200 FLORIDA GEOLOGICAL SURVEY-I3TH ANNUAL REPORT

era and even some of the species composing it are very widely distributed.
 Marginal and shore vegetation (fig. 20). In shallow margins of lakes and along rivers where they are not subject to much fluctuation, as near their mouths, we commonly find a type of vegetation intermediate between the preceding and the saw-grass marshes (described a little farther on), and grading into both. It consists mostly of a few coarse monocotyledons with hollow or spongy stems or petioles, like maiden cane (Panicum hemnitonion), saw-grass, wampee (Pontederia), and Sagittaria lancifolia. Then above the usual water level on sandy and peaty shores of lakes we find a greater variety of herbs, mostly monocotyledons, often with a few scattered shrubs among them. A list of characteristic plants of such places was given in the 3rd Annual Report, page 267.
 Grassy dunes. On dunes where the sand is constantly moving, apparently not so much on the east coast as on the west coast, there is a sparse vegetation of coarse. grasses and other herbs, chiefly seaoats (Uniola panicidata) and other plants belonging to families well represented in tropical America. These renew their foliage every year, necessitating comparatively rapid growth and presumably indicating moderately fertile soil, though the bulk of vegetation per square yard or acre is not large on account of its very open structure. A little farther back from the shore, where the sand is not moving perceptibly, and much of the plant food has been leached down beyond the reach of roots, the vegetation is of a much slowergrowing type, described below under the head of shrubs.
 Salt marshes (fig. 3). These are characteristic of shallow bodies of salt water protected from wave action, where the vegetation builds up a foundation of muck just about to high tide level. The characteristic plants are coarse grasses and rushes, with a few-scattered bushes. In warmer climates the woody plants become larger and more numerous, until the marshes are replaced by mangrove swamps (described farther on).
 Saw-grass marshes (fig. 36). When a lake or a large embayment of one becomes filled with peat, especially if the water is a little calcareous, the vegetation is often composed almost wholly of saw-grass (Cladium effusum or Mariscus Jamaicensis), an evergreen sedge several feet tall. The same species also forms a fringe