i96	PLANT WINNING OF TIDAL LANDS
such cases the lines of irrigation are determined more by the accidents of colonization than by any special pre-existing features in the relief.
   On salt marshes where Fucoids (e.g. Fucus limicola) play an important part, these plants are especially conspicuous at the lower levels, and their occurrence synchronizes with the pre-Salicornia phase and persists into the Salicornia phase itself (Plate XV, p. 172, below). As the level rises and other halophytes come in, the Fucoids tend to disappear, or at best to survive only locally. During the period of their dominance, however, they are of great importance as accretors of silt (cf. p. 172).
   In due course the Salicornia phase gives place to the next succession, and this is by no means the same in all cases. It is possible also that the Aster is one of the plants liable to become prominent in salt marshes undergoing down-grade changes or retrogression, and if this be the case a certain difficulty will be found in determining the status of an Aster marsh when its previous history is unknown. Although the detailed study of plant assemblages is relatively recent, more attention has been given to their progressive development (i.e. first establishment to climax) than to their degeneration. Sometimes the history of a vegetation comes to an end suddenly by erosion, as when a marsh is undercut by a meandering creek, whilst in other cases a vegetation may slowly go back in its footsteps, with variations. It is these latter retrogressive phases, whether in salt marsh or other vegetation types, that have been so little studied, and which are liable, in the absence of more precise data, to be confounded with true “successions”.
   In other cases, and this is the more typical history of muddy salt marshes, the Aster enters only in moderate degree, and is accompanied by a variety of halophytes, such as Plantago maritima, Spergularia media, Statice Limonium, Armeria, Tri-glochin, and Glyceria maritima. The last-named grass where present tends to form a plexus everywhere, and here, as in more sandy marshes, is a most important component. Under the sway of these perennial plants the marsh grows up into typical high marsh or “mixed salting”, the Salicornia being