pistia: pantropical floating plant forming a rosette of wedge-shaped leaves; a widespread weed in rivers and lakes
aquatic plant: a plant that grows partly or wholly in water whether rooted in the mud, as a lotus, or floating without anchorage, as the water hyacinth
zona pellucida: thick membrane around the mammalian ovum; can be penetrated by one sperm in the fertilization process; usually remains around the fertilized egg until it is implanted in the wall of the uterus