Flowers are a plant’s way of sexual advertisement. They are evolutionarily designed to attract animals that visit the flower in hopes of collecting nectar or pollen to eat, inadvertently accomplishing pollination. Insects are the most common type of animal that provide this service for flowers, although some insect visitors are thieves, taking the food but without pollinating.
To accomplish pollination, the foraging insect, as it rummages around inside a flower, incidentally gets pollen on its head or body or legs from the male parts (called anthers) of a flower and accidentally brushes off the acquired pollen on a receptive surface (called a stigma) of the female part of a flower. Many flowers have the means of avoiding self-pollination (within the same flower or between flowers on the same plant) and promoting cross-pollination from another plant, which creates greater genetic variation among offspring, one of the chief advantages of sexual reproduction. But that’s another, long story, so here I’m focused just on some behavioral interactions of insects and flowers.
Flowers have evolved many ways of controlling the visits of their pollinators, so the insects enter the flower in a particular way that effectively removes pollen from the anthers and deposits pollen on a stigma. The variety of ways in which plants do this could be the subject of several books; indeed, Darwin wrote one just about The Various Contrivances by which Orchids are Fertilised by Insects.
I’m not about to write a whole book here, so I’ll just describe a few ways some of our local flowering plants accomplish pollination. Open, saucer-shaped flowers are available to most insect visitors. A rose, for example, produces anthers and stigmas right in the middle of a circlet of petals, and all an insect has to do is walk around in the middle of the flower, sipping nectar and casually picking up or depositing pollen when it happens to contact the anthers. Nothing to it! Almost any bug can do it.
Much more interesting and intricate mechanisms of pollination exist in our local flora. For example, twayblade orchids grow profusely in young conifer forests, such as in Gustavus or near Eagle Glacier cabin. The flowers are tiny, pollinated by very small bees and flies. When the insect seeks nectar, it triggers the explosion of a drop of sticky stuff that picks up pollen on its way out of the flower and sticks the pollen to the head or eye of the insect, where it is cemented. Some parts of the flower actually move apart in order to make space for the exploding drop and pollen to emerge and fasten to the insect. Later, when the insect visits another twayblade, the pollen is detached somehow and deposited on the stigma, sometimes leaving the congealed sticky blob behind on the insect. Darwin spent a lot of effort figuring this one out!
A very different pollination mechanism is used by lupines, which are pollinated by bees. The sexual parts are hidden away in a fold between two fused petals in the lowest part of the flower. A bee pries open the fold as it probes for nectar. When it does so, out pop the sexual parts, and pollen can be dusted on the bee or brushed off the bee onto the stigma. When the flower has been visited, the uppermost petal has turned from white to pinkish-purple. The color change is a signal to future bee visitors that the nectar is depleted and the bee should visit other flowers on the stem.
Louseworts, in contrast, hide the sexual parts in a little hood in the upper part of the flower. So a visiting bee has to get into the flower in a certain way, often wedging open the hood, in order for its body to contact anther or stigma. One of the louseworts in Southeast, however, does it differently, as described the next paragraph.
“Buzz pollination’ is one of the most interesting and common pollination mechanisms in our area. In this process, a bee (usually) lands on the flower and buzzes in a special way. Its major wing muscles are temporarily inactivated, and its body just vibrates very rapidly, repeatedly hitting part of the flower. The vibrations shake dusty pollen onto the bee’s body, to be deposited eventually on a stigma. (Obviously, this technique doesn’t work with sticky pollen). Buzz pollination is how shooting stars, tomatoes, blueberries, wintergreens, and many other flowers get pollinated. Buzzing may also contribute to pollination even in open, saucer-shaped flowers such as salmonberry and roses.
Bog laurels grace our muskegs with their pink, wide-open flowers. If you look closely at a young flower, you would see that the female parts are in the center. But the male parts consist of arched, white filaments leading from the flower center to small, dark blobs (the anthers) that are nestled in pockets on the surface of the petals. When a bee lands on the flower, the spring-loaded filaments straighten, raising the anthers, and shaking pollen on the buzzing bee. So you can tell if a bee has been there by looking at the position of the filaments (arched or straight) and seeing if dark anthers remain in the petal pockets.
I’m sure there are other cute tricks by which our local plants contrive to deliver pollen from flower to flower. See if you can find some!
On the Trails in Juneau 