696 Part IV. Chapter 7. 
Rhizophora mangle bordered by Laguneularia racemosa and Avicennia nitida. The northern side 
of Man Key is occupied by a broad area of mangrove vegetation from which Laguncularia race- 
mosa is absent. Woman Key is characterized by extended areas of the mangrove associated with 
Laguncularia racemosa and Avicennia nitida while Ballast Key has only a small corner occupied 
by such plants. Boca Grande Key with the exception of the narrow area of sand is a mangrove 
island. All of the islets of the Marquesas group of keys with the exception of one small key are 
characterized by extensive growths of the mangrove, while the keys of the Tortugas group, as 
also Bird and Loggerhead keys are destitute of the mangrove. The small keys north and west 
On the seaward side, banks of shell are formed and the interior gradually becomes a typic hammock. 
Mud Flat and Strand Formation. Along the edges of the mangrove 
thickets are mud flats which are found on many of the islets which form 
the Florida keys. 
The vegetation on the mud flats of Key C. consists of Salicornia ambigua, Batis maritima, 
Monanthochlo& littoralis, Sesuvium portulacastrum. On Man Key, the tenants of such flats are Bor- 
richia arborescens, Pithecolobium guadalupense, Avicennia nitida, Alternanthera brasiliana, Batis 
maritima, Conocarpus erecta, Euphorbia (Poinsettia) havanensis, while on the mud flats of Woman 
arp 
Key are separated colonies of Suriana maritima, Sesuvium portulacastrum, Cyperus brunneus, Atri- 
plex cristata, etc. 
The islet of the Marquesas Group designated B. by LAnsınG has an association of mud flat 
plants, viz., Dondia linearis, Laguncularia racemosa, Avicennia nitida, Distichlis maritima. "The 
mud flat vegetation is not typically developed into a formation on any of the other islets of the 
Marquesas group, except oh Round Island where Laguncularia racemosa and Salicornia ambigua 
are characteristic while on Marquesas J. the mangrove border, while characterized by Avicennia 
nitida, here shows for the first time in the group a full substitution of Conocarpus erectus for 
Laguncularia racemosa. This formation is unknown on the islets of the Tortugas group. 
Historically the sea-strand formation is an old one floristically speaking. 
The east coast of Florida, as far south, as Biscayne Key (lat. 26° 75’ north), 
is lined by low sand dunes ranging from ten to thirty feet in height. Shoals 
are formed out at sea by the action of currents and tides; and when these 
emerge and become dry, such dune-building plants as Sesuvzum portulacastrum, 
Jva imbricata, Cakile maritima, Panicum amarım take possession of them and 
help to form a low line of dunes. Umzola paniculata is the main sand-binding 
grass to be found on the top and seaward side of the first line of dunes. 
Here it forms almost 75 per cent of the vegetation and may be properly 
termed the Uniola association. Species of Spartina, Panicum, Euphorbia 
and Opuntia are found mingled in this association together with /fomoea 
pes-caprae, Yucca, Croton maritimus, Serenoa serrulata (see plate VI, p. 306). 
At the base of the main line of dunes, according to WEBBER '), are a number 
of dune-builders: Panicum amarum, Ipomoea pes-caprae and littoralis, Iva 
imbricata, Cakile maritima etc. - 
ı) WEBBER, H. J.: Strand Flora of Florida (1898): see Bibliogr. p. 71. 
