How does eel grass use surface tension?

By Chelsie Vandaveer

July 24, 2003

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Variously called eel grass, tape grass, or ribbon grass, the Vallisneria species Linnaeus are popular plants in aquaria. Members of the Hydrocharitaceae, the frog-bit family, there are eight or ten species of these submerged aquatics worldwide. Called 'Vals' by aquarium keepers, there are numerous hybrids and 'sports' (cultivars) available ranging from striped to twisted leaves.

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Vallisneria are dioecious; pistillate (female) and staminate (male) flowers are borne on separate plants. The tiny pistillate flower (~5 millimeters across) is borne at the end of a long peduncle that grows upward until it reaches the surface of the water. The flower consists of a long, cylindrical floral tube within a spathe. The tip of the floral tube opens--three thick, oblong sepals and three cleft stigmas. Two of the sepals sit on the surface of the water, the third held above the water.

Up to 200 staminate flowers are enclosed in a spathe on a submerged, short stalked inflorescence. When the minute staminate flowers (~1 millimeter) are mature, the spathe opens. The flowers break free and float to the surface. When they reach the surface, the three spoon-shaped sepals open, holding the stamens free of the water. (Aquatic and Wetland Plants of Southeastern United States, Robert J. Godfrey and Jean W. Wooten, The University of Georgia Press, 1979)

Because of surface tension, the pistillate flower forms a depression in the surface of the water. The male flowers often have one stamen that is sterile (no pollen) which acts like a sail; they ride like boats over the water. If they sail close to a female flower, the male flowers slide down into the depression where the fertile stamens come into contact with the cleft stigmas. (Biology of Plants, Peter H. Raven, Ray F. Evert, and Susan E. Eichhorn, 5th ed. Worth Publishers, 1992)

Once the female flower is pollinated, its peduncle twists. This spiraling pulls the flower back under the water where the 250 to 500 seeds can mature in relative safety.

The Center for Aquatic and Invasive Plants with the Institute of Food and Agricultural Sciences of the University of Florida has several great photographs taken by Vic Ramey of the American tape grass (Vallisneria americana Michaux). To view the photographs, click on the link:

http://aquat1.ifas.ufl.edu/vaampic.html

The photo on the right shows the twisted peduncles of pollinated flowers. Click on the thumbnails to enlarge the images. Note the last thumbnail is a herbarium illustration and shows a female flower in the upper left corner and two views of a male flower with two stamens on the right.

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