Page:Popular Science Monthly Volume 85.djvu/223

Rh upon the southeast reef-flat are distributed, and in order to gather a census of the corals, a line was surveyed across the reef, and at intervals of 200 feet, squares of 50 feet on the side were staked off and all the coral heads on each square were counted.

The shallows within 350 feet of the shore lack corals, the bottom being covered with a thin layer of limestone mud which supports a vigorous growth of a short-bladed eel grass, Posldonia australis.

On the square whose center was 400 feet from shore, only three small coral heads were found, and these were growing upon loose corroded limestone blocks which had been washed shoreward from the outer parts of the reef-flat. As one goes outward, however, the coral heads steadily increase in number becoming a maximum at 1,425 feet from shore where there were 1,838 coral heads on the square. Even the crest of the lithothamnion ridge 1,750 feet from shore had 201 living coral heads clinging to the bottom of its shallow tide pools, although the ridge itself was here fully six inches above the level of the lowest tides.

There must be some cause for this tendency on the part of the corals to grow best at considerable distances from shore. The truth of the matter is that corals are very sensitive to changes of temperature and an ocean as cold as 56° or as warm as 98° F. would be fatal to all the reef-building forms. The more delicate corals such as the finelybranching Seriatopora or the "stag horns," formerly known as Madrepora