464 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 52 



The manner in which they do their work in extending areas of 

 land above water will be described first. Plate L, figure i, shows a 

 young mangrove growing in the water some feet away from shore, 

 at Northwest Point, Virginia Key, Bay Biscayne. In many localities 

 along the keys young mangroves may be seen with their terminal 

 leaves protruding above the water at distances of only a few feet up 

 to several hundred feet from the shore. An especially good example 

 of this may be seen along the southern shore of the Marquesas. 

 Plates XLVi, figure 2, and xlvii, figure i, representing the Miami 

 River, illustrate how the young mangroves extend into the water 

 area. When they have grown sufficiently for the development of a 

 tangle of roots, they catch and hold sediment and any floating debris, 

 by the successive accumulation of such material ultimately bringing 

 the level of the land above that of the water. 



The process by which they build new land is as follows: Behind 

 the keys, in the regions of slack water, deposition of sediment is 

 taking place, forming banks of soft calcareous ooze. After these 

 shoals have been built to within about a foot of the water-level (at 

 low tide), young mangroves begin to catch and grow. Plate h, 

 figure 2, represents a single young mangrove growing on a shoal 

 north of Pigeon Key in water about one foot deep. Plate l, figure 

 3, represents two young plants from the same locality, held up by the 

 boatman. Plate Li shows a further stage in the development of a 

 mangrove key, the young plants being more numerous and larger in 

 size. The plants become still more numerous, further increase in 

 size, and ultimately form a mat of interlocking roots and branches 

 resulting in keys such as those represented on plate Lii. When the 

 plants become thick they catch and retain sediment and ocean drift, 

 and are a constructive agent in the formation of land. 



After a time, whether it be a newly formed key or the margin 

 of a land area, the mangroves, by the accumulation of sediment and 

 drift, form land, and thus cut off their roots from the necessary sup- 

 ply of sea water, causing their own death. The land surface then 

 acquires another vegetation. But the marginal fringe of mangroves 

 persists to protect the young island from the erosive action of the 

 ocean waves, and young mangroves spread seaward to add new land 

 to that already formed. 



Thus these plants are among the most important constructional 

 geologic agents of southern Florida. 



