ALL coral reefs provide a great deal of limestone in one form or another. These dead results of the activities of living organisms accumulate on the beaches in the coral seas and range in size from a boulder, five or six feet across, down to the finest sand. One of the very common kinds of this debris is a sort of shingle consisting of sticks of coral limestone up to an inch in diameter and maybe a foot or so long, but usually less. When a coral reef grows to the level of low-water this debris begins to accumulate behind the reef; that is, away from the prevailing wind. The finer material is carried farthest, but eventually a wall or rampart of coral shingle is formed on the surface of the reef.
In the shelter of this wall the young plants of the red mangrove, carried out in the first instance by flood-water from the mainland rivers, are able to establish themselves and produce an almost impenetrable forest miles out at sea. On the rampart itself, the white mangrove is found. Theof this species also germinate on the tree, but, owing to their small size, have no chance of occupying any territory below low-water mark. The species is, therefore, restricted to a narrow strip of shingle at and just below high-water mark. Other species with intermediate lengths of occupy intermediate levels.
Nevertheless, this amphibious plant population is no more fixed and stable than any other. The system, like so many others, contains the seeds of its own decay. The red mangrove has a very successful arrangement of buttress or stiltby which it is enabled to withstand water movements in a soil of soft mud. When the forest is established these roots, which commonly grow out from the trunk of the tree
at an angle of 45 degrees from a height of several feet downwards, form a densely interlacing entanglement over the whole area. This provides a means whereby the level is gradually raised by the accumulation of entrapped material, thus giving opportunity to species of higher levels to establish themselves. At the same time the shingle is driving in from the windward side, thus killing off the mangrove swamp at its fringes. As the ground rises, low-level species give way to higher-level species and new areas of dry land arise from the sea.
If we go on to examine the ground as it goes downwards into deeper water, we shall soon find that plants without the-habit become increasingly common and plants with it increasingly rare. However, there are which grow and form extensive meadows under twelve feet of sea-water. The eel grass or grass-wrack, common on the shores of the British Isles, and indeed of the temperate Atlantic generally, is a -plant growing in the sea. In the tropics under-sea meadows of sea-grasses are the grazing grounds of turtles and other marine animals. Yet the seed-habit, so common on the dry land, is very much the exception in the sea. It is obviously true to say that for plants living in the sea there is no water problem. It is only when a living organism is exposed to the atmosphere that the danger of drying up becomes a serious menace to its continued existence. By means of the seed-habit the young plant survives the period between dependence on its parent for water supplies and getting them for itself.
The plant populations of the sea have to face just the same sort of competition for every available square inch of ground as we see happens on the land. It is one thing to get there first and get started : quite another to hold the territory against the better-equipped species which may come along later. Broadly speaking, the more favourable the locality the greater the number of species we shall find occupying it: the worse the place, the fewer the species. For instance, if we examine the vegetation of a breakwater, at Lyme Regis say, we shall find the space between the tide-marks occupied by brown seaweeds, leathery plants well-equipped to withstand the daily exposure to sun and air to which the tidal movements subject them. Looking closely, we shall find that there are only four or five species of larger plants, each of them occupying a horizontal strip or zone, pretty clearly defined
in relation to the level of the tides. Once we get below the level of low-water the number of species becomes greater, and any zoning there may be is much less obvious.
Investigation of the social relations of under-water plants is obviously a much more difficult matter than that concerning land-plants. Something can be done under favourable conditions by means of diving gear of a simple sort, but most of our knowledge has been gained by means of dredging, grabbing or trawling. All of these methods have very serious limitations; grabbing gets too little material from too small an area; dredging and trawling cover a larger area and get more material, but then material from different levels will be mixed indiscriminately. Nevertheless we do know that plants in the sea are sorted out by their particular requirements in quality and quantity of light. Here it may be remarked that the four greatof spore-bearing plants in
the sea (Alga)1—commonly referred to as blue-greens, greens, browns and reds—have different capabilities in respect of light; their colours are the expressions of that fact.
The details need not concern us; all we need to realise is that the quality and the quantity of light available at different depths in the sea differ likewise. We are fairly safe in saying that brown plants need red light and are, therefore, not plants of deep water. The same is usually true of the green plants. In temperate seas they are comparatively insignificant and restricted to comparatively shallow water. In tropical seas, however, they are important elements in the population, and have been taken from water more than 300 feet deep—in fact the greatest depth record is held by a green plant. In northern waters the deep water is occupied by the red plants which are capable of using green and blue light.