NO. 1877 GEOLOGIC WORK OF MANGROVES — VAUGHAN 463 



islands or keys on which mangroves are practically the only vegeta- 

 tion. It is estimated that perhaps between one-half and one-third of 

 the total key area is occupied by these plants. 



The mangroves, where they are fully developed, form dense mats 

 of vegetation, with interlocking branches above and interlocking 

 roots below. The roots constitute an interesting and geologically 

 important feature of the plant. Besides the single root or tuft of 

 rootlets given off from the basal end of the young plants, there are 

 other roots originating above ground, at higher levels from the plant 

 stem. These grow downward and imbed their lower ends in the soil, 

 thus adding to the support of the plants. The roots arising in the 

 manner just indicated multiply and form a root tangle above the 

 ground. The various stages of root development are illustrated by 

 text figures 79 and 80, and by plates xlvii, xeviii, figure 2, and xlix_, 

 figure I (the illustrations are cited in an order to indicate a develop- 

 mental series). The tangles of roots are geologically important in 

 catching and holding debris washed among them by currents and 

 waves. 



The three modes of occurrence of mangroves — along the river 

 banks, around the margins of keys having their land surface above 

 water level, and the purely mangrove keys — are illustrated by the 

 plates. Plates xlvi and xevii represent the banks of the Miami 

 River and illustrate the river mangroves. Plate xeviii illustrates the 

 shore of the western side of the cape east of Bay Biscayne ; plate 

 XLix, figure I, depicts Pigeon Key ;^ plate xlix, figure 2, the Mar- 

 quesas, and plate Lii, figure i, the southern end of Old Rhodes Key — 

 all keys margined by mangroves. Plate lii, figure 2, represents a 

 mangrove-covered key between Key Largo and Old Rhodes Key. 



It has already been stated that these plants may initiate the forma- 

 tion of islands or they may be active in increasing land areas. The 

 process may now be sketched as follows : 



The fruit of the mangrove is an elongate body, from six inches to 

 a foot long, about half an inch thick, with a pointed distal, and an 

 enlarged and heavy proximal end, the calyx still adhering to the latter. 

 These cigar-shaped bodies drop into the water and are carried hither 

 and thither by the waves and currents, to settle on any soft bottom 

 where the water at low tide does not exceed about one foot in depth. 

 They sprout and quickly take root. Text figures 79 and 80 represent 

 a series of young mangroves, ranging from pods plucked from the 

 trees to specimens with a considerable development of roots and 

 several young branches. 



' A small key north of the western end of Key Largo. 



