Ivan Kuzmin / Alamy
Why do Venus flytraps and pitcher plants trap and digest insects if they are fully capable of photosynthesis? If this is an adaptation to ensure there is always food, then why don’t other plants do the same? (continued)
David Muir
Edinburgh, UK
Plants require 17 essential nutrients. Through photosynthesis, carbon and oxygen are derived from carbon dioxide in the air, and hydrogen comes from water. The other 14 elements: nitrogen, phosphorus, potassium, calcium, magnesium, sulphur, iron, zinc, copper, boron, nickel, molybdenum, manganese and chlorine, would normally come from the soil for land plants. Aquatic plants get their nutrients directly from the water they live in, or from the substrate in which they are fixed. For these land and aquatic plants, the benefits of evolving insect-trapping mechanisms isn’t worth the cost.
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Evolution can bring about adaptations that are beneficial, but only if the advantages outweigh the disadvantages
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Carnivorous plants generally grow in boggy places, often sunny, where they can photosynthesise. But such environments are nutrient-poor. To catch animal sources of essential elements, such plants have evolved adaptations: snap traps (Venus flytrap), bladder traps that use a vacuum to suck in prey (bladderworts), flypaper traps that use a glue (sundew), lobster-pot traps that use angled hairs to direct prey one-way to their demise (corkscrew plants) and pitfall traps that send prey down into a swimming pool of digestive enzymes (pitcher plants). For these plants, the benefits of evolving insect-trapping features is well worth the cost.
Under Mother Nature’s cost-benefit analysis of natural selection, evolution can bring about adaptations that are beneficial, but only if the advantages outweigh the disadvantages.
Rob Leah
London, UK
Most carnivorous plants live in acidic, boggy soils, which are notoriously poor in various nutrients – particularly nitrogen needed to make proteins. They therefore evolved to trap and digest insects, to provide the nutrients that weren’t available from the soil.
A surprising number of plants are weakly carnivorous (such as tomatoes, which have stems covered in sticky hairs that can capture small insects and absorb the nutrients from their decomposition), but presumably, the evolutionary cost of being actively carnivorous only makes sense in very specific environments.
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