THE PLANT WORLD 9 



becomes detached from the parent. The young plants thus formed 

 grow rai)idl Y and soon are able to continue the reproductive process. In 

 addition to this method of development the plant emits numerous aerial 

 roots from all parts of the main branches ; these roots elongate rapidly, 

 and attach themselves by rootlets as soon as the apex penetrates solid 

 ground, whether above or beneath the surface of the water. If the depth 

 of the latter exceeds a foot, the roots usually branch, and thus serve to 

 support the plant in its progressive march seaward. The seeds retain 

 their vitality after long immersion in salt water ; and the projecting 

 radicle very soon secures attachment upon a small portion of the reef 

 that has been exposed at low tide. Here, then, is the beginning of an 

 island ; as soon as the plant has gained sufficient size to produce aerial 

 roots, flowers and fruits, the formation of a thicket ensues with surpris- 

 ing rapidity, even though the inchoate island may yet be partly sub- 

 merged at high water. Many of the smaller keys are still in this initial 

 stage of formation, consisting of from one to several acres of mangrove 

 thickets, with no other vegetation and but little solid ground beneath. 



This intertwining mass of stems cannot fail to arrest and hold a 

 large amount of tidal drift, consisting of decaying algse, sponges and 

 hj'droids, fragments of the reef, shells, etc.; and this organic matter, as 

 it decomposes and becomes amalgamated, affords a rich culture medium 

 for the seeds of other plants which ultimately secure lodgment on the 

 islet. The black button wood {Laguncularia racemosa), and the white 

 button wood {Conocarpus erecfa), two shrubs belonging to the Combre- 

 taceae, and ha^'ing a habit similar to the mangrove, soon establish them- 

 selves, and by their annual contribution of dead leaves and twigs the 

 interior of the island is built up so that it gradually dries out. Under 

 these conditions another tj'pe of vegetation begins to thrive, and ulti- 

 mately the interior of the island is converted to " hammock " land. 

 The mangrove thicket, it must be remembered, is continually enlarging 

 around the shore Kne, as the roots push further into water, while it 

 simultaneously dies out in the interior. Thus the area of a small ke}"- 

 increases wonderfully within a few years, so that it is difficult to pre- 

 serve absolute accuracy in maps of this region. There are many islands 

 whose dimensions at the present time are almost sufficient to vrarrant 

 the establishment of jjlantations, that are entirely omitted from the 

 older charts, or are indicated merely as shoals. 



As might be expected, the soil constituents are similar on all the 

 keys, disintegrated coral rock forming the basis of all soils, the admixture 

 of humus varying in proportion to the extent of vegetation and the eleva- 

 tion above tide water. Although the depth of soil, even in the best 

 plantations, is inconsiderable, it reaches a maximum of fertility owing 

 to the large proportion of lime salts combined with the natural leaf 



