Nature is full of all kinds of neat little tricks that are essential for species’ survival. One of the neatest tricks of nature is pollination. Most people understand the basics of pollination, in that a pollinator has to visit a flower, and in the process takes pollen from one plant to another, and that is a very simple way of describing the process. However, upon investigation, it turns out that pollination is not necessarily an easy or simple process, because flowers have to be receptive for pollination, for one, and they also need a way to make sure they are receiving pollen from only their species. As such plants use a combination of tactics such as color, smell, and nectar (the key ingredient in the making of honey). Interestingly enough, while nectar is a pleasant treat and we think of it as important for honey production, not all nectar is good. Some of it is toxic to not only humans but also to insects that might visit the plant.
Either way, it is primarily a means to an end that ensures the plant’s survival and ability to reproduce, not the survival of the pollinator, although in many cases, symbiotic relationships exist between plants and their specific pollinators.
One of the tricks that pollinators have to increase their likelihood of being pollinated is something called nectar guides. To the human eye, these nectar guides may appear as stripes, spots, or a combination of things. Some nectar guides may be completely invisible to humans. So how is this significant? Many pollinators such as bees, butterflies, and moths can see light in the ultraviolet range of the light spectrum. What may appear as plain brown dots or stripes to us, appear to an insect as a giant neon sign saying, “Eat at Joe’s.” Let’s look at some interesting examples.
One easy example that comes to mind are rhododendrons and azaleas. Whether it’s the typical cultivated varieties or native species such as Catawba rhododendron or rosebay, all of these species have nectar guides. If we look closely at the flowers of these plants, we can clearly see the nectar guides. Rhododendrons and azaleas have five-petals that make up a shallow-throated corolla. Inside the base of the corolla is where the nectar can be found. Blocking the way to the nectar is a collection of pollen-bearing stamens and a singular pistil which must receive pollen for the ovule inside to be fertilized. If we imagine the petal arrangement as star-shaped, on the uppermost point of the star is where we would find the nectar guides. They show as multiple spots which may be yellow, green, or maroon. The nectar guides tell the pollinator (usually a bee) that the flower has nectar, but in order to get to it, the bee must pass over the stamens which have sticky clumps of pollen on them. The pollen sticks to the bee and gets transferred to the pistil. After all of the available nectar has been removed, it is believed that the nectar guides “turn off,” meaning they no longer reflect ultraviolet light, essentially telling any other bees that might visit the flowers that “Joe’s is closed.”
Irises are another genus of plants that effectively use nectar guides. Their nectar guides consist of lines on what are referred to as the falls (modified sepals) that point in the direction of the nectar.
Typically, within the center of the lines is an ultraviolet-reflecting patch of color. In this case, the nectar guides serve as a landing pad, or runway if you will. The insect pollinator will walk up the falls, crossing over the hairy stigma (pollen-capturing part of the pistil) and push up a flap where the anther is bearing the pollen. The insect must do this in order to get to the nectar. The insect will then back out of the flower and carry the pollen to the next plant where the pollen will be brushed onto the sticky stigma.
Thus, irises are not self-pollinating. On a side note, tree-frogs use this floral trick to their advantage, perching in the center of the flower and awaiting unsuspecting insects coming in for landings. Very cool stuff!
One of my favorite examples of nectar guides is how they are used by lady’s-slipper orchids. Lady’s slippers have a complex flower. The flower consists of a lower pouch called a labellum. This pouch has lines that point towards the center of the pouch. The flower gives off a fresh scent that is detected by the pollinating insect. Upon entering the flower, instead of being rewarded with nectar, the insect finds itself trapped. The flower doesn’t produce nectar, but uses the nectar guides to entice the insect to come to the flower. Once inside, the insect can’t go out the way it came into the flower. Instead, it must use the lines as a map for the exit. The lines point the way to the top of the flower, where a small hole can be found. As the insect climbs upward towards the hole, it climbs over the pollen-bearing anthers dusting itself with pollen. Upon reaching its only way out, the insect must squeeze through the very narrow hole, and in doing so, brushes the pollen onto the stigma of the flower, pollinating it in the process. This is what I like to refer to as one of nature’s dirty little tricks.
As you go out this summer, enjoying the wildflowers, you will find many examples of this fascinating adaptation, from honeysuckles to lilies. One of the things that we often fail to realize about the natural world is that it comes down to survival at all costs. Nature is mostly a very unfriendly world, particularly when we look at some of these adaptations. It’s almost like reading a spy novel because you have assassinations, violence, sex, poisonings, romance, and all the other things that make such novels intriguing. Even the things we might think are quite delicate and harmless, may be something that could very easily kill us. For me, this is what makes nature so interesting.
Until Next Time!
Clint Calhoun teaches high school science and outdoor education classes at Lake Lure Classical Academy and has worked as a naturalist and biologist in Hickory Nut Gorge for over 25 years.
