<P> The leaf blade is divided into two regions: a flat, heart - shaped photosynthesis - capable petiole, and a pair of terminal lobes hinged at the midrib, forming the trap which is the true leaf . The upper surface of these lobes contains red anthocyanin pigments and its edges secrete mucilage . The lobes exhibit rapid plant movements, snapping shut when stimulated by prey . The trapping mechanism is tripped when prey contacts one of the three hair - like trichomes that are found on the upper surface of each of the lobes . The mechanism is so highly specialized that it can distinguish between living prey and non-prey stimuli, such as falling raindrops; two trigger hairs must be touched in succession within 20 seconds of each other or one hair touched twice in rapid succession, whereupon the lobes of the trap will snap shut, typically in about one - tenth of a second . The edges of the lobes are fringed by stiff hair - like protrusions or cilia, which mesh together and prevent large prey from escaping . These protrusions, and the trigger hairs (also known as sensitive hairs) are likely homologous with the tentacles found in this plant's close relatives, the sundews . Scientists have concluded that the snap trap evolved from a fly - paper trap similar to that of Drosera . </P> <P> The holes in the meshwork allow small prey to escape, presumably because the benefit that would be obtained from them would be less than the cost of digesting them . If the prey is too small and escapes, the trap will usually reopen within 12 hours . If the prey moves around in the trap, it tightens and digestion begins more quickly . </P> <P> Speed of closing can vary depending on the amount of humidity, light, size of prey, and general growing conditions . The speed with which traps close can be used as an indicator of a plant's general health . Venus flytraps are not as humidity - dependent as are some other carnivorous plants, such as Nepenthes, Cephalotus, most Heliamphora, and some Drosera . </P> <P> The Venus flytrap exhibits variations in petiole shape and length and whether the leaf lies flat on the ground or extends up at an angle of about 40--60 degrees . The four major forms are:' typica', the most common, with broad decumbent petioles;' erecta', with leaves at a 45 - degree angle;' linearis', with narrow petioles and leaves at 45 degrees; and' filiformis', with extremely narrow or linear petioles . Except for' filiformis', all of these can be stages in leaf production of any plant depending on season (decumbent in summer versus short versus semi-erect in spring), length of photoperiod (long petioles in spring versus short in summer), and intensity of light (wide petioles in low light intensity versus narrow in brighter light). </P>

Five characteristics of living things displayed by the venus flytrap