The Pollination of Vallisneria Spiralis

Abstract

Vallisneria has long been counted one of the classic examples of cross-pollination. Living vegetatively as a submersed aquatic, its dioecious flowers are brought together at the surface of the water in most ingenious fashion. These highly specialized flowers present the strongest contrasts, not only in size and structure, but in behavior as well, and give this plant its rank as one of the climax types with respect to floral differentiation. Specializations of such evident advantage for cross-pollination in a form so admirably situated for vegetative propagation seem to emphasize the importance of sexuality, or at least of seed production, in the higher plants. While the general method of pollination in Vallisneria is well known, many interesting facts seem never to have been published, and the underlying principle has not been emphasized. The figures current in textbooks are highly generalized, and some of them are far from accurate. The story which they are intended to illustrate is likewise incomplete or in some cases highly distorted. In any event, neither figure nor story has done justice to the intimate history of pollen transfer in this remarkable plant. It will be noted at once that the following account diverges radically from that suggested by KERNER'S (i) beautiful and widely copied figure. A comparison shows that these differences relate not only to the size and structure of the flowers, but are even more fundamental in character. KERNER emphasizes the fact that pollination is brought about through the contact of flowers floating on a level water surface; there follows an outline of a method of pollen transfer through the special agency of the surface film of water. The general drawing (pl. IX) is based on photographs of living flowers, measurements, and camera drawings of parts.