492 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



TREE THAT MIGRATED INTO THE SEA 



Botanists are in disagreement over what constitutes a mangrove, 

 since the name "mangrove" itself relates more to habit than form. 

 Generally speaking, mangroves are plants that can live in loose, satu- 

 rated, salty soils ; they have respiratory roots and produce seeds that 

 are more or less viviparous — that is, the seeds germinate while still at- 

 tached to the parent tree. Some scholars suggest that mangroves are 

 primitive plants that evolved in shallow seas and never managed to 

 move inland. But as mangroves belong to the highest order of plants, 

 the fruit-bearers, the prevailing opinion is that these unique trees 

 developed on land and later began their return to the sea. 



In one sense they have completed their return to the sea and might 

 be considered marine plants ; for the red mangrove is capable of living, 

 growing, and reproducing without the aid of either "land" or rainfall ; 

 it can exist hundreds of miles from the nearest true shoreline (but 

 must, of course, be in quite shallow water for its roots to find a 

 foothold). 



But in another sense mangroves cannot be considered strictly marine 

 plants, for they grow best in brackish water — near the mouths of 

 rivers, for example — and can sometimes be found in virtually fresh 

 water. Laboratory tests, in fact, show that some mangroves can live 

 and grow for years in water that has no detectable trace of salt. 



The way in which the red mangrove, Rhizophora mangle^ propa- 

 gates itself is quite remarkable. When each fruit matures, the embryo 

 of a new plant begins to grow inside it. A slender, spikelike root 

 emerges and grows to a length of 6 to 12 inches while still hanging 

 from the tree. Wlien it falls, the seedling, which resembles a green 

 wooden dart whose lower end has been dipped into copper paint, may 

 stick into mud and send out rootlets and soon become anchored to the 

 spot. But most seedlings will float away with the tide before second- 

 ary roots develop. 



The seedlings may drift for hundreds — or even thousands — of miles, 

 remaining alive for a year or longer and sometimes even producing 

 secondary roots and top growth while afloat. At first they float in a 

 horizontal position, but as they grow older they swing more and more 

 to the vertical. After a month most are floating vertically, the root 

 end downward, ready to take root if it comes into contact with the 

 bottom. 



The saltier the water, the longer the seedlings will float. Even 

 those that eventually sink may occasionally survive, since tests show 

 they may live a year or longer while totally submerged. If they sink 

 in water no deeper than 2 or 3 feet, they may root in the bottom, send 

 a thin shoot to the surface, and then produce leaves. 



The mortality of seedlings is undoubtedly high. In one afternoon 

 of prowling through a Florida mangrove swamp at low tide, hundreds 



