botanic garden botanics carnivorous catch danger devour digests eating flies flower flowers fly Flytrap glasgow green grip insect insect eating nature nutrition pitcher plant plant plants purple red sarracenia purpurea savage trap appearances attracted attraction 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 prey proteases rim sarracenia species unusual variations variety various vegetation