By John Palka — Posted September 1, 2019
The back wall of the house where we spend each summer is lined with plants that have been among my favorites since my childhood—hollyhocks (Alcaea, most often A. rosea, Family Malvaceae). The stems grow tall enough to project above the rain gutters. The buds at the top give way to open flowers below, and then to maturing fruits near the bottom. Every morning bumblebees arrive to frolic in the flowers, flitting from one to another. A garden paradise awaiting a close look!
A HOLLYHOCK FLOWER UP CLOSE
A closer look at a single flower from this bouquet will start us on our exploration.
The petals are colored so as to attract pollinators from a distance, and also to guide them into the interior of the flower to where the nectar is to be found. In many species the color patterns of flowers include components that are only visible in ultraviolet light, a range of wavelengths that we cannot see but insects can. So, there may well be more to this photograph than meets our eye! For examples of flowers that have prominent patterns in the UV, look here.
In the center of the flower you see a star-shaped figure and within it a pale tuft. The star is the five nectaries that produce the sweet nectar that pollinators love. We will look at them in more detail shortly. The nectaries are actually located on the sepals, the green, leaf-like structures that form the outside layer of the flower. In this picture, the greenish tinge of the nectaries is due to those sepals, visible between the bases of the petals. The tuft is made up of the male and female reproductive structures, the stamens with their slender filaments and pollen-bearing anthers, and the styles with their sticky stigmas whose role is to capture pollen so that the eggs hidden deeper inside can be fertilized.
Now here comes a bumblebee (most likely Bombus flavifrons, Family Apidae), a pollinator par excellence.
The tuft of styles and stamens provides a highly effective support for bumblebees like this one, so they are not dependent on the smooth surfaces of the petals. You can see that the hairy surface of the bumblebee’s body is covered with white pollen grains, very likely brought in from another flower. As the bumblebee maneuvers around the flower, some of these grains will be transferred to the sticky surfaces of the stigmas, thus initiating a process that culminates in fertilization. It’s a long way from where the pollen attaches to where the egg is waiting to be fertilized, so each pollen grain grows out a tube that makes its way inside the full length of the style all the way to the egg. This pollen tube carries within it the cell nucleus that will cross over into the egg and actually carry out fertilization.
Looking elsewhere in the photograph, you can see unopened buds on the left and one just starting to open on the right. The green, leafy structures pressed up against the outer surfaces of the petals are the sepals on which the nectaries form.
NECTAR AND NECTARIES
Nectaries make nectar, so let’s ask—what exactly is nectar ? Nectar is the main reward that a flower provides to an insect, or a bird, or even a mammal that comes to visit and, in the process, picks up the flower’s pollen and takes it to another flower where fertilization has a chance to occur. Nectar contains sugars, so it tends to be very sweet, but it often contains other valuable chemicals like amino acids, vitamins, fatty acids, and so forth.
What exactly is a nectary? Functionally, it is any structure that produces nectar. Structurally, while there is diversity, most nectaries are glands containing clusters of ordinary looking cells. They usually release their nectar via pores that resemble the stomata of leaves. The nectar accumulates into a droplet that can be sucked up by a pollinator. This is the case in hollyhocks.
Remember the star-shaped arrangement of nectaries at the base of a hollyhock flower that we saw earlier? Each wedge-shaped arm of the star is a space between the bases of the adjacent petals. The edges of the petals are lined with hairs, and the two sets of hairs from facing edges interlace to loosely cover the space. In the photograph above, I have pulled the petals apart slightly so you can see this arrangement better. Under the covering layer of hairs is a droplet of nectar produced by a gland situated at the base on the underlying green sepal. Each nectary of a hollyhock, then, is a complex, layered structure: on the bottom, at the surface of the sepal, is the nectar-producing gland; then a droplet of nectar; then a layer of presumably protective hairs produced by the edges of the overlying petals. The whole structure is visually conspicuous and helps to draw the pollinating bumblebees to it.
The pictures below show bumblebees in action, using their probosces to probe nectaries by pushing through the layer of hairs to reach the droplet of nectar underneath.
MALES AND FEMALES
From the point of view of the bumblebee, the point of visiting a flower is to take advantage of a good source of nectar. From the point of view of a flower like a hollyhock, however, the point of being visited is to achieve fertilization, preferably between the flowers of two different plants. Now here is a conundrum. Each individual flower has both male and female parts. Why doesn’t the bumblebee simply transfer pollen from a single flower’s anthers to its own stigmas so that the flower fertilizes itself? Biologically this would be a bad idea, of course, the most direct kind of incest you could imagine. Some species do rely on self-pollination, but most (including hollyhocks) avoid it because cross-pollination promotes genetic diversity and this is a good thing. So, with the anthers and stigmas of a hollyhock flower being so close together, and with bumblebees regularly distributing the pollen that clings to their hairs, how can self-pollination be avoided? Here is how hollyhocks do it.
A closer look at a sampling of hollyhock flowers reveals something unexpected—many of the flowers show stamens only, as if they had no female parts. Each individual stamen in the flower below is composed of a slender filament carrying a bulbous anther at its tip. The anthers contain the pollen grains that will soon dust the body of a visiting bumblebee.
Other flowers, like the one below, show both stamens and styles with their sticky stigmas, as if they were both male and female.
The paler threads at the tip of the tuft are the stigmas, formed at the ends of the styles that are out of view in this picture. The tiny bumps on their surfaces are pollen grains that have stuck there. The slightly darker threads below the styles are the stamens.
This difference between individual flowers reveals part of the mechanism for avoiding self-pollination. Many flowers, including hollyhocks, avoid self-pollination by the simple expedient of having the male and female parts mature at different times. In some species the female parts mature first so, with no pollen yet present within their own flower, they get fertilized by pollen from a different flower. In other species the male parts mature first and release their pollen. By the time the female parts are receptive, the pollen of the same flower is gone and only pollen from another flower can do the job. This is the case in hollyhocks, whose anthers mature before the stigmas do. The first of our two pictures shows a younger flower with mature stamens only, the second picture shows an older flower in which the stigmas have emerged while the anthers have already shed their pollen.
Hollyhocks also take advantage of a second mechanism, less widespread in the flower world than is asynchronous maturation. Remember, in many of the flowers the styles/stigmas can’t even be seen. The flowers look as if they have formed only stamens. Where are the female parts?
This is quite a remarkable story. The styles develop within the interior of a hollow cylinder formed by the filaments of the stamens, fused together edge-to-edge. In the beginning the styles are shorter than the stamens are, so their receptive tips, the stigmas, are hidden within the stamen cylinder and are unavailable to any pollen in their vicinity, including the flower’s own pollen. This is what you see in the photograph above. The anthers are just opening and shedding their abundant pollen. The few pollen grains within the tube were probably brought there by me while I was cutting the stamen cylinder open for photography.
Later, after the flower’s pollen has been shed, the styles lengthen. Their stigmas emerge from the stamen cylinder and are now ready to receive pollen. Because the flower’s own pollen has been shed (notice that there is hardly a pollen grain to be seen except those attached to the sticky stigmas) and is thus no longer available for fertilization, pollen landing on the newly-emerged stigmas has to come from a different flower!
There is one more rather remarkable piece of flower “behavior” that relates to hollyhock fertilization. In the photograph above the styles are somewhat curved, but not in a way that would bring the stigmas into contact with the anthers. Now look at the photograph below (it glows so much because the flower was back-lit by the afternoon sun).
All of the stigmas are curved over, and you can see that most of them are actually contacting the anthers below them. Why would this be? After all, we have just seen two mechanisms—delayed maturation of the styles and stigmas relative to the anthers, and the hiding of the female parts within the stamen cylinder until the anthers are spent—whose function is precisely to avoid contact and hence self-fertilization?
Here is the favored explanation. If a bumble bee successfully deposits pollen grains from another flower on a freshly emerged stigma that is projecting freely above the anthers, the pollen tubes of these foreign grains will start to grow immediately. The styles curve over only later. For this reason, pollen deposited on them from the flower’s own anthers will be late starting and will be outcompeted by the more desirable foreign pollen. In this way, self-fertilization is minimized. If, however, no bumblebee successfully pollinates a given stigma with pollen grains from another flower, then pollen grains from its own flower will have no competition and thus will have a chance to fertilize the flower’s egg. This self-fertilization is better than no fertilization at all!
THE BROADER PICTURE
We biologists are curious creatures; we forever want to peer below the surface of whatever has caught our attention. In the story of the hollyhock and the bumblebee, for example, the details we have already seen would prompt us to ask questions like these:
- The male parts of a hollyhock flower mature before the female parts do. What mechanisms actually bring about this sequence?
- Pollen grains develop inside tightly closed anthers. How do they ever get out?
- The end of a style, and the stigma continuous with it, grow straight out from inside the stamen cylinder. What makes them grow, especially at the right time?
- Later they curve over to contact the anthers. How is curving initiated at the right time, and what mechanism produces it?
- Bumblebees are strongly attracted to hollyhock flowers. How does attraction actually work? What happens in the brain?
- Bumblebees go back to their nests for the night and emerge to forage again in the morning. How well do they remember where they were the previous day? How far away can a bumblebee be and still detect a hollyhock plant?
There are hundreds of questions like this, and answers to them bring us ever-closer to a personal engagement with the natural world. Find your own questions, and then see if you can find a few answers as well!