Plant carnivory is a rarity, occurring in only about 550 out of approximately 250,000 plant species. The carnivorous habit is not obligate, and carnivorous plants can grow without an insect meal, depending instead on photosynthesis and minerals supplied from the soil. In general, carnivorous plants grow in sunny areas, and in mineral-deficient, sometimes sandy soils. Often these soils have standing or gently moving water, with any dissolved minerals from the soil being easily carried away by the flowing water. The carnivorous plant habitat is typically low in nitrogen and phosphorus and, some reports suggest, in potassium as well. In this sort of habitat, plants that have alternative strategies for obtaining essential minerals are at a competitive advantage. The capture of insects and other animals thus provides carnivorous plants with a supplemental source of essential nutrients.
The carnivorous habit depends on an ability to trap prey. In the vast majority of carnivorous plants, the trap represents a modification of the entire leaf or of structures borne on the leaf. Given this rather straightforward requirement of a trap, it should be easy enough to characterize a plant as carnivorous, or not. However, the picture is not so simple: many plants can trap insects yet are not considered to be carnivorous. What truly distinguishes a plant as carnivorous is not only a trapping ability, but also a mechanism to digest prey and to absorb the prey's nutrients.
Digestion implies an ability of the plants to break down the trapped prey into its component chemicals, to be able to absorb them as nutrients. It is the specifics of this digestion that have caused some controversy. Some workers consider a plant to be carnivorous only if it has an inherent ability to digest prey—that is, if the plant itself produces enzymes to break down the insect. Other plants, sometimes considered to be semicarnivorous, are able to trap prey but depend on the assistance of other organisms, usually, but not always, microbes, to digest the captured insects. However, for this short article, a plant is considered to be carnivorous if it traps and has a means, of its own making or not, for digesting prey.
The trap of a carnivorous plant is a true marvel, designed to attract, capture, digest, and then absorb nutrients from the prey. Traps can be grouped by whether they are "passive," with no or slowly moving "parts," often relying on gravity to aid in capture of the insect, or "active," exhibiting some sort of usually rapid movement. Perhaps the most familiar examples of passive traps are the sundews (Drosera) and the pitcher plants (Sarracenia and Darlingtonia in temperate climes, and Nepenthes in the tropics).
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