172	PLANT WINNING OF TIDAL LANDS
removal of the mussels, or by their destruction in situ by the application of copper sulphate.
   The most important of the bedded-in Algas are Fucus vesiculosus, form limicola, Vaucheria Thuretii, and Microcoleus chthonoplastes, each of which deserves a word of description.
   Fucus vesiculosus limicola is really a form of the common Bladder-wrack seaweed (F. vesiculosus), which occurs on every rocky shore. It appears to originate from the parent type in a purely vegetative manner, and assumes its remarkable growth habit in response to the special marsh conditions. Fucus limicola occurs on the upper 3 feet of the low marsh, the
Fig. 4a.—Fucus vesiculosus, form limicola, bedded in mud and proliferating- to the surface. (After Dr. Sarah Baker.)
upper limit of its range being at or slightly above the high-water mark of the neap tides. It occurs in part on bare mud (as in Plate XV, lower photo.), and also mingled with the pioneer colonists of the high marsh. It is distinguished by the narrow segments of its frond, by its sterility, and especially by the liability of the segments to be spirally coiled. Lying prone on the surface it becomes embedded in the mud, through which, however, it always pushes up numerous branches (fig. 42). Within the limits of its zone the plant carries on in this vegetative fashion, the parts that become embedded undergoing decomposition and enriching the soil. As level rises and other plants come in it assumes the secondary role of undergrowth; eventually it is crowded out by taller plants. In the history of marsh development Fucus limicola plays an important part in catching and holding silt, and also in the enrichment of the soil.