SOUTHEASTERN CLIMAXES. 237 



record the flora in such a way as to determine later invaders, and to study 

 the spread of species when once established. The conclusion is reached that 

 the order of precedence in the invasion of the wave-formed sand keys of Florida 

 has been substantially as follows: 



(1) Sesuvium portulacastrum; (2) Cakile fusiformis; (3) Euphorbia buxifolia; 

 (4) Cenchrus tribuloides, Cyperus brunneus; (5) Uniola paniculata; (6) Andro- 

 pogon glomeratus; (7) Suriana maritima, Tournefortia gnaphalodes; (8) Bor- 

 richia arborescens, Iva imbricata; (9) Ambrosia hispida. 



On the mangrove-formed strands, the order of invasion from the mangrove 

 nucleus toward the strand appears to be: 



(1) Rhizophora mangle; (2) Avicennia nitida, Laguncularia racemosa; (3) 

 Conocarpus ereda, Batis maritima; (4) Salicornia ambigiia, Dondia linearis. 



E. Bessey (1911 : 268) has studied the origin of the island-like hammocks of 

 broad-leaved trees found in the pine woods of Florida: 



It is thought that these are due to the biotic reaction of broad-leaved shrubs 

 or trees which find favorable places for reproduction here and there in the pine 

 woods. The increasing shade augments the humidity as well as the water- 

 content. Such areas in consequence become favorable for the reproduction 

 of other trees. More and more trees and shrubs appear, and the space beneath 

 grows up to underbrush. The humidity and water-content are further in- 

 creased and maintained in such a way that the hammock becomes larger and 

 larger and more permanent. This process continues until the drier conditions 

 outside check the growth of the marginal individuals. It is interesting to 

 note also that the hammocks are so dense that the temperature never drops 

 to freezing within them, as it frequently does in the pine forest outside. 



Harper (1911 : 515) has explained the broad-leaved forest vegetation of 

 islands and peninsulas in Florida as due to fire: 



Such areas in the midst of the pine woodland are known as hammocks, and 

 it is assumed that they represent the normal or climax vegetation of the region 

 the pine woodland being a secondary development due to fire. Pinus palus- 

 tris, the long-leaved pine, is very little affected by fire after it is a few years 

 old. In consequence, in southern forests periodically swept by Stie, it is 

 practically the only tree that can maintain itseK. In the case of islands, fires 

 woidd be relatively infrequent and the original vegetation would persist. 

 Much the same conditions prevail on peninsulas where the approach could 

 usually be made from only one direction. In the case of islands or peninsulas 

 with a pine barren covering, the immunity from fire would allow the humus 

 to acciunulate to such degree that the seeds of hardwoods brought in by 

 birds or squirrels would secure a start. As they grew they would make suffi- 

 cient shade to prevent reproduction of pines, while at the same time improving 

 the conditions for then- own reproduction. In the course of time, the pine 

 vegetation would yield entirely before the hardwoods. A reciprocal relation 

 is then established between the hardwood or climax forest. The latter con- 

 tains very Uttle herbaceous vegetation and the humus is too damp or too 

 decomposed to burn readily. The interesting assumption is made that fire- 

 produced pine barrens are often prehistoric in origin, and that the fires that 

 originally caused them were due to lightning. 



Harper (19112, 191 1^ 1914) has also discussed certain other vegetational 

 problems of the same general region, and has indicated the successional 

 relations. 



