Okay. So, when we left Part I, I was promising jokes, light refreshments, and nectar guides. Help yourself to some soda/cookies/nuts/whatever, and then we'll get started.
Nectar guides are, like they sound, markings on the flower which guide pollinators toward the nectar. In some plants (e.g. black-eyed susan, Rudbekia hirta, or Gazania), the flower guide is simply a dark spot1 around the center of the flower. Other plants make the whole flower dark but make the pollen stand out very brightly (Zantedeschia does this in UV, as does Glechoma hederacea), and still others draw lines or spots which point toward the nectar (Digitalis, Streptocarpus, Geranium, Viola).
I admit to being kind of puzzled about this, initially. To my thinking, the flower itself should be advertisement enough. I mean, bees aren't usually that much smaller than a lot of the flowers they pollinate, so if they wandered around on the flower for a second or two they would find the nectar anyway, whether there were "EAT AT JOE'S" signs or not. And it's not like their eyesight is particularly bad, considering that they can clearly manage to see small distant objects pretty accurately in order to find the flowers at all.2 So what does the plant get out of this? What, for that matter, do the bees get out of it?
My best guess is that it's worth the effort for the flowers because there are a limited number of bees, and each bee can only visit a limited number of flowers in a day (or season, or whatever). If you want all the flowers in a field to have the best possible chances of being pollinated, it's helpful to make it as obvious as possible where the nectar is, to minimize time spent fumbling around trying to find the nectar. It may not seem like that big of a deal, but if you're competing with other plants for pollinators, any advantage you can get over your competition, everything else being equal, is going to be helpful. If you can get eighty pollinations in an hour when everybody else is only getting seventy-eight, you will inherit the earth. So it can be worth it to become an air traffic controller.
And if you're a bee? Well, obviously you can collect more nectar in a given amount of time if you don't have to fumble around looking for it, which enables you to bring back more nectar, which means you can raise more baby bees. That much is pretty straightforward, actually.
This waste-of-time theory has been observed in the wild, on a Delphinium species which produces both normally-colored flowers with nectar guides and albino flowers with no markings: both hummingbirds and butterflies took longer to pollinate the albino flowers. The researchers went the extra step to verify that this was because of the lack of guides, and not just that the hummingbirds and butterflies were less enthusiastic about white flowers, by painting nectar guides on the petals, and painting other petals on the same flower blue, and then timing the pollinators again. They even made sure that the change wasn't just because the pollinators were attracted to the paint, by painting blue flowers that were already blue and comparing the response.3 (ref.: .pdf file)
Having said all this, nectar guides are not a universal thing. Nectar, in fact, is not a universal thing either. Flowers that are pollinated at night don't really need guides, for obvious reasons.4 Cultivars don't necessarily have the same nectar guides as their ancestral species, and may not have nectar guides at all. Streptocarpus 'Falling Stars,' above, has much less pronounced nectar guides than 'Purple Martin' or 'Tanager.' We also had a fourth variety at work, 'Snow Bunting,' that was pure white. (Not only could I never get a decent picture of it myself, but I can't find one on-line, either.) I'm fairly sure it didn't have any markings at all.5 So nectar guides are not mandatory, just helpful.
As with human air traffic control, certain conventions have arisen to make everything run smoothly.6 Nectar guides are often in specific colors, either yellow or "negative ultraviolet."7 In some plants, the nectar guide, but not the rest of the flower, changes color when the flower has been successfully pollinated, signalling pollinators to move along to the next flower and not waste their time with this one.
Streptocarpus nectar guides are relatively obvious and straightforward, at least in most cultivated varieties, though as with most things, if you get to digging around in the literature, you find some cool details. Most cultivated Streptocarpus are hybrids, with the bulk of their genes coming from the lavender-colored S. rexii, which displays very prominent nectar guides and is pollinated by bees. However, not all Streptocarpus species are bee-pollinated: some, like S. dunnii, are pollinated by birds instead, and lack nectar guides altogether. (ref.) Such species also show other signs of being specifically adapted against pollination by bees.8, 9
When I first had the idea of going with "Air Traffic Controller" for Streptocarpus, I wasn't really thinking about that choice much beyond the fact that it uses nectar guides, and I wasn't sure that was really enough to hang the choice of "person" on. Seems kind of minor, you know? But it turns out that this was even more apt than I imagined, because not only do Streptocarpus spp. use nectar guides to point pollinators to nectar, but they also use different colors when talking to different pollinators, and send them different signals about when and where to land. It really was the right choice. I love it when these profiles work out that way. (You can tell I'm excited when the footnotes have footnotes.)
Photo credits: all me.
1 Rudbekia "flowers," as well as those of Gerbera daisies, sunflowers, and many other familiar plants of the Asteraceae, are actually clusters of a large number of tiny, individual flowers. Only the outer ring of true flowers grow petals, which serve to attract the attention of pollinators (though cultivated varieties of these plants may produce inflorescences in which every flower produces petals, as for example in some of the more hideous cultivars of Echinaceaa). Although to us the petals of Rudbekia look uniformly yellow, to a bee the petal changes color about halfway out: the inner part of the petal absorbs ultraviolet and the outer part reflects it.
This is the same kind of thing people see when we look at, say, a Hibiscus that's yellow with a red center, it's just that in the case of the Hibiscus, the difference between the two parts of the flower is in the green part of the spectrum, which we're able to see. The outer part of the flower reflects both red and green (a combination our brains call "yellow"), and the center reflects only red. The same thing is happening in Rudbekia: the outer portion of the petals is reflecting red, green, and ultraviolet, and the inner portion is reflecting only red and green. There are pictures at this site which can show you what you're missing in UV wavelengths. The Rudbekia plan, with UV absorbed in the center and reflected on the outer edge, is typical of a lot of flowers, though that's not the type Streptocarpus uses.
a I know you like them, jodi, but they don't look right. . . .
2 Though their eyesight is not especially good, either: human eyes are much better at seeing detail, whether from far away or close-up, than bee eyes are. So this might be bad logic on my part.
3 This sort of double- and triple-checking, silly though it might sound to the non-scientist, is actually very important, and is called scientific control. Without it, experimental data is difficult or impossible to interpret accurately. For example, suppose I let Nina loose in the house ten different times, and each time I find her hiding in the Aglaonemas. I conclude, oh, anoles are crazy about Aglaonemas, and I post this on the blog and talk it up and publish papers and books about it and eventually it becomes my whole career, the anole-Aglaonema-attraction theory. Then somebody points out that when I did these experiments, all my Aglaonemas were in the same room, and how do I know that Nina didn't just prefer that room for some reason? And somebody else asks if I've tested this on any other anoles, and how can I be sure that Nina doesn't just have some "personal"a fascination with Aglaonemas? And somebody else wants to know whether I did this all at the same time of day: maybe she was just looking for the brightest/darkest/warmest/quietest/etc. spot she could find. And so on, and the questions keep coming and suddenly nobody wants to buy books about the special bond between anoles and Aglaonemas anymore, and my career is ruined, and the husband, Nina and I all lose the house and Nina's turning tricks in alleys in order to buy crickets, and it's all because I wasn't controlling for these variables in my original experiments.
There are some critics of science that don't understand this concept, many of them failed scientists themselves, and they will complain about how Western science is all about "controlling nature" and "making nature jump through hoops" and what have you, because they're not able to understand that there might be reasons for their experimental results that have nothing to do with their particular pet theory. I'm hoping to bring this up again in a different set of posts, eventually, so I figured I'd get the idea out there for everybody to chew on ahead of time.
a (In quotes because she's not exactly a "person," so "personal" is probably not the right word to use. Though "anoleal" doesn't sound right, either.)
4 Although I will predict that if nobody's found them yet, somebody is going to someday discover a night-blooming flower that uses texture, instead of color, as a nectar guide. There are already day-bloomers that use hairs pointing in a particular direction instead of pigments, for their nectar guides (e.g. Iris). It doesn't seem like that big of a stretch to think that a bat-pollinated flower might reflect sonar differently from different parts, in a way the bat could read. Color, after all, is just differences in the reflection of light, so why not differences in the reflection of sound? If I were an actual botanist, instead of just playing one on the internet, this would be something I'd be interested in going looking for.
5 (It was also really boring: I didn't care for it.)
6 E.g. saying "niner" instead of "nine," which seems like a quirky affectation until you imagine trying to tell the difference between the spoken words "nine" and "five" while trying to control a large, loud, potentially dangerous piece of machinery, and then all of a sudden it seems totally obvious and kind of brilliant. Similar logic underlies the use of the NATO alphabet (Alpha Bravo Charlie Delta Echo etc.) for letters.
7 "Negative ultraviolet" means "absorbs ultraviolet." Since people can't actually see ultraviolet light, we've never had to come up with a name for this color. I mean, to (non-color-blind) people, red is red, and red+green is yellow, and red+green+blue is white, but red+ultraviolet is also just red, to us, and red+green+ultraviolet is also just yellow, because the presence or absence of UV doesn't register on our eyes or brains one way or the other. Bees, on the other hand, do see ultraviolet, and so to a bee, red+green is yellow and red+green+ultraviolet is a different color, which they may not regard as being terribly similar to yellow at all. Weirdly, though, to bees, red+green+ultraviolet is the same color as green+ultraviolet, because they don't see red.a
This, incidentally, is why none of the great honeybee artists have ever been appreciated for their genius in the human world (Particularly BZbzbZZ and BzbzzbzZZbz, whose work human critics have called "derivative," "uninspired," and "cliched" despite their wide acclaim in the honeybee community. This has led to some frustrated and enraged honeybee artists stinging humans as an act of tragic revenge/suicide, made all the more tragic because they can't tell people apart from one another very well and therefore almost never sting their critics. The life of the honeybee artist is a tortured one.): we're literally not seeing the same pictures.
All of which is to explain what "negative ultraviolet" means: it's a color word we've had to make up for colors that we can't see, and would make little sense to the bees. Though very little makes sense to bees anyway, as you know if you've ever tried to talk to one.
a Which hopefully answers the question you've never thought about: how come red flowers are so attractive to hummingbirds? What's so special about red? The answer is, it's not that the hummingbirds are necessarily so attracted to red as that, if you're a red-flowering plant, you're basically invisible to the bees, so if you want to get pollinated, you have to either change colors or focus your attention on the non-bee pollinators. If the red-flowering plants are being pushed to adopt ever more hummingbird-friendly designs, then evolution is going to reward hummingbirds that focus ever more tightly on red flowers.
8 S. dunnii is also adapted in other ways to pollination by birds: besides having red flowers, the flowers are more tubular in shape, and narrower (suitable for a bird's beak, but not large enough for a bee to crawl in), and all face the plant's one large leaf, which is used by birds as a perch while they collect the nectar from the flowers. The flowers of bee-pollinated species like S. rexii face every direction. (From this paper, which contains a lot of interesting stuff relating to nectar guides and pollination.) There are a few "unifoliate" (= "one-leaf") species of Streptocarpus (S. dunnii, S. grandis, S. pusillis, S. polyanthus), which grow only one leaf, and which are a little too weird-looking to be commonly grown, though they're cultivated by people who are, you know, really into Streptocarpus and are useful in hybridization.
9 Species like S. dunnii, if not S. dunnii itself, are probably the original source for the genes which result in red-flowering Streptocarpus hybrids like 'Tanager.' You'll notice that 'Tanager's nectar guides are not as intense as 'Purple Martin's.