At first sight such swamps appear to be useless, impossible, and dangerous. But that is not the case. No one, of course, would ever willingly reside in mangrove swamps, and the mangroves themselves are of scarcely any use to man, although the bark does sometimes furnish a useful tanning material; but, indirectly, the mangroves are one of the most important of all Nature's geographical agents.
On those horrible, slimy, shifting mudbanks no other plants could manage to exist. If one looks carefully at the seaward side of the last of the mangrove swamps, then it is easy to see that they are colonizing and reclaiming the mud.
Not only do the roots depending from the branches grasp and colonize new mud, but the seedlings are also specially adapted to fulfil the same office.
They remain a long time attached to the parent fruit; they also grow to a considerable length before they fall off. When ready to fall, they have a distinct seedling stem, which swells out towards the base and ends in a pointed root. The seedling is, in fact, like a club hanging upside down and with a pointed end. When it does fall, it goes straight down deep into the mud; then it promptly forms some anchoring roots, and the young mangrove is fixed in new mud and begins to develop. So that the forest continually grows towards the sea.
Such mudbanks soon become pierced by roots in every direction. Then the leaves of the mangroves themselves, as well as silt, soil, and rubbish floating in the water, gradually accumulate about and around these roots. This must raise the level of the ground. Eventually the soil becomes hardened and is above the level of the water. When this happens, the mangrove, which likes salt water about its roots, becomes unhealthy and the ordinary jungle trees kill it and take its place. Thus in course of time, when the jungle is cleared, fertile ricefields may be thriving on what was once a pure, or rather impure, mudbank.
In this way, by the continual development of the mangroves, enormous stretches of land are being added to the continents, and the process continues so long as the character of the coast-line favours it.
The shore-line covered by these mangrove swamps is enormous. In fact, within the tropics one finds them almost everywhere along the seashore, but coral, rock, or an exceedingly dry climate such as that of Arabia or Northern Peru, prevents their growth. Central and South America, West and East Africa, India, Polynesia, Australia, and much of the Asiatic coast-line, is covered by mangroves.
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