American Wildcelery (Vallisneria americana):
Ecological Considerations for Restoration
Reproduction
Although V. americana is a plant capable of both asexual and sexual reproduction, it lives in a habitat where asexual reproduction is apparently favored (Titus and Stephens 1983).
Sexual
The pistillate flower is borne on a pedicel that increases length by cell elongation to carry the flower to the air-water interface for pollonation in late summer (Donnermeyer 1982). Male inflorescences are borne submerged in the axils of leaves. Each inflorescence consists of about 2,000 flowers, each less than 1 mm long (Wylie 1917). Upon maturity, the male flowers break from the pedicels and float to the surface (Kaul 1970). The male flowers are spread about by air and water currents on the surface where they may encounter a female flower. Male flowers may be captured inside the perianth of the female flower as it closes during temporary immersion by a wave (Sculthorpe 1967). After pollination the pedicel contracts into a spiral, thus retracting the flower underwater where fruit development takes place. In late summer or early fall, some of the fruit capsules rupture and release a gelatinous matrix containing seeds. This mass settles to the bottom in close proximity to the parent plant (Kaul 1978). Other fruits do not rupture until the plants have broken free of the substrate and floated away, thereby providing a dispersal mechanism. (J. E. Titus and C. E. Korschgen, unpublished data).
Choudhuri (1966) determined that 30-35°C (86-95°F) was the optimum germination temperature for V. spiralis seeds near Varanasi, India. He concluded that germination was favored by a shortened day but was not affected by light intensity. Muenscher (1936) however, found that seeds of V. americana exposed to bright light were slow to germinate, whereas those kept in diffuse light germinated more uniformly. Dried seeds gave almost no germination.
During the 1978 growing season in Chenango Lake, New York, only 24% of the V. americana rosettes sampled by Titus and Stephens (1983) flowered, yielding a population mean of less than 0.6% of dry weight allocated to sexual reproduction. Zamuda (1976) noted no germination from V. americana seeds in the Pamlico River estuary, North Carolina. However, at seven deep-water stations (4.2-6.2 m) in Douglas Lake, Michigan, Bromley (1967) found growing V. americana that had seed coats still attached to the rooting structures, thus indicating origin from seeds. C. E. Korschgen (unpublished data) has observed seedlings in Lake Onalaska of the Upper Mississippi River.
Asexual
Shoots of V. americana emerge in late spring from over wintering buds (morphologically called turions). Winter buds become dormant during winter and resume growth in spring when water temperatures reach 10-14°C (Zamuda 1976). Winter buds are buried in the sediment about 8-10 cm in Lake Mendota, Wisconsin (Titus 1977) and 5-27 cm (mean 15 cm) in the Potomac River (Carter et al. 1985) but depth is governed by the type of substrate. In very organic sediments, the buds might be buried more than 15 cm (C. E. Korschgen, unpublished data). In spring, the second internode of the winter bud elongates to form a stolon that carries a compact rosette of ribbonlike leaves to the sediment-water interface (Wilder 1974). In a favorable experimental environment, one winter bud may produce as many as 20 rosettes during a growing season (C. E. Korschgen, unpublished data). The ribbonlike leaves usually reach the water surface (water depth, 95-120 cm) in early to midsummer at southern Wisconsin latitudes (Donnermeyer 1982). Near the close of the growing season in late summer, the production of rosettes ceases and some rosettes develop one or more winter buds on stolons (Fig. 3) that grow down into the sediment (Titus and Stephens 1983). After winter bud formation, the remaining stem tissue degenerates, breaks free of the substrate, and floats until it decomposes (Titus and Adams 1979a).
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