8 THE PLANT WOELD 



PLANT AGENCIES IN THE FORMATION OF THE 



FLORIDA KEYS. 



By Charles Louis Poli,ard, 



THE long chain of coral islands popularly known as the Florida 

 Keys extends in a southwesterly direction from a point near the 

 head of Biscayne Bay, in more or less close conformity with the 

 coast line as far as Cape Sable, and thence extending at a tangent to the 

 westward, terminating in the cluster of small keys known as the Dry 

 Tortugas. Most of the larger islands, such as Key West, Sugar Loaf, 

 and Largo, lie near the center of this chain, the outlying westerly groups 

 being small and scattered, while the eastern ones are less numerous and 

 form practically a single series. The name "Key West" is thus, 

 strictly speaking, inapplicable, but at the time of its discovery by Span- 

 ish wreckers and pirates the island was called " Cayo huesco," probably 

 on the assumption that it was the westermost extremity of the chain. 

 Key West was not occupied by American settlers until the early part of 

 the last century. 



The Keys represent elevated portions of one continuous coral reef, 

 about 200 miles in length, the south side of which is washed by the 

 waters of the Gulf Stream flowing through the Florida straits. This 

 reef contains numerous depressions affording navigable channels to sail- 

 ing vessels i^assing from island to island ; frequently, however, it pre- 

 sents large areas of fairly level surface covered at low tide with scarcely 

 more than a foot of water, and quite impassable even for a small boat. 

 It is therefore difficult to map out a course for a steamer of any size, 

 although between the southern line of the reef and the island chain there 

 is a clear channel, used by the steamer plying between Key West and 

 Miami. This channel, being landlocked by the islands on the north and 

 the reef on the south, is clear and quiet, presenting a startling contrast 

 with the turbulent waters of the Florida straits on the other side of the 

 reef. 



While the coral polyp is naturally the active agent in reef construc- 

 tion, yet the building up of habitable islands thereon is due almost en- 

 tirely to plant factors, the most important of which is the mangrove 

 (Bhizophora Mangle) and its allies. The actual sea area annually re- 

 claimed by the growth of this remarkable plant would no doubt be sur- 

 prising if it could be correctly computed. Some explanation of the 

 means whereby this is accomplished may not be inappropriate while 

 discussing the general topic of island formation. 



The seed of the mangrove germinates within the persistent pericarp 

 while the fruit is still attached to the tree, and the radicle often attains 

 a considerable length, occasionally even penetrating the soil before it 



