the simplest pitcher plants are probably those of "heliamphora", the marsh pitcher plant. in this genus, the traps are clearly derived from a simple rolled leaf whose margins have sealed together. these plants live in areas of high rainfall in south america such as mount roraima and consequently have a problem ensuring their pitchers do not overflow. to counteract this problem, natural selection has favoured the evolution of an overflow similar to that of a bathroom sink—a small gap in the zipped-up leaf margins allows excess water to flow out of the pitcher.