MARINE MARSHES 489 



monly push their growth from the shore outward. Their roots 

 and stems, partly submerged at each tide, readily entangle sediment 

 and so the level of the deposits is rapidly raised to where it will 

 be covered only at the highest tide. At this stage portions of the 

 marsh are sometimes inhabited by enormous numbers of the fiddler 

 crab (Gelasimus). At this point the process is generally arrested 

 except for the decay of the older vegetation and the growth of 

 new crops. The appearance of the marsh and its structure are 

 shown in the following map and section, copied from Shaler (Figs. 

 no, iioa.) In cases where much sand is carried into the lagoon by 

 the tidal currents, the eel-grass-mud-flat stage may be entirely 

 omitted, the marsh-grass stage following directly upon the sandy 

 filling of the lagoon. 



The salt marsh vegetation consists chiefly of grasses and sedges. 



A. Bed rock. 6. .SantI and gravel. C. Eel grass layer. D. Upper marsb. 



Fig. no. Ideal section of a salt marsh formed behind barrier beaches. 

 (After Shaler.) 



Among the former the genus Spartina leads with seven species, 

 not all of which are, however, of marine habitat. Some species 

 are of world-wide distribution, while others occupy a restricted 

 area. In vertical range the species also vary. In the lowest zone 

 occupied by those plants, i. e., the zone limited downward very 

 nearly by half tide and upward by ordinary high tide, the tall, 

 coarse, rank-smelling salt thatch, Spartina glabra Muhl., var. altcrni- 

 flora Loisel and Merr, is the sole representative of these grasses, 

 except for occasional stragglers from the zone above. This plant 

 has large, hollow, jointed stems, broad, yellowish-green leaves, and 

 grows from two to four or even six feet high. It produces numer- 

 ous seeds, and sends strong, thick and very characteristic under- 

 ground stems in all directions into the substratum on which it 

 grows. From these, at nodal intervals, spring the long, fibrous, 

 much l)ranched roots. The underground stems, and those parts of 

 the aerial branches buried in the mud are the most frequently pre- 

 served, and these are readily distinguished from other plants by 

 their size and straw color, which is retained for a long time. The 

 aerial parts of the plant, /. c, the leaves and stems, become very 

 brittle at the end of the growing season and are broken off by 



