CORAL REEFS 165 



nutrient salts. These substances are produced as a result 

 of normal breakdown in the animal tissues. But corals 

 can exist without plants ; they may occur in dark places 

 under boulders ^vithout plants, which cannot exist in dark- 

 ness. It has been suggested that corals may feed upon the 

 zooxanthellae if animal plankton is scarce, but experiments 

 carried out during the Great Barrier Reef Expedition failed 

 to confirm this. But it is certain that the plants automati- 

 cally remove waste matter produced in the bodies of the 

 corals. In this way, by increasing the efficiency of the corals, 

 they may be an indispensable factor in the necessarily excep- 

 tional powers of growth and repair possessed by coral reefs. 



It is difi&cult to estimate the growth of corals, for this de- 

 pends so much on conditions, especially temperature and the 

 supply of food, but it has been found that the branches of 

 Acropora may increase by one or two inches in length per an- 

 num, while a mass of Porites increased its diameter by thirty 

 inches in twenty-three years. Other estimations have shown 

 that a shallow-water reef might, under favourable condi- 

 tions, grow upwards at a rate of about one foot in eleven and 

 a half years, or fourteen and a half fathoms in one thousand 

 years. In Samoa, Dr. Mayor came to the conclusion that the 

 corals added some 840,000 pounds of limestone to the reef 

 annually, although about four times this amount is removed 

 in the same period by currents, sea cucumbers and other 

 coral-boring animals. This is not the only evidence we pos- 

 sess which indicates that coral reefs, in some areas at any 

 rate, may, at the present day, be decreasing rather than 

 increasing. 



The NuUipores or calcareous seaweeds grow as sheets 

 (Plate 61), usually comparatively thin, though masses two 

 feet thick have been reported from the Indian Ocean. They 

 vary in abundance from place to place, being clearly of first- 

 rate importance in some areas, such as the Maldive and 



