
PLANT GEOGRAPHY 461 
of the sea moves. At extreme low tide the flats are exposed, 
and often even the larger channels are dry. On these mud flats 
the trees which form the mangrove vegetation find conditions 
favorable to their development; and as the seeds or seedlings of 
these species are distributed by water and can be transported 
for long distances without injury, the formation of flats and their 
seeding are almost simultaneous. When conditions are favorable, 

Fic. 517. Aérating roots of Sonneratia caseolaris in Philippine 
mangrove swamp 
new flats are formed beyond the old, and the forest advances 
year by year. The mangrove forests may contain trees more 
than a meter in diameter, and when fully stocked with mature 
timber compare favorably with the commercial forests of the land. 
The mangrove forests are characterized by the presence of 
roots that show above the surface of the ground (Figs. 197, 517) 
and contain numerous air spaces that serve for the conduction 
of oxygen to the underground root system. In the genus Rhi- 
zophora these roots take the form of prop roots and serve both as 
props for the tree and for the aération of the root system, In 
