THE MANGROVE SEED’S FUTURE DEPENDS ON THE TIDE

IT is not easy to give a comprehensive description of what is meant by mangrove swamp. The term is probably associated in the minds of many people with the steamy, muddy estuaries of tropical rivers; indeed mangrove swamps are very commonly found in such places. The only soil they have is subject to regular inundation by tidal water, which may vary in salt content from that of the open sea to being only brackish. There are tidal-lands in the British Isles which are no less interesting, but they are, perhaps, rather less easy to describe and have fewer obviously novel features. Let us begin, as before, by considering the seed, and let us remind ourselves that a seed is a very young plant in a state of arrested development. All seeds have some more or less efficient protection against mechanical damage and drying up and are therefore able to withstand a certain amount of transportation and so establish the species at some distance from the parent plant. It is a very notable fact that if we find plants on any seashore we shall find many of them have seeds which are capable of-floating unimpaired in the sea for long distances. The important plants of the mangrove swamp are able to do this and more also. Unfortunately it is not

possible to see mangroves nearer than the United States and then only in the far south. The most impressive swamps are found in the tropical regions of the Indian and Pacific oceans, but there, interestingly enough, nearly all the species are different from, though related to, those of the Atlantic.

An interesting species for us to consider is the Red Mangrove. We may properly do so because it forms practically pure forest without human interference. Besides the evaporation control by the leaves it has several other features which make it remarkably fitted for the situations in which we find it. The seed germinates before it leaves the parent plant and the young plant continues its growth until it may be eighteen inches long. When it falls from the tree it will have two chances, depending on the state of the tide. Owing to its swollen, pointed lower end, if it falls at low-water it will plant itself in the shade of its parent. Falling at high-water will give the seedling a chance of being carried away by the receding tide.

When it goes aground it will, owing to its length and its power of floating upright, be able to occupy territory not available to its associates. It is, in fact, the length of the young plant which decides the level in relation to the tide at which it can and does grow : the longer the seedling, the lower the level at which it can colonise. As there are a number of species each with seedlings of different lengths it is not difficult to see that they should occupy separate zones from just below low-water mark upwards. On the low wooded coral islands of the coast of Queensland we can find an extremely interesting arrangement of the plants due to these facts.

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