a medieval monk—hence the common name.

Photo by Bob Armstrong

Inside the flower, at the base of the sepals, lie the working parts: lots of short stamens offering pollen in their anthers, to be picked up and exported by a flower visitor (male function) surrounding a few stubby pistils that will become fruits if pollen is deposited on their receptive stigmas (female function). Inside the expanded hood lie two true, narrow petals that bear nectaries tucked way up into the top of the hood. To get to the nectar, a bumblebee has to reach or crawl inside the flower, passing over the sexual parts as it does so, picking up or depositing pollen.

As is so often the case, our local species of Aconitum (A. delphiniifolium) has not been studied, so research on other monkshood species may be used to shed light on the local species.

Japanese researchers experimented with the flowers of their monkshood species: they removed one or more sepals and watched the behavior of bumbles on the manipulated flowers, recording the amount of pollen picked up and deposited by the visiting bees. Removal of the large lateral sepals deprived a bee of her usual platform for standing in the flower while she reached up to the nectaries. This meant that her body did not contact the anthers or stigma properly, so pollen pick-up and deposition (and fruit set) was reduced. Removal of the small lower sepals had little effect except that a bee had some trouble entering the flower. Taking off the hood of the flower changed the look of the flower greatly, but bees still visited. However, sometimes the bee extended her tongue into the air instead of inserting it into the nectary, so although the pollination effectiveness of the visit was adequate, the bee often got no reward—and that would mean that the rewardless bees would be less likely to visit other such flowers. The researchers suggest that the function of the hood is to guide a bee’s tongue to the nectaries and perhaps also to maintain the concentration of sugars therein, thus keeping the bees’ interest in visiting.

In at least some species of monkshood, there is considerable variation among populations in the depth of the nectary and hence in the distance a bee has to reach in order to get nectar. A long nectary can only be reached by a long-tongued bumblebee species; short-tongued bumblebees have trouble reaching the nectar in the normal way and may become nectar-robbers, by chewing a hole in the hood and reaching into the nectary that way. Nectar robbing may reduce visits by good pollinators and has the potential to reduce both pollen export and deposition.