Chap. VI] EDAPHIC INFLUENCES IN THE TROPICS 



407 



Rhizophora Mangle (Figs. 229, 230), like R. mucronata in the eastern man- 

 grove, occupies the outer edge of the formation, whilst Laguncularia 

 racemosa appears particularly at the inner boundary and there frequently 

 forms pure mangrove. The Avicenniae assume an intermediate position. 

 Just as in the eastern mangrove, so in the western, on drier islets a few 

 additional species of plants occur which become dominant in formations 

 transitional to inland formations, and include tropical cosmopolitan species, 

 such as Hibiscus tiliaceus and Chrysodium aureum, but also some purely 



Fig. 22S. Mangrove in Samoa. Inner margin on a less saline and less wet soil. On both banks : 

 Chrysodium aureum. To the right: Hibiscus tiliaceus (?)-. In the background: Cocos nucifera. 

 From a photograph. 



western species, such as the combretaceous Conocarpus erectus. Epiphytes 

 are scarce in the western as in the eastern mangrove, and are usually con- 

 fined to a few Bromeliaceae and lichens. 



v. DISTRIBUTION OF LITTORAL FORMATIONS IN THE TROPICS. 



Open littoral formations occur throughout the tropics, and in districts 

 with a small rainfall they are almost the only ones. The close woodland 

 above high-tide mark and the mangrove growing within reach of the tide 

 are luxuriantly developed only in districts with abundant rain ; and as the 



