Meristic and Organogenetic Variation in Ruppia occidentalis and R. maritima

Abstract

Floral meristic and organogenetic variation was sampled in Ruppia occidentalis from an alkaline lake of the Nebraska Sandhills and in Ruppia maritima var. rostrata from a saline, non-Sandhills lake nearby. The androecium is meristically stable, always having two stamens, but the gynoecium is not. Seventy-two percent of the flowers of R. maritima had four carpels and the others had three, and in 80% of inflorescences the two flowers had the same number. In about one-third of inflorescences having dissimilar carpel numbers, the four-carpellate flower was uppermost. The number of carpels in each flower of R. occidentalis ranged from four to nine, averaging six, and in 57% of inflorescences both flowers had the same number; of those that did not, most had more in the lower than upper flower. Twenty-five percent of the flowers had four carpels, 8% had five, 35% had six, 15% had seven, 14% had eight, and 3% had nine. Ordered, dimerous, decussate organogenesis through the first four carpels followed the same pattern in both species, but carpels beyond four were alternate with the first four and the decussate pattern was broken. The stomatiferous dorsal lobe of each carpel produced persistent gas bubbles that probably aid flotation of the inflorescence and might function in trapping pollen as well. Only ephydrophilous pollination was observed in both species.