458 INTRODUCTION TO PLANT GHOGRAPHY 



forests inland. Exceptions may be atforded by the mouths of large 

 rivers, where the salt water is diluted with fresh, and conditions of 

 physiological drought are thereby relieved — provided cloudiness 

 reduces insolation and, consequently, transpiration. 



Although mangroves are usually serai within themselves, and in the 

 most favourable of tropical rain-forest areas appear to be succeeded 

 by freshwater swamp-forest {see pp. 461-3) or perhaps sometimes 

 by tropical rain forest, in other instances there is no certainty that 

 they can develop alone, by mere accumulation of silt or humus, 

 into normal land vegetation or even littoral forest. In such instances 

 the more advanced and stable ' inland ' types would seem best 

 considered as edaphic climaxes, that appear likely to persist in the 

 absence of disturbance. 



The other vegetation-types of tropical sea-shores are more or less 

 comparable with those of temperate regions. Thus the beach 

 between tide-marks is usually devoid of vegetation on sandy or 

 shingly shores and bears only Algae on rocky ones, while even above 

 high-water mark on exposed coasts the sandy tracts are often poorly 

 vegetated, as are the outermost dunes. However, these last tend 

 to be bound by Grasses such as Spinifex Iittoreus, whose rhizomes 

 give off tufts at intervals, much as do many of the sand-binding 

 Grasses of temperate regions. Many other littoral plants of the 

 tropics adopt a similar trailing habit — including Pes-caprae {Ipomoea 

 pes-capiae), which forms a particularly characteristic and widespread 

 vegetation-type. Such plants also have the important faculty of 

 being able to groM- out of the sand when covered by its drifting, 

 and in dry climates their areas of prevalence may extend far inland. 

 Some other colonists have prop-roots that grow down and anchor 

 them in the shifting sand, and almost all have a very deep and 

 extensive root system. In more sheltered situations, shrubs often 

 become numerous, as may in time small trees, such as Screw-pines 

 {Pandanus spp., Fig. 159, A). Farther back still — or in quiet creeks 

 sometimes near high-tide mark — a closed woodland is typically 

 formed in areas of sufficient rainfall. In subtropical regions, as for 

 instance the extreme southeast of Iraq bordering on the Persian 

 Gulf, there may be salt-marshes reminiscent of those of temperate 

 estuarine flats. An example is seen in Fig. 159, B, where the planted 

 groves of Date Palms {Phoenix daciylifera) seen on the horizon afford 

 a characteristic relief. 



Littoral woodlands developing out of reach of the highest tides, 

 for example on sandy and gra\ elly shores that still retain an abnormal 



