408 



ZONES AND REGIONS 



[Pt. Ill, Sect. I 



atmospheric precipitations decrease they become lower in stature, less close, 

 and poorer in species. Besides this, littoral woodland is exclusively tropical, 

 or is represented in temperate zones only by a few tropical emigrants of 

 stunted form, whereas the open formations agree oecologically with those 

 of higher latitudes. 



Up to the present time the distribution of mangrove alone has been 

 studied in detail. Within the tropics its distribution nearly agrees with 



that of rain-forest. 

 The mangrove is 

 absent or poorly 

 developed on 

 coasts the inland 

 vegetation of 

 which possesses a 

 xerophilous char- 

 acter, except where, 

 as at the mouth of 

 the Indus and other 

 large rivers, there 

 is a considerable 

 addition of fresh 

 water to the sea- 

 water. This cor- 

 relation, in spite of 

 the continuously 

 very wet condition 

 of the substratum, 

 is quite compre- 

 hensible when the 

 distinction is re- 

 called between 

 physical and 

 physiological dry- 

 ness. Sea- water is 

 physiologically 

 dry 1 , so that plants 



that meet their demands for water from it are exposed to the danger of 

 excessive transpiration, and consequently climatic factors counteracting 

 transpiration must necessarily favour the development and spread of 

 mangroves. The air in districts near the sea, even with scanty rainfall, 

 is probably as humid as in those with abundant rain ; on the other 

 hand, cloudiness, which reduces the heating of the foliage due to insola- 



1 See p. 4. 



Fig. 229. Mangrove in Florida. View from exterior. Rhizophora 

 Mangle. From 'Garden and Forest.' 



