4(54 ECOLOGICAL STUDY OF SALTAEARSH VEGETATIOX. 



common vegetative response to bad drainage is shallow rooting; 

 the Grey Mangrove also reacts to imperfect oxidation by the 

 production of special breathing organs (1) . Massed assemblages, 

 with a rhizomatic connection and tolerance of the deposit of 

 considerable quantities of silt on their formations, are charac- 

 teristic adaptations of our marsh plants to resist tidal and stream 

 invasion . 



The Habitat. 



The local marshes are the result of hydrodynaniic action, 

 stream and tide each bearing its quota of soil particles to form 

 a mud bank at the head of the estuary. When the alluvium, 

 usually a clayey deposit, has attained an altitude upon whieb. 

 vegetation may exist, floating seeds of the Grey Mangrove settle 

 on the mud and the surface is eventually covered with a forest 

 of these trees. By obstructing the tide and stream flow they 

 compel each in turn to deposit a portion of its burden of silt 

 in their immediate vicinity, thereby accelerating the uplift of 

 the marsh. The landward margin of the 1 marsh is built up by 

 stream deposit and as the banks advance upon the plain, the 

 halophytic vegetation is driven seawards, until it reaches the 

 region where the tidal scour is sufficiently energetic to preclude 

 further encroachment. In many local marshes the littoral mar- 

 gin is moulded into a crescent by the sweep of the tide, and 

 frequently bisected by a channel flowing inland and connected 

 more or less directly with the stream formed by the drainage 

 collected in the adjacent country. 



The littoral soil consists of loose black mud, whose stabilisa- 

 tion is largely the work of Algas which form a filamentous net- 

 work on the surface, and eventually provide a modicum of humus 

 for the succeeding herbs. This muddy area is frequented by a 

 crab, of which Mr. C. Hedley (18, p. 46) writes: "Between 

 the falling and the rising tide, it burrows and builds with such 

 energy that the whole field is covered with little pits and heaps 

 of mud pellets like worm castings on a lawn." The analogy 

 may be advanced a step further, as the crabs perform a similar 

 beneficent service in the aeration of the marsh mud to that under- 

 taken by the earthworms in ordinary garden soil. The various 

 formations, by collecting soil and debris, finally raise the marsh 

 until it becomes unfitted for their further occupation and are 

 eventually destroyed by the conditions in whose production they 

 have played such a prominent part. 



