TROPICS AND SUBTROPICS. 275 



Atriplex, etc. The forest and scrub vegetation is described at length, but 

 without the indication of formations or associations. 



Pearson (1899) has discussed the rdle of fire in the production of patanas, 

 grassy slopes, and plains in the mountain regions of Ceylon: 



The patanas are burned annually to provide fresh grazing for cattle. The 

 fires often burn the trees at the edge of the forest, and the general opinion of 

 foresters is that the patanas are spreading at the expense of woodlands. 

 The decay of the grasses produces a sour humus in which trees estabHsh them- 

 selves with difficulty. It is by no means hard to find seedUngs of forest trees 

 establishing themselves among the patana grasses, and experience shows that 

 where the patanas in the vicinity of the western forest are protected from grass- 

 fires, the forest slowly establishes itself upon the patana. The sharp boundary 

 once established by fire would so gradually become irregular by the advance 

 of forest growth that only careful observations over a long period would detect 

 a change. He further noted that while fire produces grassland from open 

 savannah forest, it produces scrub or "chena" from the low-country forest, 

 which possesses an undergrowth of shrubs. 



Ule (1900) has described rock moors, high moors, and meadow moors in 

 Brazil: 



The wet rock-fields in the neighborhood of timber-line are covered with 

 Sphagnum, accompanied by a number of dominant shrubs belonging to the 

 Myrtaceae, Melastomaceae, and Ericaceae. The high moors contain Poly- 

 trichum, as well as several species of Sphagnum. The chief flowering plants 

 are Baccharis, Senecio, Hippochoeris, Utricularia, Eriocaulon, Rhynchospora, 

 and Carex. The meadow moors are dominated by Cyperaceae, together with 

 Sphagnum, grasses, and a number of other phanerogams, of which Eryngium is 

 the most characteristic. 



Hope (1902) gives a comparative accoimt of the "sadd" of the Nile, and of 

 similar vegetational obstructions in other waters: 



The "sadd" consists chiefly of Cyperus papyrus and Vossia procera, with 

 Herminiera in some places, whUe Pistia stratiotes, Utricularia, Azolla, Ottelia, 

 and Aldrovandia are smaller constituents. These often form a large part of 

 the mass. The swamps of Lower Bengal are very similar, containing Vossia, 

 Trapa, and Pistia. The American " sadds" are of very different composition. 

 In the rivers of Guiana, the obstructions are caused chiefly by Montrichardia 

 arborescens and Panicum elephantipes; in Florida, Eichharnia spedosa is the 

 dominant species. These are developed in situ, while in the Nile the plants 

 are forced from the lagoons into the river by floods and mud. 



Ruber (1902) has foimd the pioneer vegetation on alluvial isles of the Amazon 

 to consist of Montrichardia arborescens or Drepanocarpus lunatus: 



These are followed by Rhizophora mangle racemosa, which in turn gives way 

 to other trees which can reproduce in its shade. The actual succession is also 

 indicated by the zonation of the isles, Montrichardia and Drepanocarpus 

 alternating in the outer zone, followed by a Rhizophora zone, or a zone of 

 Raphia, and one of Mauritia, and then by the forest nucleus. The floating 

 vegetation of the earlier stages consists principally of Eichhornia. 



Penzig (1902) has recorded the fiuther changes in the developii^ vegetation 

 of Krakatoa and the two neighboring islands observed in 1897. The number 

 of species had increased from 26 (Treub, 1888) to 62. On all three islands, 

 the Pes-caprae formation was dominant, mangrove vegetation was still lacking, 



