Charles Darwin and his family were enchanted by the small carnivorous plant “Drosera”, also known as sundew, and were fascinated by the way Drosera captures prey. The mechanism of capturing prey involves many processes that we commonly think are unique only to animals – like the use of electrical signals, hormones or the production of digestive enzymes. Drosera lives in wetlands that are usually very poor in nutrients and are important for our fight against climate change. However, carnivorous plants are threatened to leave our planet forever, due to loss of their habitat by the input of nutrients through fertilizers and pollution, selling of plants collected in the wild and the draining of wetlands for exploiting peat often used in gardening soil.
We all know Charles Darwin from his famous book “On the Origin of Species”, published in 1859, which with its ideas about evolutionary adaptation and natural selection became the foundation of modern evolution theory. I can guess that in your mind an image appears with an ape on the left, a human being on the right and all intermediary steps in between (- and maybe the pinnacle of evolution totally to the right: a dancer, a climber or a person sitting at his computer). Yes, this is the Darwin we know! But there is also another side of Darwin that we often forget, but which was similarly groundbreaking for modern science. Indeed, Darwin seemed to care much more about these observations than about his work that made him so famous, as he himself proclaimed “I care more about Drosera than the origin of all species in the world”. “Drosera” was kind of an obsession for the whole Darwin family. Charles’ granddad Erasmus Darwin wrote funny-to-read poems, in which he basically humanises plants and describes their sex life. In “The Love of the Plants” he described Drosera as the “queen of the marsh” “with a slender waist” that has a “zone of diamonds trembling round her brows”. Charles Darwin was under Drosera’s spell for incredible 16 years until he published his first book about her and similar species. 2 of his 10 children even joined him in working on this book. His wife seemed to become jealous about Darwin’s little affair as she expresses in a letter to a friend: “At present he is treating Drosera just like a living creature and I suppose he hopes to end in proving it to be an animal”. So, who is “Drosera” to whom Charles devoted all his time? It is a little plant that even today fascinates young and old: the carnivorous sundew!
Why exactly was Charles so much devoted to this little plant? Darwin was especially fascinated by his observation that plants do not simply exist in a vegetative state, but actually take decisions, for example by growing towards a light source. A plant catching prey and therefore turning the table of nutrition around where usually animals feed on plants, was another example of a plant being actually very similar to animals.
The miracle of the trap
If Charles could travel through time and appear today, I think he would be delighted to see how far we have come today to understanding his little sweetheart. Although not as famous in pop-science as the big bang theory, we now know that plants actually share a lot of characteristics that one associates rather with animals. This becomes clear when we have a look at how sundew actually captures prey: sundew is covered with lots of tentacles with glands on the tip that produce mucilage (similar to glands in our intestine) droplets. This is reminiscent of dew in the morning that gave sundew its name. As soon as a fly gets into contact with the mucilage, it is trapped and naturally tries to get out of the trap. The fly however does not know that this behaviour makes things worse. The struggling leads to electrical signals – similar to the electrical signals we have in our nerves – that travel from the tip to the base of the tentacles and induce the bending of the tentacle, bringing the prey from the periphery of the plant towards the leaf center. The fly comes in contact with more tentacles in the center of the leaf, leading to more electrical signals and inducing the bending of tentacles that were previously not in contact with the fly. The leaf now surrounds the fly which is totally covered in mucilage. The electrical signal stimulates the accumulation of a hormone – yes, plants also produce and use hormones just like use -, called jasmonic acid. The hormone as well as chitin, which comes from the skeleton of the fly, induce the production and secretion of digestive enzymes. Basically, the leaf becomes the stomach of the plant. Finally, the nutrients coming from the fly are taken up by the leaf transforming the stomach to an intestine.
Sundew is one of 583 plant species currently known to be carnivorous. The way they hunt insects is incredibly diverse. Apart from the sticky trap of sundew, some of them have snap traps like the well-known Venus fly trap. Others even live under water and suck up swimming insects like a vacuum cleaner. But why do plants evolve these bizarre strategies? Well, they all have one thing in common: they live in areas where the soil is poor in nutrients and therefore nutrient uptake from the root is not sufficient for the plant to survive. As if this wasn’t a difficult life already, some carnivory plants only survive in even more extreme environments: the Venus fly trap for example lives in hot, nutrient-poor wetlands where fires ravage other taller growing plants every 4-5 years. As you can imagine these ecosystems are rare. The Venus fly trap for instance exists only in a very tiny area at the border between North and South Carolina in the USA. Besides, carnivorous plants have severe disadvantages compared to other plants. They grow very slowly, are tiny and some of them – like the sundew due to the mucilage reflecting parts of the sunlight – have a poor efficiency of photosynthesis, which every plant needs for producing their energy. Therefore, outside of their niche, they are quickly overgrown by other plants.
The International Union for Conservation of Nature (IUCN) publishes every year the red list, enumerating species that are threatened to leave the planet forever. Only 17 % of known carnivorous plant species were analysed so far, but 56 % of these are threatened. The biggest problem for carnivorous plants is pollution by agriculture and urbanisation which lead to an influx of nutrients to their nutrient-poor environment. Other plants now become better adapted as they can more easily extract nutrients from the soil than carnivorous plants which have reduced roots. In addition, carnivorous plants are still collected in the wild and sold on markets to customers, although we know how to propagate some carnivorous plant species. Taking care of carnivorous plants is very difficult, often leading to the carnivorous plant’s death in the customers’ homes. Of course, carnivorous plants can be a fascinating means to excite for example a child’s interest in taking care of a plant (I had one myself I have to admit – that died within a year). But do you really think that it needs to be a carnivorous plant? If you anyway decide for a carnivorous plant, you should inform yourself whether the carnivorous plant was really grown in a gardening center or whether it was selected from the wild.
Chose the right gardening soil to protect the climate and the queen of the marsh
Another risk factor, especially for our little sweetheart “sundew” is the draining of wetlands for example to win peat which today is excessively used in gardening soils. Peat is actually a very slowly growing moss that did not fully decompose in the oxygen-poor environment of wetlands. As the moss takes up CO2, but never returns all the carbon to the atmosphere, these wetlands are important to fight climate change as they are carbon sinks. When peat is used in gardening soil, it quickly decomposes fully emitting CO2. Therefore, the use of peat in gardening soil destroys the beneficial effect of peat. Since peat is used virtually in the whole gardening sector, peat does not regrow as fast it is exploited. More and more wetlands need to be drained to win peat. In Germany, the consumption of peat is so high that local peat resources, which are only allowed to be exploited if the wetland was already drained before, are not enough. Now, peat is transported all the way from East European countries to Germany adding even more harm to the climate. Alternatives for gardening soil actually do exist. Therefore, if you are a passionate gardener, try out the gardening soils that do not contain peat. But be careful while buying your soil: You really need to read the ingredients because many peat-reduced gardening soils still consist of 80 % peat. In addition, peat can be sold as “organic” soil, because “organic” is not a protected trademark and does not necessarily need to be sustainable. Taking this into account, keep enjoying gardening using non-peat gardening soils, so that the homes of the queen of the marsh is still being protected.
To continue reading the scientific articles (most relevant):
David E. Jennings, Jason R. Rohr (2011); A review of the conservation threats to carnivorous plants; Biological Conservation Vol 144:5, https://doi.org/10.1016/j.biocon.2011.03.013
Mithöfer A. What Darwin only divined: unraveling the hierarchy of signaling events upon prey catch in carnivorous sundew plants. New Phytol. 2017;213(4):1564‐1566. doi:10.1111/nph.14463
Krausko M, Perutka Z, Šebela M, et al. The role of electrical and jasmonate signalling in the recognition of captured prey in the carnivorous sundew plant Drosera capensis. New Phytol. 2017;213(4):1818‐1835. doi:10.1111/nph.14352
Rutishauser, Rolf. (2009). Vom Milch trinkenden Sonnentau (Drosera spec.) zum schlafenden Wassersalat (Pistia spec.): Charles Darwin als Botaniker. 10.5167/uzh-25234.
Weblink about peat use in Germany: https://www.landwirtschaft.de/diskussion-und-dialog/umwelt/torf-unersetzlich-oder-verzichtbar
Thanks to Bergadder on pixabay for providing the cover image.
Hallo Nils,
die “andere” wissenschaftliche Arbeit von Darwin ist dir in dieser Kürze gut gelungen. Dies gilt auch
für die Fangfunktion des Sonnentaus. Sehr anschaulich beschrieben. Das Ganze bekommt seine
ökologische Bedeutung mit dem Torfabbau und dem heutigen Gartenbau. Ja, die Menschen
wissen vieles, sehen es auch ein und handeln aber “sündhaft”.