Dominant in the terrestrial realm, flowering plants are familiar to everyone. In the marine environment, however, they are definitely in the minority. Only a few truly marine species—the seagrasses—live in the Pacific Northwest. Members of this group all have flowers (see photograph of Phyllospadix sp., SW4), leaves, stems and roots. Their roots derive nutrients from surrounding soft sediments such as sand and mud.
Seagrasses are flowering plants that have migrated from the land back into the ocean. In this sense, they can be considered the whales of the flowering plant world! Actually they are not true grasses and consequently are not placed in that family, the Poaceae. Rather, they are placed in two families: the Potamogetonaceae (34 species) and the Hydrocharitaceae (14 species), for a total of 48 species of grass-like marine flowering plants worldwide. Seagrasses are especially diverse in tropical to warm temperate seas.
Ecologically, seagrass meadows play several important roles. Their root systems stabilize bottom sediments, and their leaves slow water currents and promote sedimentation. Seagrasses are also important primary producers, and they provide crucial habitat for fish, invertebrates and marine birds.
Other flowering plants encroach into the marine environment, primarily as salt-marsh inhabitants. This topic is outside the mandate of this book, but two species are included primarily because they also dwell in coastal salt marshes, along rocky shores.