PHYSIOLOGY OF THE RED MANGROVE. 653 



by their structural adaptations and, particularly, vivipary, consti- 

 tute "the mangrove" in the sense of the French and German 

 botanists, only comprise in the American mangrove thickets of 

 Florida, four species. These are the red mangrove, Rhisophora 

 mangle L., the black mangrove, Avicennia nitida Jacq., the white 

 buttonwood, Conocarpiis erectus Jacq., and Laguncularia racemosa 

 Gartn. This last species is not contained in Harshberger's lists and 

 perhaps was not seen by him, although it occurs quite abundantly 

 in the keys, particularly the more southern ones. Dietrich Brandis, 

 writing on its range in Engler and Prantl, says, however, that on 

 approaching its northern limit it becomes merely a low shrub, and 

 hence easily overlooked. At Ragged Keys, for instance, trees were 

 observed three to four meters tall growing on the outer edge of 

 the fringe vegetation associated with Rhisophora. Laguncularia 

 grows in fairly deep water along shore with the red mangrove, 

 while Conocarpiis and Avicennia are, for the most part, in shore 

 on ground that is only submerged at high tide, or not reached 

 by the daily tides at all. On approaching a mangrove island or 

 shore this feature is easily seen, the rich olive or bright green of 

 the two species growing in deeper water is noticed as a dense wall 

 about two meters tall with a line of brown along the water's surface 

 which is composed of the tangle of aerating prop roots of Rhiso- 

 phora, and the small knotty pneumatophores of Laguncularia. In 

 the background, stretching above these two outer species, appears 

 the silvery white and light green of the Avicennia and the Cono- 

 carpus. At some places, however, Avicennia grows out in fairly 

 deep water and produces its large area of apogeotropic slender 

 yellowish-brown aerating roots also. 



Physiological Considerations of the Ecology. 



The adaptations of mangroves to their environment have been 

 grouped by Warming (/. c, p. 236)'^° under several heads as fixa- 

 tion, respiratory roots, germination and vivipary, means of migra- 

 tion, and xerophytic structures. This last heading is best illus- 

 trated in the leaves, as being perhaps the most plastic organ and 

 hence most easily adaptable. The structure of the leaves of Rhiso- 



