26 THE GEOGRAPHY OF PLANTS. 



able particular. It is well known, that the seeds 

 of most plants do not germinate or commence 

 growing till placed in the ground, and warmth 

 and moisture induce the germ to sprout. But 

 the seeds of the mangrove germinate as they 

 hang upon the tree, and push forth a rootlet, 

 which gradually lengthens until it readies the 

 marshy surface beloAv ; or, if too high on the 

 tree for this, after pushing out to a considerable 

 length, it drops from the tree ; the thick pointed 

 root fixes itself in the mud, and leaves imme- 

 diately spring from the opposite extremity. 

 In this way, so dense a forest is formed, in a 

 few years comparatively, from a single trunk, 

 that the foliage entirely intercepts the rays of 

 the sun, and, preventing the exhalation of the 

 foul vapours that arise from the marshy surface 

 beneath, becomes one of the most unhealthy 

 places in a tropical climate ; indeed, so pesti- 

 lential are many of the forests, as to render it 

 .almost certain death to botanize among this 

 luxuriance of nature. The network of roots of 

 such a spot intercepts and collects all the solid 

 matters brought down by the rivers at the 

 mouth of which these are often found, not only 

 on the marsh, but often growing so far into the 

 water that their trunks are covered with shell- 

 fish. They not only thus extend along the 

 coast, but actually in many instances form new 

 ground for themselves, and as a large part of 

 this consists of decomposing vegetable matter, 

 under the heat of a tropical sun, pestilential 

 vapours are continually arising, rendering it 



