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If there was anything that enchanted Charles Darwin as much as ‘The origin of species’ it would have to be, as he himself called it - the ‘meat eating’ carnivorous plants. He seemed to be specifically fascinated by the common sundew plant (Drosera rotundifolia), conducting several experiments on them. He even published his results in a book titled - Insectivorous plants (Darwin, 2011), which contains some of the most detailed hand-drawn illustrations of sundew and other related plants. So what was it that drew Darwin specifically to these plants? - While most plants were perfectly content with using sunlight for their food requirements, these plants additionally consumed moving, crawling, living beings!
Carnivorous plants (Król et al., 2011), also called insectivorous plants, in addition to performing photosynthesis, trap and absorb nutrients from insects and other small organisms. What is noteworthy is that eating animals does not provide the plant with energy as its energy requirements are met by photosynthesis (just like other plants). Carnivorous plants are found growing in nutrient-poor habitats like flooded swampy areas, boggy areas (wet and muddy grounds) near streams, still waters, rocks etc. Thus carnivory provides the plant with an improved supply of nutrients like nitrogen and phosphorus, which is otherwise absent in the soil they grow in.
There are about 600 species of carnivorous plants and they all employ the same strategy:
They attract their prey using visually appealing leaves or flowers and sweet-smelling/tasting nectar. Once the prey has been lured, it’s time to trap it! Carnivorous plants have been observed to have 5 different types of traps - adhesive traps, snap traps, pitfall traps, suctions traps and tubular eel traps. The trapped animal is then digested by digestive enzymes which are produced by the plant. In some cases, however, the plant depends on other animals for performing digestion on its behalf. In such cases, the plant is not considered to be a true carnivore.
Here in this article, we will take a deeper dive into the 5 different trapping mechanisms employed by these deceptively genius plants.
Adhesive traps (Adlassnig et al., 2011), as the name suggests, use sticky hairs or tentacles to trap their prey. If we carefully examine the plant kingdom, we would find several plants (even the non-carnivorous variety) that have sticky plant parts that help capture prey. Not all of these plants necessarily uptake nutrients from the prey, some simply use it as a form of protection. An interesting example would be the sticky sage (Salvia glutinosa). This plant, which has some beautiful yellow flowers, is covered with sticky hairs all over the plant body. Small insects easily get trapped in these hairs, but since the hairs are believed to be mostly protective in function, the plant is not considered to be carnivorous.
However, if we were to consider the plants belonging to the Roridula genus, we would see a slightly different picture. This insect-trapping shrub (commonly called the fly bush) has leaves that contain sticky tentacles that capture insects, sometimes as big as wasps, and derive nutrition from them. However, they depend on other organisms for the digestion of their prey as they do not produce their own digestive enzymes. In fact, an insect called Pameridea roridulae is found exclusively on this plant, feeding off the other trapped insects, without itself getting trapped! The plant then feeds on the faeces of this insect to obtain nutrition. Quite a ‘shitty’ situation isn’t it? Now considering that this plant doesn’t produce its own digestive enzymes, it is not classified as strictly carnivorous. Instead, these types of plants are classified as protocarnivorous and are regarded as the ancestors of carnivorous plants.
Coming to strictly carnivorous plants, we would have to mention Darwin’s favourite - the sundew plant (Drosera species).
I care more for Drosera than the origin of species... it is a wonderful plant...— Charles Darwin
Plants belonging to this genus come in all sizes and shapes. They can vary from small ground forms (1-5 cm), to vines, to bushes, to plants with leaves up to 50 cm in size! These plants have leaves covered with tentacles, which are actually glands that secrete a glistening, glue-like substance that not only attracts the prey (usually small and sometimes large insects) but also traps it. Once trapped the tentacles tend to tighten around the prey, making escape more difficult. These glands also secrete digestive juices that then help the plant digest and absorb nutrients from the prey.
A smart plant you say? Not smarter than the parasitic ants and caterpillars that can escape the sticky, digestive glue of the plant and steal up to 70 percent of the captured prey! Funnily, every organism in nature seems to have the ability to outwit another.
The first plant that might come to our minds when we think of insectivorous plants is probably the elegant venus flytrap (Dionaea muscipula). This plant, along with a few other carnivorous species, contains specialised leaf traps (Gibson and Waller, 2009) which snap shut when triggered by their prey, enclosing the prey within the trap.
The leaf margin of the venus flytrap, which contains teeth like projections, secretes a substance that reflects UV light. This attracts crawling prey like ants, spiders, grasshoppers, beetles and some flying insects into the leaf.
Projecting from the inner surface of the leaf are thin trigger hairs. If an insect crawling on the leaf triggers 2 separate hairs within 1- 20 seconds, the trap snaps shut and it takes just 0.3 seconds for it to do so! Once the trap is shut, the marginal teeth interlock. Any struggle by the insect just tightens the trap further, making escape impossible. The plant can then take up to 7 days to digest the insect. Once digested, the trap reopens, waiting for its next victim to be lured into its little evil game.
So if you were an insect, and you see a shiny attractive leaf with teeth like protrusions, FLY AWAY!
Pitfall traps are the kind of traps we see in carnivorous pitcher plants (Bauer et al., 2015). These contain cone-shaped leaves, called the pitcher, filled with digestive fluid called the pitcher fluid, used to trap and digest animals. There are over 100 species of pitcher plants with traps that are remarkably similar to each other. However, the size of the traps can greatly vary, with the capacity to hold a few microliters of pitcher fluid to over 1 litre. Hence, the kind of animals they feast on can also greatly vary, from small insects to vertebrates like salamanders (and even bats and rats).
A pitcher plant doesn’t really need to do much to capture its prey. It just uses gravity to allow animals to stumble into its trap. A typical pitcher has the following regions:
However, interestingly, there are many organisms (Adlassnig et al., 2010) that not only survive the trap but also thrive in the pitcher fluid, using it as a nutrient-rich soup. A good example would be Nepenthes ampullaria, where over 100 tadpoles have been observed to develop simultaneously! These animals can either help the plant in its digestion or can simply steal away its food.
Suction traps (Westermeier et al., 2017), as the name suggests, use ultra-fast suction to capture their prey and are displayed by the more recently evolved group of carnivorous plants. We see this type of trap in the genus - Utricularia (also called bladderworts). The genus contains several species, of which some are terrestrial and some are free-floating aquatic forms.
The trap consists of a small, hollow, bladder-like bag, 1-5mm in size. Suction is created in the bladder by pumping water from inside the bladder to the outside. This creates a hydrostatic pressure in the trap, which deforms the walls of the trap, which now store elastic energy. The trap door, which is motile, seals the trap making it watertight. The trap is now said to be in the ‘catch’ state. When a prey (usually small terrestrial and water insects, and related organisms) touches the trigger hairs present close to the trap door, the trap door opens, the trap walls relax, allowing water and the prey to rush into the trap. The door now closes shut once again. All this occurs in a few milliseconds making it the fastest motile trapping system (Poppinga et al., 2017) in the plant kingdom. What next? The prey is of course cooked and consumed, thanks to the digestive juices secreted by the trap. Indeed, a hopeless situation to be stuck in.
Eel traps (Carmesin et al., 2021) resemble the traps used to catch eel (hence the name) and are probably the least understood type of carnivorous trap. This type of trap is seen in the species belonging to the Genlisea genus. The traps are actually modified leaves, which function as roots growing beneath the ground. So this is the only type of trap that captures small soil insects present underground.
The trap consists of two helically wound trap arms with small openings, which mimic the interspaces between soil particles. Insects lurking in the soil tend to casually walk into the openings, not realising that there is no way back! The prey is now retained in the trap by hair that points upwards, allowing only unidirectional movement. It is then forced to enter a Y shaped junction which acts as the mouth of the trap. Next, it enters the neck of the trap, which produces mucus allowing smooth movement of the prey. Finally, the prey enters the digestion chamber, called the vesicle, which secretes digestive enzymes and absorbs nutrients from the prey. A casual stroll for the insect turns into a deadly nightmare!
Carnivorous plants are perhaps one of the finest examples of how evolution has shaped the landscape of life on our planet. Organisms that evolve unique, creative ways to survive their harsh habitats manage to survive, and for a plant, capturing insects is no doubt innovation at its best! We humans, on the other hand, are certainly not going to stop with simply marvelling at the genius of these plants. Biological principles are now routinely being used in engineering technology. We may not be far away from seeing engineered analogues of insectivorous plants in fields for controlling pests and reducing the need for pesticides. Indeed, we humans are no less innovative!