Carnivorous plants derive some or most of their nutrients from trapping and consuming animals or protozoans, typically insects and other arthropods. They are adapted to grow in places where the soil is thin or poor in nutrients such as bogs and rock outcroppings.
In carnivorous plants, the leaf is not just used to photosynthesise, but also as a trap. Changing the leaf shape to make it a better trap generally makes it less efficient at photosynthesis. For example, pitcher plants have to be held upright so that only their opercula directly intercept light. The plant also has to expend extra energy on non photosynthetic structures like glands, hairs, glue and digestive enzymes. To produce such structures, the plant respires more of its biomass. Hence, a carnivorous plant will have both decreased photosynthesis and increased respiration, making the potential for growth small and the cost of carnivory high.
The archetypal carnivore, the Venus flytrap, grows in soils with almost immeasurable nitrate and calcium levels. Plants need nitrogen for protein synthesis, calcium for cell wall stiffening, phosphate for nucleic acid synthesis, and iron for chlorophyll synthesis. The soil is often waterlogged, which favours the production of toxic ions such as ammonia which can be used as a source of nitrogen by plants, but its high toxicity means that concentrations high enough to fertilise are also high enough to cause damage.
It has been suggested that all trap type carnivorous plants are modifications of a similar basic structure, the hairy leaf. Hairy leaves can catch and retain drops of rainwater, especially if shield shaped or peltate, thus promoting bacteria growth. Insects land on the leaf, become mired by the surface tension of the water, and suffocate. Bacteria jumpstart decay, releasing from the corpse nutrients that the plant can absorb through its leaves. This foliar feeding can be observed in most non carnivorous plants. Plants that were better at retaining insects or water therefore had a selective advantage.
Pitcher plants are carnivorous plants whose prey trapping mechanism features a deep cavity filled with liquid known as a pitfall trap. It has been widely assumed that the various sorts of pitfall trap evolved from rolled leaves, with selection pressure favouring more deeply cupped leaves over evolutionary time.
The traps of the bladderworts may have derived from pitcher plants that specialised in aquatic prey when flooded. In terrestrial pitchers, Escaping prey have to climb or fly out of a trap, and both of these can be prevented by wax, gravity and narrow tubes. However, a flooded trap can be swum out of, so in some plants a one way lid may have developed to form a door. Later, this may have become active by the evolution of a partial vacuum inside, tripped by prey brushing against trigger hairs.