appearances attracted attraction carnivorous carnivorous-plants clumps clusters coloring colors colouring colours cover digest digested digesting display edge edges enzymes flora funnel herbaceous insect-trap insects nectar-like-secretions nutrient-source nutrients operculum perennials pitcher pitcher-plant plant prey proteases rim sarracenia species trap unusual variations variety various vegetation attract-prey carnivore devour distinctive flowers passive-trap pitcher-plants pitchers pitfall-traps plants rims slippery veins