37^ George E. Nichols, 



dominated by the rank-smelling salt thatch {Spartina glabra 

 alterniflora) . In typical cases this grass forms a fringe along 

 the outer edge of the marsh. It predominates from about a 

 foot above mean low tide level upward to within a few inches of 

 mean high water mark, its actual vertical range being scarcely 

 two feet. Except for Vaucheria and certain other filamentous 

 algae, which commonly thrive on the muddy substratum, the salt 

 thatch is ordinarily the only plant present in this outermost 

 zone {Spartina glabra association-type). 



The salt meadow association-types. — By the time the surface 

 of a marsh has been built up to such a height that it is submerged 

 for only a few hours daily, the pioneer association-type has 

 given way to salt meadow : the rank, but open growth of tall, 

 coarse salt-thatch has become superseded by a sward of lower, 

 finer grasses, predominantly the salt meadow grass {Spartina 

 patens). Along with the salt meadow grass in this association 

 {Spartina patens association-type) commonly grow in greater 

 or less abundance : Distichlis spicata, Triglochin maritima, 

 Plantago decipiens, and Linioniiim caroliniamtm. 



At this point there is one feature which is almost universally 

 associated with salt marsh building and which demands a few 

 words of comment. During the elevation of the substratum 

 there may arise in various ways, which will not be discussed in 

 detail here (but see in this connection. Yapp and Johns '17; 

 Johnson and York '15, pp. 22, 25, etc.; Harshberger '16), 

 sloughs and depressions which become generally distributed 

 throughout the higher parts of the marsh. Here the depressions 

 may be deep or shallow ; they may be filled much of the time 

 with more or less stagnant water or may be merely muddy. In 

 the majority of these so-called "pans" the difference in level 

 between their bottoms and the higher surfaces of the surround- 

 ing meadow is but a matter of inches or even fractions of an 

 inch, yet they present an environment for plants which is quite 

 different from that afforded by the higher, better drained areas. 

 The pans may be quite barren of vegetation or they may be well 

 populated, but their plant cover is usually in marked contrast 

 with that of the surrounding meadow. Especially characteristic 

 of such situations are Salicornia europaca and Spergularia 

 canadensis, which, one or both, may be the only forms present 

 or which may grow in association with such species as Scirpus 



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