"Harperella is an aquatic, primarily annual herb. The inflorescence is an umbel of small white flowers quite similar to other members of the carrot family. The most distinguishing character of the plant is its unusual leaf form. Leaves are reduced to a hollow, septate central stalk, or "rachis leaf"—an adaptation to a semi-aquatic habitat. Harperella blooms from May through October. Population sizes may vary dramatically from year to year in response to water levels. The Oklahoma population was found along a river in the southeastern part of the state and included two stands of approximately 500 plants." The link in the first line above will take you to the original article.