A submersed Old World Plant(Hydrilla verticillata)having whorled,lance - shaped leaves and unisexual,solitary,axillary flowers.

• 软水草一种生在水中的东半球植物(黑藻黑藻属)，叶片呈螺纹形、矛状，雌雄异株，花为单生、腋生