The carnivorous plant with catapult traps

Drosera glanduligera (pimpernel sundew) shown here in a figure from the
new paper describing the fast-acting catapult traps (Poppinga et al., 2012).
Creative Commons Attribution License.

As usual, I'm late to the party and many other outlets have already written about this fascinating new paper published in PLoS ONE; such is the nature of the captivating world of carnivorous plant research. I'll try to place the new article in a bit more context.

First, the study organism: Drosera glanduligera, the pimpernel sundew, an Australian native, was first described in 1844 by Johann Georg Christian Lehmann who was cataloging and describing new species found in the collections of the botanical garden that he established and directed in Hamburg (now called Alter Botanischer Garten Hamburg). My botanical Latin needs a good refresher, but at a quick glance it appears to me that Lehmann noticed enough to write down that the marginal tentacles of this species are larger, but that's the only observation he recorded. Indeed, as you can see in the photo above, the marginal tentacles are much longer than the glue-tentacles closer to the center of the leaf lamina (more on this later).

Drosera regia, king sundew.
My plant in cultivation.

It turns out that, at least with this species, nothing more was recorded on what is apparently a fascinating trapping mechanism more analogous to the fast snap trap action of the Venus flytrap swiftly snatching a meal as seen here. That is until Richard Davion began making new field observations in 1974 when he wasn't even ten years old yet and published them in the 1990s in the relatively low circulation newsletter of the Carnivorous Plant Society of New South Wales. This underscores the importance of publishing new observations in journals where your work might get noticed. Davion later contacted two authors of the current study, the Hartmeyers, in 2003 and asked them to corroborate his findings. Within two years they confirmed Davion's observations and presented a film titled, "Drosera: Snap-Tentacles and Runway Lights," at the International Carnivorous Plant Society conference in 2006 that summarized their findings. (The video can be seen in full on YouTube courtesy of the ICPS.)

Snap tentacles: Everyone is excited about this research - and rightly so - but what exactly are snap tentacles and how do they differ from regular tentacles on sundews? On most sundew species like this Drosera regia above and to the right, there is one kind of tentacle: a few-celled stalk supporting a multicellular, glandular, globular head. They vary in length from the center to the edge of the leaf but not in overall morphology. They produce and rely on a viscous mucilage to retain captured prey until the tentacles slowly move (in some species, if at all) and direct the prey toward the digestive glands at the center of the lamina. This can take minutes, even half an hour depending on temperature. The snap tentacles, on the other hand, produce no mucilage and typically have a faster movement. The multicellular head is modified and looks more like a spatula or a pillow resting on oversized spoon (or maybe like a catapult?). They quickly flip unsuspecting prey up and into the center of the remarkably sticky mucilage produced by other tentacles. In milliseconds the prey can reach a maximum velocity of 0.17 m/s and a maximum acceleration of 7.98 m/s. Before it can think or react, it's deposited in the lamina where it's immobilized and often suffocated by the mucilage. Any struggling is futile as other slower tentacles reposition the prey nearer the digestive glands. A few days later after the plant's enzymes have done their work, meal time is complete and new leaves are unfurling, awaiting new dinner guests.

About two years ago, two of the authors of the current study, Siggi and Irmgard Hartmeyer, published their findings on over 100 Drosera taxa in the Carnivorous Plant Newsletter while investigating snap tentacle morphology. They concluded that many species of Drosera from multiple points on the established phylogeny of the genus have snap tentacles of some kind at some point in their ontogeny that vary in how swift the response is. It's important to note, however, that in the 2010 paper they set aside the tentacles of Drosera glanduligera as something wholly different since it was the fastest and the mechanism wasn't quite clear. This may be the reason why we can refer broadly to snap tentacles and flypaper traps on all the species but this one alone has been granted the new term catapult-flypaper-trap.

For their part of the new study, Irmgard and Siggi cultivated the plants, captured new HD film, and created this documentary to accompany the paper's release. Now this is effective science communication (in German with captions in English). If you want to skip the beginning and see the plant in action, make your way to about the 5:00 mark.

So that's cool! A catapult that helps deliver prey in the center of the trap. What else did they find? I'm glad you asked. These tentacles move by some pretty awesome mechanisms since they're moving so quickly - did you watch the video? It completes that swing from laying on the ground to delivering the prey to the hungry center of the leaf in as little as 75 milliseconds. In their investigation, the research team found that these snap tentacles actually deform beyond the ability to "reset" in a resting position to fire once again like the Venus flytrap can. They hypothesize that the cells in the hinge zone actually buckle from the stress involved with the movement - nature's one use only device.

In the discussion, they spend a good bit of real estate on hypothesizing on the mechanism involved: is it rapid water movement from one side of the tentacle to the other or loss of turgor pressure in combination with what would essentially be the release of stored potential energy by a sudden geometric change or curvature inversion not unlike this child's toy (oh, you know you remember annoying your parents with one of those.) But they didn't find any noticeable inversion. This may not be all that different from other tentacle movement (and the initiation of movement in the Venus flytrap) where the acid growth hypothesis is supported. Simply, in acid growth a signal would cause H+ ions to be pumped out of the cell into the cell wall space where proteins in the cell wall matrix known as expansins loosen at an acidic pH (higher H+ concentration). That allows the cells to increase in size very rapidly. If you do this on only one side of the tentacle, the lower surface, the result would be rapid bending inward. There are ways to inhibit the transporters associated with acid growth, but it might take the skills of a fine surgeon to delicately and strategically place the inhibitors on the tentacles without prematurely triggering them!

Regardless, there are plenty of possibilities here for future research, much of which was identified by the authors themselves in the last few paragraphs. Congratulations all around for such an attention-grabbing paper that was even blamed, in part, for slowing down the PLOS website:

Ha! Our site slowdown blamed on mega-popularity of PLOSONE's carnivorous plant bit.ly/RlVHLu and Cannibis papers bit.ly/S3Oy53
— PLOS (@PLOS) September 27, 2012

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References:
Poppinga, Simon, Siegfried R. H. Hartmeyer, Robin Seidel, Tom Masselter, Irmgard Hartmeyer, & Thomas Speck (2012). Catapulting Tentacles in a Sticky Carnivorous Plant. PLoS ONE, 7 (9): e45735. 10.1371/journal.pone.0045735

Hartmeyer, Irmgard, & Siegfried R. H. Hartmeyer (2010). Snap-tentacles and runway lights: summary of comparative examination of Drosera tentacles. Carnivorous Plant Newsletter, 39 (4), 101-113.

New study on carnivorous plants makes headline writers batty

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Before we get started, let me say that I forgive those who write headlines like pollution makes carnivorous plants go vegetarian and carnivorous plants go vegetarian in response to pollution and new study finds that pollution turns carnivorous plants into vegetarians. They know not what they do. It's also tempting to go after the flashy, attention-grabbing headline. Just try to do a better job next time, ok? And while we're on the topic, let us thank those that presented reasonable titles, like the one by Liat Clark at Wired.co.uk: carnivorous plants capture less prey in polluted bogs. Thank you for getting it a bit better! It would also be a terrible oversight if I neglected to mention that the Southern Fried Scientist took this issue to task a few days ago. Bravo!

So, on to the science at hand. What's all the fuss about?

Drosera rotundifolia at Triangle Lake Bog, Ohio. Photo by kitkor.

Drosera rotundifolia, the round-leaved sundew. Or common sundew. Or "bloody hell that thing is everywhere." And it is: North America, Europe, Asia... Here in Ohio it is the most common species of Drosera that you'll bump into - the other being Drosera intermedia, but several sites it had been known from have now been developed. For those unfamiliar with carnivorous plants, you might be peripherally aware that it is thought that these species have evolved in nutrient-poor environments. Given this idea and our knowledge that most species possess a good deal of phenotypic plasticity in response to environmental cues, researchers decided to further test earlier experimental observations that Drosera rotundifolia reduced its investment in carnivory (as measured by stickiness in units of force used to remove a piece of filter paper from the leaf) when grown in the presence of more nitrogen (Thorén et al., 2003).

Here in the new study, the researchers, a team including J. Millett of Loughborough University, B. M. Svensson and H. Rydin of Uppsala University, and J. Newton of the Scottish University Environmental Research Centre, were more interested in the relative amount of nitrogen that came from prey captured by normal means and from the roots as a result of increased nitrogen available from atmospheric deposition due to increased air pollution.

Briefly, the authors identified three bogs in Sweden that represented a gradient of mostly pristine to somewhat polluted in terms of nitrogen deposition. Fifteen specimens were removed from the bogs, dried, and analyzed for stable isotopes of nitrogen. Once they had their isotope data, all they did was subtract surrounding Sphagnum isotope data from Drosera and divide that by (insect - Sphagnum), where insect represents the mean isotope number for prey captured on the plant at the time of collection. And there's an easy ratio!

So conclusions from this? Well, the authors state it very clearly in the abstract, which many of the headline writers must have missed: "Drosera rotundifolia plants in this study switched from reliance on prey N to reliance on root-derived N as a result of increasing N availability from atmospheric N deposition." (emphasis mine) No, headline writers, these plants were not "OMG BECOMING VEGETARIANS!" Wouldn't that be a plant eating plant matter? And, as strange and wonderful as nature is, we have two possible examples in Nepenthes ampullaria and Utricularia purpurea where the former seems well-adapted to catch leaf litter and the latter appears to primarily cultivate algae in its bladder-like aquatic traps. No, dear headline writers, increased pollution will not turn Drosera rotundifolia into a vegetarian. It may, however, given this work and that before it, be the cause of changing priorities in nitrogen uptake from primarily prey-derived to primarily root-derived. It should be noted, however, that the authors did not set out to assess prey capture rates in these areas, so any statement has to be carefully worded and specifically related to nitrogen assimilation from different sources. We don't know if the plants in areas with more nitrogen capture fewer arthropods. It's entirely possible that the plants that incorporate more nitrogen from their roots capture the same number of prey but preferentially assimilate the nitrogen from the roots.

More troubling, however, is that with increased nitrogen availability in these once off-limits landscapes, opportunistic species may find it easier to overcrowd the poor little perennial carnivorous herbs. (Of course, the increase in nitrogen in this study was not very large and probably would not be enough to allow non-bog-adapted species to thrive.) Most carnivorous plants are low to the ground and depend on high light conditions to thrive; if shaded too much, they may soon succumb to succession. Of course this is only a hypothesis and needs to be studied! I wonder what the headline writers will say then...

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References

Millett, J., Svensson, B., Newton, J., & Rydin, H. (2012). Reliance on prey-derived nitrogen by the carnivorous plant Drosera rotundifolia decreases with increasing nitrogen deposition New Phytologist, 195 (1), 182-188 DOI: 10.1111/j.1469-8137.2012.04139.x

Thoren, L., Tuomi, J., Kamarainen, T., & Laine, K. (2003). Resource availability affects investment in carnivory in Drosera rotundifolia New Phytologist, 159 (2), 507-511 DOI: 10.1046/j.1469-8137.2003.00816.x

A sundew makes a hasty retreat

I'm not yet ready to send out the heralds and call this a success on my first go at cultivating tuberous sundews, but I'm closer now than I was before. If you recall, I originally purchased a lovely specimen of Drosera peltata from California Carnivores in January 2012 and first posted about it in March. It started out as a cute little rosette of carnivorous leaves, then bolted to produce two lovely 6-inch tall stems bearing those irresistible peltate leaves. And then throughout the last few months it was happily going about the business of, well, what sundews do best: capturing prey to collect nutrients.

Most of the tuberous sundews are native to Australia where the winter is rainy and the summer is hot and dry as a bone. This lineage of sundews has evolved the handy adaptation of giving up trying to survive as a full-fledged leafy herb during that hot, dry, unforgiving summer. Instead, they retreat into the soil, packing up their nutrients into root structures called tubers, not unlike a potato in many ways though much smaller.

In just the last few weeks as we approach the hottest late May and early June weather in the Northern Hemisphere here in Ohio, this particular specimen I had was finally ready to make its scheduled retreat. The leaves and stem quickly browned from the tips in a matter of days, my cue to stop watering and let the soil go bone dry lest the tubers succumb to rot as they form. And then, a few weeks later, out of curiosity and because I knew the soil surface in the pot was much too hard for the new growth to break through next year, I dug through the soil to find the tiny tubers:

Those little cream-colored pearls are definitely not perlite! A closer look:

In the above photo, the two tubers toward the top were still attached to the root, the dark brown object leading from center to the bottom right. It was a bumper crop! After sifting through the remainder of the soil, I found ten tubers in all:

The next challenge will be keeping them in a nice, dark place until next fall when they begin to stir. I think the hardest part will be remembering that I have them stuffed away somewhere!

And also, thanks to my friends at Botanical Oddities, I now have tubers from Drosera auriculata. Thanks, guys! Here's hoping I have success with both as I imagine the difficulties of growing tuberous sundews arise when preparing the new soil mix - the sand can't be too sharp or the new growth will be torn up on its several inch ascent from below. It will, at least, be a fun challenge.

Drosera peltata bit off more than it could chew

I found this grisly scene out in the greenhouse when I got home today. It looks like my Drosera peltata got a little overzealous, trapping this fly (ID? I'm hopeless at identifying anything with legs) by the head. It's at the end of this tuberous sundew's growing season, so all of that energy derived from this meal will be going directly to the tuber that should be forming several centimeters below the soil.

Drosera - Reach out and touch someone grab someone by the head.

The shield sundew

Encouraged by a Facebook post from California Carnivores and on a whim I decided to get my first tuberous sundew a few months ago. These are plants with a decidedly curious habit unfamiliar to those of us where winter normally equals dormancy. During the winter, more accurately described as the wet season, the plant will spring up out of the soil and produce first a flat rosette of leaves and then begin to bolt, sometimes attaining a height of 50 cm. Once the high heat normally associated with the Australian dry season arrives, the plant withers and retreats to a tuber some 4 to 6 cm underground. I suppose this unfamiliar habit is the reason why tuberous sundews get the reputation of being quite difficult to maintain - they must be kept wet but not soaked in the winter and nearly bone dry in the summer. Luckily, California Carnivores occasionally stocks Drosera peltata, the shield sundew, so named for the shield-shaped leaves, reportedly one of the easiest tuberous sundews to grow. A beginner's plant, if you will.

So far, I'm thrilled with it! I just hope that I'm able to keep the tuber viable through the summer.

As a bonus, I also received the above dainty flowering plant, Utricularia bisquamata. Known as a prolific weed of the carnivorous plant world, I'm not sure if I should torch it or try to contain it. If I don't do something, it is nearly guaranteed that it will end up taking over every single pot in my collection.