io6 PLANT GEOGRAPHY 



canopy overhead. Unbranched Tree-ferns and Monoco- 

 tyledons, such as Palms, Bamboos, and Plantains, are 

 abundant; the stems of others often have prominent 

 vertical buttress-roots, as in the Silk-cotton trees (Erio- 

 dendron); and the whole jungle is interlaced with a 

 tangle of woody climbers or lianes, such as Ficus, 

 Bauhinia, Bignonia, Paullinia, and Lygodium. Epiphy- 

 tic Aroids and Orchids cover the boughs, and parasites are 

 also numerous. The leaves are often leathery and 

 glossy: they often taper into " drip-tips," and "in the 

 shade are often red, especially on their under surfaces. 

 Flowers and fruit are often borne on the main stem, 

 from dormant buds. The ground is often carpeted with 

 ferns, especially Hymenophyllum, or Selaginella ; but 

 there is a general absence of greensward. 



The following Orders are mainly restricted to this 

 zone, which is estimated to comprise two-fifths of the 

 land of the globe and to have also two-fifths of the 

 Phanerogamia : Anonaceae, Menispermaceae, Bixaceae, 

 Guttiferae, Bombaceae, Malpighiaceae, Simarubeae, Bur- 

 seraceae, Meliaceae, Caesalpineae, Passifloraceae, Ebenaceae, 

 Gesneraceae, Artocarpeae, Pandanaceae, Zingiberaceae, 

 and Marantaceae. Dipterocarpaceae and Nepenthaceae 

 are confined to the Indian Monsoon Region, Pandanaceae 

 to the Old World, and Cinchoneae, Lecythideae, Marc- 

 graviaceae, and Cyclanthaceae to South America. 



MANGROVE-SWAMPS. A striking characteristic of the 

 humid portions of the Tropical Zone is the occurrence of 

 Mangrove-swamps along its coasts. A few species of 

 trees, belonging chiefly to Rhizophora, Avicennia, and 

 Sonneratia, grow on saline tidal mud-flats, supported on 

 spreading " flying-buttress " roots from their main stems, 

 and " pillar " roots descending vertically from their 

 branches. They have also ascending aerating roots, 

 like the " knees " of the Deciduous Cypress of Louisiana. 

 Their seeds are often " precocious " or " viviparous," 

 germinating while still in the undetached fruit and at 

 once forming a long radicle by which, on falling, they 

 are anchored in the mud. Growing in salt water, Man- 

 groves are distinctly xerophytic, their leaves being 

 leathery, with a thick cuticle. On sandy shores, both 

 in the Old and New World tropics, the Mangrove-swamp 

 is commonly replaced by the wide-spreading runners of 

 Ipomcea Pes-capvce Roth. Further inland, where the 



