Plant Biomimicry: Thigmotropism
Rebecca Eagle, November 13, 2017
Over the past three years in our program, I’ve had many opportunities to converse with interested folks about the wonders of plants. Plants do some pretty miraculous things, no doubt. At the very core of their existence, they are required to survive ‘in place’. How many other living organisms on Earth can claim this feat? Very, very few things can accumulate life’s requirements (reproduction and resource acquisition), without movement. Not to mention, plants also must adapt to local conditions: contamination, weather, drought/flooding events, and more. While animals, insects, and birds can move when their environment gets unfavorable, plants must shelter in place and utilize strategies that they’ve evolved over the millennia of time they’ve been on this planet.
A favorite plant of many inquisitors of plant biomimicry is the Venus Fly Trap (Dioneae muscipula). Why wouldn’t someone
admire this plant?! It eats meat, but cannot move from place because it lacks musculature and because it needs to stay rooted in the ground to obtain water, minerals, and necessary stability to stay erect. Many are surprised to learn that this insectivore is native to our own United States, found chiefly in wetlands of the Carolinas. Let’s discuss the biology of the Venus flytrap, and then talk about its inspiration for design applications.
The Venus Fly Trap lives in nutrient-poor wetland soils, particularly low in nitrogen and phosphorus. Plants require these elements and all plants have strategies that allow them to acquire them from their environments—sometimes in very unique ways! Remember, though, that plants can’t move. They rely on things that are accessibly near them. Soil and the atmosphere being the mediums for most plants, cannot be relied on by the Venus flytrap. This constraint doesn’t faze it! Other organisms come to plants, right? Aphids, pollinators, nectarivores, and other critters visit plants for meals of all cuisines (vegetation and nectar), and this carnivorous plant evolved to capture the nutrients and energy from these insects to ensure its survival throughout time! (The first written documentation of the Venus Fly Trap was noted in 1760 in North Carolina, by North Carolina Colonial Governor Arthur Dobbs ). A question I am frequently asked is whether the plant does photosynthesis. Yes, the Venus flytrap does have the same anatomy and physiology required to be in the Kingdom Plantae. It is not uncommon to hear that they rely solely on insects for nutrients, but this is not true. Insects are merely the back-up mechanism for the minerals that a play would obtain from the soil, not the CO2 or sunlight energy obtained above ground.
We get it, the Venus flytrap eats insects for nutrients… but how? (Video: 4-minute YouTube video of Venus flytrap in action). When a larger-sized insect (flies, ants, spiders, grasshoppers, i.e.), lands on the inside of the leaf blade, the weight of it will eventually trigger minute hairs. These trigger hairs will respond (0.1 second response time), by closing the trap. Ideally, the prey will be inside, but, as you can see in the suggested video, this mechanism is not fail-safe. As is in nature and life, sometimes we lose the game.
The response of the trigger hairs is an example of a nastic movement and thigmotropism. Thigmotropism is the act of responding to the direct stimulus of touch, such as a fly landing on the inner leaf blade and bumping into one of the two or three trigger hairs. Nastic movements are controlled by hormones, more so than by a direct stimulus. Once the direct stimulus causes the thigmotrophic response, auxin (a plant hormone) stimulates cell expansion as a rapid growth response. In short, the cells inside the leaf of the Venus flytrap are told to swell up quickly, which causes the leaf blades to close. This is the same physiological response and movement that is witnessed when a flower of an angiosperm plant opens and closes in response to light! (As an aside, auxin does some pretty rad things in plants that I encourage you all to read about in your down time!).
Bio-inspiration from the Venus Flytrap
How could we not be inspired by this amazing plant?! I’ve talked in previous posts about some possible applications for designs based on the mechanisms of the Venus flytrap: baby gates, pet gates, sensors for factories, sensors for home safety, etc. I won’t rehash that conversation. The general idea involved here is the passive sensing with quick response that uses only clean energy.
While the response mechanism is certainly worthy of investigating, I would add in the importance of Life’s Principles as an additional means of bio-inspiration from the Venus flytrap. If we consider the rationale involved in utilizing insects for survival, we witness the ability of this natural organism to obtain its needs from the local environment in absence of the preferred mechanism for sequestration. As far as I know, the Venus flytrap isn’t shipping in her flies and spiders from the west coast. She has found a way to survive and thrive with what is near to and available to her. She is substituting a necessary product for another locally obtained product. She carefully considered her choices and chose to adapt and evolve, rather than die.
Of course I am getting a bit anthropomorphic here, but my goal is to encourage companies to look at the bigger picture of what is important to the planet, to its business, and to its customers. The amount of money and natural resources that are invested in product development could, perhaps, be re-evaluated to better meet the needs of the business by responsibly utilizing local supplies, rather than shipping them into the area. The re-evaluation might discover that the location of the business itself is better suited to be near the customers it most services—avoiding the strain of shipping far distances.
As I leave you, to spend more time preparing for my upcoming comprehensive exams, I would like to mention this quote I read in Botany for Gardeners (Capon, 1990). The preface of the quote describes the means by which antifreeze was developed, inspired by ‘leaf antifreeze’ (increasing the concentration of sugars in the protoplasm to lower the freezing point inside the cells). “Plants have been ahead of human invention by several million years.” Consider this as coming from a 1990’s book for gardeners, not for engineers, designers, or biomimicry-enthusiasts. This is written by someone who just appreciates plants for the value they bring to all of us in such a variety of ways. I encourage all of you to continue to read about the wonders of plants and be inspired by all the great things they do… all without leaving home!
The habitat of the Venus flytrap is limited to a small area of the Carolinas. Modern development threatens this already minuscule area with increasing take-over. Consider visiting the website of The Nature Conservancy to learn more about the plant that Charles Darwin has called “the most wonderful plant in the world.”1
 The Nature Conservancy, accessed November 13, 2017. https://www.nature.org/ourinitiatives/regions/northamerica/unitedstates/northcarolina/explore/venus-flytrap-brochure.pdf?redirect=https-301
 Capon, Brian. (1990). Botany for Gardeners: An Introduction and Guide. Portland, OR: Timber Press. pp. 86.
“Plants are amazing!” This is something I hear a lot from non-botanists. Of course, I know plants are awesome, but every time I turn around, I learn something new and exciting. This semester was no exception. Tasked with a project in my Biomimetic Design class, led by Dr. Petra Gruber, I walked into the meadow to find inspiration– literally.
On a very wet, cold, rainy day in October, I walked to a meadow within our field station property (Bath Nature Preserve, Bath Twp., Akron, Ohio) and found a section to investigate. Indian grass (Sorghastrum nutans) towering over my head, I decided to stop at 20 steps and set up a 1m x 1m plot to sample. October in a meadow doesn’t give you very much to identify, but goldenrod (Solidago spp.) and Indian grass (S. nutans) were plentiful among a few baby asters, Galium spp. (aka ‘Cleavers’ or ‘Bedstraw’), wild strawberry (Fragaria virginiana),clumps of unidentifiable grass and moss. I measured heights of stems and area covered, took the percent coverage to determine how much each species covered the plot,and took several picture views for record. After returning to campus, I created a hand-drawn schematic of the plot.
A few weeks later, I returned to the same plot. Apparently my methods of counting and direction are spot-on because my last step landed on a pen I had dropped on that rainy day a few weeks earlier! If you’ve ever done field work, you understand how amazing it is that I found a PEN in the middle of a meadow over 2 meters high! This time I was there to measure the ability of the meadow to hold a load. I admit, I didn’t think the stems would hold up… being so late in the year and being dried out. As usual, though, plants are amazing and surprised me yet again!
I decided to test the load by creating a 1m x 1m foam board that was sturdy, yet lightweight. I placed the board directly over the plot, placing flags on each corner. The flags allowed for a visual cue to observe movement of bot
h plants and the board, as well as giving a reference point at which to measure the height of the board after each addition of weight. After the foam board was placed on top of the plants, I measured the height at each corner (flag) for the “initial” height. I added one heavy book and measured the height at each corner. Subsequently, I added increasing weight and measured the heights. At 3 books (6.7kg), the system (the meadow plot) could no longer hold the weight. Because this was the same plants were used over the entire experiment, I believe more weight can be held by the plants in true form.
So how does this happen? Plants are amazing. In the meadow, plants grow up to 10 feet below ground (roots) and above ground. You can imagine how secure this makes these cantilever beams! Here, the Indian grass and Goldenrod grew 1.5m to 2.5m above ground. The stems reached diameters of 2-5mm. You may wonder how the stems did not break when the weight was added. Galileo was the first to record these observations, noting that bending is resisted in the outer layers, not the inner stem as some might think. Several studies have investigated this design, including F.O. Bower (1930) who compared plant stems to concrete, saying, “Ordinary herbaceous plants are constructed on the same principle. The sclerotic strands correspond to the metal straps, the surroundin
g parenchyma with its turgescent cells corresponds mechanically to the concrete.” Equisetum (Horsetail) is another champion plant for many reasons, but here, in this context, it’s a biomechanic superstar. “The hollow stem of Equisetum giganteum owes its mechanical stability to an outer ring of strengthening tissue, which provides stiffness and strength in the longitudinal direction, but also to an inner lining of turgid parenchyma, which lends resistance to local buckling. With a height >2.5 m isolated stems are mechanically unstable. However, in dense stands individual stems support each other by interlacing with their side branches, the typical growth habit of semi-self-supporters.” (Spatz, Kohler, Speck 1998). Again, plants are amazing.
After doing some mathematical calculations (very much estimated
in this case because of the imprecise nature of this ‘experiment’), it is expected that a single Goldenrod stem can support >118% of its biomass! Now, we’re not talking about the strength of steel or lead, but we can see that plants offer us new possibilities when we are designing or constructing new things! Imagine a support feature that is hollow inside and allows for storage in the “stem” as well has having the strength to support weight. Think on a smaller scale: imagine a space in which a stiff, lightweight outer covering is needed to secure something. Imagine the many possibilities that plants offer us to grow using Life’s Principles.