Twinspurs & Their Pollinators

© El Grafo / CC-BY-SA-3.0 (via Wikimedia Commons)

Garden centers and nurseries always have something to teach me. Though I am largely a native plant gardener, the diversity of plant life offered up for sale is always a bit mind boggling. Perusing the shelves and tables of myriad cultivars and varieties, I inevitably encounter something new and interesting to investigate. That is exactly how I came to learn about the twinspurs (Diascia spp.) and their peculiar floral morphology. Far from being simply beautiful, these herbaceous plants have evolved an interesting relationship with a small group of bees.

Diascia whiteheadii. Photo by Ragnhild&Neil Crawford licensed under CC BY-SA 2.0

The genus Diascia comprises roughly 70 species and resides in the family Scrophulariaceae. They are native to a decent chunk of southern Africa and have adapted to a range of climate conditions. Most are annuals but some have evolved a perennial habit. The reason these plants caught my eye was not the bright pinks and oranges of their petals but rather the two spurs that hang off the back of each bloom. Those spurs felt like a bit of a departure from other single-spurred flowers that I am used to so I decided to do some research. I fully expected them to be a mutation that someone had selectively bred into these plants, however, that is not the case. It turns out, those two nectar spurs are completely natural and their function in the pollination ecology of these plants is absolutely fascinating.

Diascia rigescens photo by Dinkum licensed under CC BY-SA 3.0

Not all Diascia produce dual spurs on each flower but a majority of them do. The spurs themselves can vary in length from species to species, which has everything to do with their specific pollinator. The inside of each spur is not filled with nectar as one might expect. Instead, the walls are lined with strange trichomes and that secrete an oily substance. It’s this oily substance that is the sole reward for visiting Diascia flowers.

Diascia megathura (a) inflorescenc with arrows indicating spurs and (b) cross sectioned spur showing the trichomes secreting oil (Photos: G. Gerlach).

If you find yourself looking at insects in southern Africa, you may run into a genus of bees called Rediviva whose females have oddly proportioned legs. The two front legs of Rediviva females are disproportionately long compared to the rest of their legs. They look a bit strange compared to other bees but see one in action and you will quickly understand what is going on. Rediviva bees are the sole pollinators of Diascia flowers. Attracted by the bright colors, the bees alight on the flower and begin probing those two nectar spurs with each of their long front legs.

If you look closely at each front leg, you will notice that they are covered in specialized hairs. Those hairs mop up the oily secretions from within each spur and the bee then transfers the oils to sacs on their hind legs. What is even more amazing is that each flower seems to have entered into a relationship with either a small handful or even a single species of Rediviva bee. That is why the spur lengths differ from species to species - each one caters to the front leg length of each species of Rediviva bee. It is worth noting that at least a few species of Diascia are generalists and are visited by at least a couple different bees. Still, the specificity of this relationship appears to have led to reproductive isolation among many populations of these plants, no doubt lending to the diversity of Diascia species we see today.

Diascia 'Coral Belle' Photo by KENPEI licensed under CC BY-SA 3.0

The female bees do not eat the oils they collect. Instead, they take them back to their brood chambers, feed them to their developing offspring, and use what remains to line their nests. At this point it goes without saying that if Diascia were to disappear, so too would these bees. It is incredible to think of the myriad ways that plants have tricked their pollinators into giving up most, if not all of their attention to a single type of flower. Also, I love the fact that a simple trip to a garden center unlocked a whole new world of appreciation for a group of pretty, little bedding plants. It just goes to show you that plants have so much more to offer than just their beauty.

Photo Credits: [1] [2] [3] [4] [5] [6]

Further Reading: [1] [2] [3] [4]

The Rose of Jericho

To survive in a desert, plants must eek out an existence in specific microclimates that provide conditions that are only slightly better than the surrounding landscape. Such is the case for the Rose of Jericho (Anastatica hierochuntica). This tenacious little mustard is found throughout arid regions of the Middle East and the Saharan Desert and it has been made famous the world over for its "resurrection" abilities. It is also the subject of much speculation so today we are going to separate fact from fiction and reveal what years of research has taught about this desert survivor. 

Natural selection has shaped this species into an organism fully ready to take advantage of those fleeting moments when favorable growing conditions present themselves. A. hierochuntica makes its living in dry channels called runnels or wadis, which concentrate water during periods of rain. It is a desert annual meaning the growth period of any individual is relatively short. Once all the water in the sandy soil has evaporated, this plant shrivels up and dies. This is not the end of its story though. With a little luck, the plants were pollinated and multiple spoon-shaped fruits have formed on its stems.

Photo by Phil41 licensed under CC BY 1.0

As the dead husk of the plant starts to dry out, its branches curl up into a ball-like mass with most of the fruits tucked away in the interior. There the plant will sit, often for many years, until rain returns. When rain does finally arrive, things happen fast. After all, who knows how long it will be before it rains again. Thanks to a quirk of physiology, the dried tissues of A. hierochuntica are extremely elastic and can return to their normal shape and position once hydrated. As the soil soaks up water, the dried up stems and roots just under the surface also begin taking up water and the stems unfurl.

To call this resurrection is being a bit too generous. The plant is not returning to life. Instead, its dead tissues simply expand as they imbibe liquid. Water usually does not come to the desert without rain and rain is exactly what A. hierochuntica needs to complete its life cycle. Unfurling of its stems exposes its spoon-shaped fruits to the elements. Their convex shape is actually an adaptation for seed dispersal by rain, a mechanism termed ombrohydrochory. When a raindrop hits the fruit, it catapults the seed outward from the dead parent.

Photo by Roland Unger licensed under CC BY-SA 3.0

If rains are light, seeds do not get very far. They tend to cluster around the immediate area of their parent. If rains are heavy, however, seeds can travel quite a distance. This is why one will only ever find this species growing in channels. During the rare occasions when those channels fill with water, seeds quickly float away on the current. In fact, experts believe that the buoyancy of A. hierochuntica seed is an adaptation that evolved in response to flooding events. It is quite ironic that water dispersal is such an important factor for a plant growing in some of the driest habitats on Earth.

To aid in germination, the seeds themselves are coated in a material that becomes mucilaginous upon wetting. When the seeds eventually come into contact with the soil, the mucilage sticks to the ground and causes the seeds to adhere to the surface upon drying. This way, they are able to effectively germinate instead of blowing around in the wind.

Again, things happen fast for A. hierochuntica. Most of its seeds will germinate within 12 hours of rainfall. Though they are relatively drought tolerant, the resulting seedlings nonetheless cannot survive without water. As such, their quick germination allows them to make the most out of fleeting wet conditions.

Photo by Nikswieweg at German Wikipedia licensed under CC BY-SA 2.0 DE

Occasionally, the balled up husks of these plants will become dislodged from the sand and begin to blow around the landscape like little tumbleweeds. This has led some to suggest that A. hierochuntica utilizes this as a form a seed dispersal, scattering seeds about the landscape as it bounces around in the wind. Though this seems like an appealing hypothesis, experts believe that this is not the best means of disseminating propagules. Seeds dispersed in this way are much less likely to end up in favorable spots for germination. Though it certainly occurs, it is likely that this is just something that happens from time to time rather than something the plant has evolved to do.

In total, the Rose of Jericho is one tough cookie. Thanks to quick germination and growth, it is able to take advantage of those rare times when its desert environment become hospitable.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2] [3] [4]