Observations on the vegetative development, flowering and seeding of cymodocea nodosa (Ucria) ascherson on the Mediterranean coasts of France

Abstract

Along the Mediterranean coasts of France, Cymodocea nodosa (Ucria) Ascherson occupies wide expanses between depths of 0.5 m and 18 m. The stem of C. nodosa shows dimorphism. This is manifest: (1) as vigorously vegetative plagiotropic rhizomes which produce several secondary ramifications per year, and in which the growth rate can reach 1.80 m year −1 in seagrass beds of low density; and (2) by orthotropic axes which ramify little, grow slowly (1–2 cm year −1), and produce flowers. The dimorphism may be switched by environmental factors. Furthermore, C. nodosa shows a marked seasonality in growth, with rapid growth during April-May, and very much slower growth from October-March. This seasonality is also shown by changes in the intensity of development of both ramifications and adventitious roots, as well as by gradual and regular variations in the length of internodes on the rhizome. These seasonal changes also apply to the number of leaves in leaf shoots, and to the length and width of these leaves. As a result, the leaf-area index varies from a maximum of 3.90 in spring to a minimum of 0.32 in early winter. Flowering is annual and may be abundant, particularly in dense beds (up to 226 flowers m −2). Flowers develop terminally on orthotropic axes, and growth of these axes continues by the development of the subjacent axillary bud. In the sediment of C. nodosa beds, seeds are found throughtout the year, but germination of seeds has been observed only occasionally on the coasts of France.