242 SEA AND LAND 



march in such a sturdy way against the sea that they can 

 resist waves which occasionally attain the height of four or five 

 feet. Much of the Everglade district has been won from the 

 condition of shallow water to the state of swamp by their action 

 during relatively modern times. Their rate of advance over 

 the shallows is much more rapid than that of the grasses 

 which form the northern marine marshes, and is limited only 

 by water which is too deep for their root-like runners to 

 descend through it and fix themselves upon the bottom. 

 Where the water which they seek to penetrate is tolerably 

 deep, their growing ends are apt to be swayed by the current 

 so that they can not readily attain a foothold. Moreover, they 

 are more or less eaten by certain fishes which have a fancy for 

 vegetable food. 



In regions which are sufficiently warm to permit the free 

 development of mangroves, they take possession of all the mud 

 flats which in more northern districts would be occupied by 

 grass marshes. The result is that margins of the harbors 

 have a totally different aspect from that which is presented by 

 the shores of the havens in higher latitudes. Their general 

 effect, however, is the same as that which is brought about by 

 the lower growing plants which dwell between high and low 

 water mark. They narrow the channels of the reentrants, and 

 by concentrating the tidal flow into a small space serve some- 

 what to increase the scouring action of the currents through 

 the acceleration of their velocity which is thus induced. 

 Owing, however, to the fact that the mangroves elevate the 

 marshes somewhat above the level of high tides, these trees 

 by debarring the tidal water from entrance to extensive dis- 

 tricts are on the whole more deterimental to havens than are 

 the grassy marsh growths. The last named type of vegetation 



