Sundews

The carnivorous sundew which you see in the featured photo is properly called the Drosera burmanii or Burman’s sundew. It is mistakenly called Burmese sundew sometimes. The story behind the name took me back to the origins of modern biology. It was described in detail in a book by the Dutch physician Johannes Burman, who spent a few years in Sri Lanka. His assistant in the production of this book, called Thesaurus Zeylanicus, was Carl Linnaeus. This was 1735, and Linnaeus had just published his own book, the Systema Naturae, which was to change the world by inventing a new way of naming all living creatures. Linnaeus’ naming system is the one all biologists and amateur naturalists follow. Darwin, in his book Insectivoruous Plants, remarked on the common trapping mechanism through the sticky “dew” which you can see in the photo, and classified all sundews into one family, which is still called the Droceraceae.

Darwin was a wonderful naturalist and asked most of the questions which keep drocerologists busy till today. Are the sundews selective about their prey? In the nearly three centuries that have passed since the first descriptions of sundews, all evidence indicates that they are generalized carnivores. They feed on whatever gets stuck in their dewy glue. How large can their prey be? Darwin believed that they feed on fairly small animals. Strangely, there have been no measurements of their prey since 1925; and that was the first one since Darwin himself. So, if you happen to take photos of any sundew with its prey, you will add substantially to the sum total of human knowledge. I scanned about 20 plants quickly, and if they had captured prey, then they were too small for me to see with my unaided eye (the photo here shows a small insect stuck to the plant). Are all carnivorous plants related? Fossils and genetic data seem to say that carnivorous plants evolved six times independently from completely different origins; so the bladderworts and sundews that I saw were not related.

A question that Darwin never asked is why a plant would turn from photosynthesis to carnivory. The general observation that these plants grow in nutrient-poor soil was taken as enough of an answer. However, there are other entirely photosynthetic plants which grow all around the sundew, so this is not a complete answer. Studies show that carnivorous plants grow and spread better once they get enough prey. Notice the bits of green on the leaves of the D. burmanii? These contain the usual chlorophyll that allow plants to use sunlight to make sugar. The density of spiny hairs is much smaller on this portion than in the red part of the leaves. So they also do photosynthesis, but they are less efficient at it. The complete story of carnivory versus photosynthesis is not yet known.

In the same patch of ground where I took the other photos, I also saw several flowering specimens of Drosera indica (one example in the photo above). This was first named by Linnaeus in 1753 based on a drawing of a plant collected in Sri Lanka. A field study in 2013 by Allen Lawrie found that there are actually 11 different species which were conflated into the single species D. indica. Are there several unrecognized species hiding behind this one label in India? I do not know the answer.

While we talk about names, you will notice that I have called these plants carnivorous instead of following Darwin and calling them insectivorous. This is because detailed counts of prey species indicate that sundews feed on anything the right size, without specializing in insects. I’ll not say much more about D. indica because I’ve already written about it in another post.

Are D. burmanii and D. indica in competition when they grow in the same patch of ground? Studies of prey captured by other carnivorous plants which grow together show that they capture the same species, and so may be considered as competing. I know of no studies of prey species among the sundews of the Sahyadris, but there is no reason to believe that they are exceptions. In that case the spreading stalks of the indica with their larger numbers of leaves possibly give it a photosynthetic edge. Maybe that is why they are more common. I wish I knew a professional ecologist who could answer these questions.

Carnivores in the wild

When I listen to fellow tourists during trips to jungles, it is clear that the stars are the carnivores: tigers, leopards, even wild dogs. In fact I often meet people who do not enjoy the strangeness of tropical jungles, and consider a trip wasted unless they see a tiger close up. So it is also during a visit to the kilometer-high plateau of Kaas: the carnivorous plants of this inselberg enjoy stardom, but are seldom seen or recognized. These stars are the sundew (featured photo) and the small purple flowers of the bladderwort (below right).

The bladderworts, such as the Utriculata purpurea whose purple flowers you see in the photo alongside, are the tiger of this jungle. These bladderworts are rootless plants that float on patches of water. I saw them in growing in close conjunction with other small herbs. Since I was a tourist and not a biologist, I did not try to separate them out to check whether they are free floating. The close association with other plants makes it likely that they float on a mat of vegetation. A recent study found large communities of living algae and rotifers in the bladders of some individuals of this species growing in Florida. So it is possible that Seeta’s tears, as this species is called locally, derives a benefit from supporting an ecosystem within itself, rather than eating everything. To push the analogy further, U. purpurea, may not be the Kumbhakarna it is often thought to be, waking from sleep during the monsoon to eat everything that lands in the bladder. By supporting communities of living creatures, it may be closer in function to Seeta’s mother: the earth itself.

At the moment I care more about Drosera than the origin of all the species in the world.

Charles Darwin (19860) in a letter to Charles Lyell

Carnivorous plants are usually found in wet and nutrient poor soils. Like many of these plants, the sundew is poor at competing with neighbouring plants for light. We found three of them, probably Drosera Indica, in patches of ground below the roots of Topli Karvi bushes. I often wondered about whether the sundew is beneficial to humans or not, and was pleased to see a positive answer. Apparently Drosera preferentially eat mosquitoes, midges, and biting flies, which would otherwise proliferate in these wetlands, and therefore pose public health hazards.

While watching tigers you may also watch birds. While trying to take the featured photo of the sundew, I thought I saw a little tuft of grass growing next to it. When I looked at it again I thought that maybe it was not grass but a lily. I took a photo, and later saw that I was mistaken again. The tiny flowers at the tips of some of the stalks were clearly protected by a single modified leaf. This is called a spathe. Its presence means that the plant belongs to the family Araceae and so is cousin to Irises and Crocuses. I wish I’d looked more carefully at it in the field and tried to take a closer photo of the unfurled spathe and the inflorescence which it protects. Not having made this observation, I have no identification. Please help, if you can.

Just in case you like to read about plants, I will leave you to settle down to read this interesting scientific review of the state of our knowledge of carnivorous plants.