PHYSIOLOGY OF THE RED MANGROVE. 667 



been observed by numerous other naturalists. This effect is mainly 

 the addition to and the extension of the land areas in the regions in 

 which mangroves grow. As remarked in the preceding chapter, 

 White Water Bay has been almost filled up by the activity of this 

 plant and many islands and keys have been elevated from merely 

 submerged coral shoals and reefs to a condition of dry and now even 

 habitable land. This growth of the land area may be very well 

 studied in Florida and the Keys and several geographers and geol- 

 ogists have commented upon the large role played by Rhisophora 

 in the geological history of Florida. This history is rather recent, 

 as geologists have discovered by borings and other investigations, 

 and Britton^*'^ says that all the flora of Florida and the Bahamas 

 has developed since Tertiary times. More recently Phillips^^- and 

 especially Vaughan^'^''' have added to the knowledge of the forma- 

 tion of land in Florida by their observations. Vaughan has spent 

 much time in the tropics and the particular region concerned in this 

 paper, and so his work may be regarded as of extraordinary value. 

 He takes the view that one half to one third of the total area of 

 the Florida Keys is occupied by the mangrove and in the work of 

 forming islands there are several stages which may be noted. From 

 a geologist's point of view the roots, of course, are the most im- 

 portant part of the tree to be considered, since it is in the tangle 

 of roots that the debris washed in by the currents is held. 



The three ecological formations are recognized, i. e., the banks 

 of the rivers, margins of keys, whose surfaces are already elevated 

 above sea-level, and the pure mangrove islets. In all of these, but 

 particularly the two latter ones, seedlings are noted at a distance of 

 a few feet to several hundred feet from the shore. This fact is of 

 important significance in the formation of land. This process, to 

 quote from Dr. Vaughan's paper at length, is as follows : " When 

 they (the trees) 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 



161 Britton, N. L., Science, XXL, April, 1905, p. 628. 



162 Phillips, O. P., " How Mangrove Trees Add New Land to Florida," 

 Jour, of Gcog., H., 1-4. 



163 Vaughan, T. W., " The Geologic Work of Mangroves in Southern 

 Florida," Smithsonian Misc. Coll., Vol. LH., Quart. Issue, Vol. V., p. 461, 1910. 



