Lupine puzzles

banner petals and a naming mystery

Lupine plants are common around here, and perhaps you’ve noticed that the uppermost petal (the so-called banner petal) is sometimes white and sometimes dark pink or almost purple, while the side petals are typically blue. If you look at the whole inflorescence, you quickly notice that the newer, white banners are on the upper flowers, whereas older, pink or purple banners are on the lower flowers. There’s a story about why the upper banners are white and the lower ones are pink or purple.

Many kinds of flowers exhibit color changes, and studies of several species have shown that the color change serves as a signal to visiting insects, directing them to the most rewarding flowers. For example, in certain species of Lantana, the young flowers in a cluster are yellow but turn red as they age. Clusters that retain the old, red flowers are more attractive from a distance than those without the red flowers, but close up, pollinating insects clearly discriminate between yellow and red flowers, favoring the yellow ones that still have nectar.

lupine-by-bob-armstrong
Photo by Bob Armstrong

Similar reports can be found for lupines, in which the color of banner petal changes. In the case of our local lupines, the story is that the white-bannered flowers have not yet been visited by pollinating bees (or perhaps were visited only very recently and there has not been time to make the color change), whereas those with darker banners have already been visited, and potentially pollinated, and were no longer worth visiting. One local observer has noted that bees do seem to favor the white-bannered flowers. Nice story!

But then, one day I noticed that bumblebees were regularly visiting the flowers with dark pink banners. So either the bees were really naïve, perhaps recently emerged workers with no experience, and had not yet learned to read the signals (a real possibility, but how could I tell!) or else those older, darker flowers still had something to offer to a bee.

I decided to look more closely. I collected a sample of young, white-bannered flowers and another sample of darker, older flowers, and dissected them. As expected, the white-bannered flowers seldom had pollen deposited on the female parts and there was lots of pollen still in the pollen sacs. However, the darker-bannered flowers did not conform to the story’s expectations. Indeed, the female parts of the darker flowers sometimes bore a wad of orange pollen, but many of those older flowers had no pollen visible on the female parts and presumably had not been pollinated. Furthermore, those older, darker flowers often still contained a full load of pollen on the male parts, showing that no bee had come to take pollen away.

The upshot of these observations is that the story of the color change in the banner petal as a signal that guides a bee to the most appropriate flower may be just a story, in the case of our local species. Or else, we have a lot of very naïve bees. At least, a more thorough investigation is warranted.

There’s a second puzzle about lupine too: As an adjective, the word ‘lupine’ means wolfish or wolf-like. Now what in the world (or out if it) do wolves have to do with these flowers? Maybe nothing; maybe the name is just a distortion of some earlier word. In any case, I have found no good, clear explanation for the name of this plant.

A different sort of confusion arose during a recent visit to Cowee Meadows, where we noted at least seventy species of flowering herbs and shrubs along the trails and in the expansive meadows. This is a really rich place! This year we found a flower we hadn’t seen before. Down along the rocky beach, we found a stand of white, daisy-like flowers. These appear to be a species of Chrysanthemum, sometimes called the Arctic daisy. These daisies have several other common and scientific names, so the taxonomy is bewildering. Whatever it is called, it is a native species that is not to be confused with the common invasive oxeye daisy that occurs as a weed along roadsides and other disturbed sites. In any case, finding a new (to us) species was fun!

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