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?
Mike Follows
Sutton Coldfield, West Midlands, UK
Carnivorous plants like the Venus flytrap (Dionaea muscipula) and tropical pitcher plants still get most of their energy from photosynthesis, just like other green plants. The insects they catch are mainly a source of nutrients rather than energy.
Although photosynthesis provides energy, plants also require essential nutrients – especially nitrogen and phosphorus – to build proteins, including enzymes, genetic material such as DNA, and new cells and tissues. These plants typically grow in habitats such as bogs, swamps and acidic wetlands where the soil is extremely poor in nutrients.
Their insect-catching adaptations help compensate for this deficiency. By trapping and digesting insects, the plants obtain nitrogen and other minerals that are scarce in the soil.
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By trapping and digesting insects, carnivorous plants obtain nitrogen and other minerals that are scarce in soil
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However, producing traps, digestive enzymes and highly specialised leaves requires substantial energy and resources. In nutrient-rich environments, ordinary leaves are generally more efficient for growth and survival. As a result, carnivorous plants are mostly confined to nutrient-poor habitats, where the benefit gained from capturing prey outweighs the cost of maintaining these specialised features.
Jonathan Wallace
Newcastle upon Tyne, UK
Photosynthesis is the process by which green plants use sunlight, water and carbon dioxide from the air to make carbohydrate. This provides the energy source needed to power the plant’s metabolism as well as a key building block for its tissues to grow.
However, it doesn’t supply all the necessary ingredients for the plant to survive and grow –also required are phosphorus, nitrogen and potassium, as well as a host of other elements in small quantities.
In most plants, these nutrients are obtained from the soil via the roots, but Venus flytraps, pitcher plants and other carnivorous plant species tend to grow in nutrient-poor wetland habitats where they aren’t present in sufficient concentration to meet the plant’s needs.
To compensate, the plants have evolved ways of catching and digesting insects to provide the necessary nutrients. Presumably, for plant species that usually grow in richer soils, there is little or no advantage to be gained from supplementing their nutrient input in this way.
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