GG8 BOWMAN— ECOLOGY AND 



the level of the land above that of the water. . . . 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 up to within a foot of the water level (at low tide) young 

 mangroves begin to catch and grow. . . . The plants become still 

 more numerous and ultimately form a mat of interlocking roots and 

 branches resulting in keys. . . . When the plants become thick they 

 catch and retain sediment 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 this cuts oft their roots from the necessary 

 supply of salt water causing their own death. The land surface 

 then acquires another vegetation, but the marginal fringe of man- 

 groves 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 constructive geologic agents of southern Florida." 

 The process, as thus described, of course, takes place according to 

 geological periods of time and the death of the Rhizophoras as indi- 

 cated above by Vaughan, due to the cutting off of the salt-water 

 supply does not take place quickly, for the trees may persist for 

 years in such a situation without the tides actually bathing the 

 roots, as the ecologic observations and the physiologic experiments, 

 set forth in preceding chapters, have demonstrated. 



Economic Aspects. 

 The uses to which the Rhizophora may be put are various, 

 though, on a whole, its importance has not been large in its appli- 

 cations to man's needs. The chief use has been as a source of tannic 

 acid in the past, although another sphere of usefulness has been 

 lately found for the trees which is perhaps destined to become of 

 great importance in tropical coastal engineering. By the natives it 

 was used for the tannin contained in the bark, as mentioned in the 

 ancient chronicles of Abou '1 Abbas en-Nebaty and Ray, etc. Some 

 travellers tell of its being used by natives as food, for the starch 

 in the hypocotyls, e. g., Ovieda, Ray, etc., while many have observed 



