COLONIZATION OF MUD
195
(Zostera marina and Z. nan a), a curious plant with ribbon-like leaves and creeping rhizomes. The latter form a dense plexus, holding the plant in place in the mobile ground. Propagation is carried out freely by seed in the localities we have examined, and this is doubtless supplemented by fragments of rhizome broken loose in heavy weather. Zostera, which occupies the lowest zone of all phanerogamic halophytes in Britain, is remarkable in being one of the very few plants whose flowers are adapted to cross-pollination by the agency of water.
   When the mud has been sufficiently consolidated, Zostera may be followed by one of the annual species of Salicornia, e.g. Salicornia annua. Henceforward the history of development follows substantially that described below for firm mud.
   Another plant which follows Zostera is Spartina Townsendii, and we should not be surprised to hear that it could colonize soft mud even in the absence of this plant. It is adapted to the habitat both by its creeping rhizomes and by its ample lacunae. The history of the spread of this remarkable grass in the Southampton region and its importance as a land builder have already been fully described (pp. 175-183). Another plant often colonizing soft mud is Scirpus maritimus (Plate XVIII, p. 180, marked Sc. in upper photo.). In some districts (e.g. Poole Harbour) it is cut for thatching.
   Firm mud, generally consolidated by the action of Algae, sooner or later will show a thin scattering of Salicornia (e.g. 5. annua). In successive years this covering, renewed annually from seed, becomes more and more compact, until a pure sward of Marsh Samphire, the first “succession” of the series, is established.
   If the area be at all extensive the marsh will be irrigated by creeks, the original lines of which are frequently laid down in advance of the appearance of the vegetation covering.
   Where flats undissected by creeks and previously bare produce rather suddenly stretches of Salicornia, the vegetated areas become appreciably convex by accretion within one or two years. The tidal waters running over such areas naturally follow the less densely vegetated and slightly lower regions between, which thus become the natural primitive creeks. In