RATE OF GROWTH OF REEF ORGANISMS 413 



fathoms of water, and it was covered astern of the middle by thick- 

 branching Madrepora, the branches of which were twelve inches in 

 diameter and sixteen feet in height. This makes a rate of growth 

 for this coral of three inches per year. 



Darwin cites some experiments made on the east coast of Mada- 

 gascar in December, 1830, by Dr. Allan, of Forres. Twenty corals 

 were weighed and placed apart on a sand bank in water 3 feet deep 

 at low tide, each of them reaching in the following July nearly to 

 the surface and being quite immovable, while some had grown over 

 the others. A block of concrete taken at Fort Jefferson, Tortugas, 

 and which had been in the water only twenty years, had a specimen 

 of Mccandrina labyrinthica growing on it, which measured a foot in 

 diameter and four inches thick in the most convex part. (Dana- 

 20:124.) At Key West, a Maeandrina had grown to a radius of 

 six inches in twelve years. An Oculina diffusa planted as a germ 

 in this same region grew in 14 years to a height of four inches and 

 a breadth of 8 inches, while a Mccandrina clivosa in the same time 

 reached a height of two and a quarter inches and a breadth of seven 

 and a half inches. 



The rate of growth of coral reefs is, however, much slower than 

 the rate of growth of the corals and other organisms composing 

 them, since these grow only in isolated patches. Estimates on the 

 rate of increase in height of reefs allow two hundred years for every 

 foot of increase. 



Compacting Agents of Coral Reefs. The isolated coral heads 

 of the reefs do not of themselves produce the solid limestone 

 masses characteristic of these structures, unless they are bound 

 together either by chemical precipitation of lime in the interstices 

 or by organic agencies. On Funafuti (74:1^7) it is the lichenous 

 Lithothamnion which performs this office most eft'ectively, cement- 

 ing the coral masses to each other and to the reef as a whole. 

 This nullipore will attack both living and dead corals and will cause 

 every interstice as well as every space between the branches, if 

 present, to be transformed into one solid mass. Thus the coral mass 

 will be permanently added to the reef, enlarging it by the amount 

 of its bulk when taken possession of by the Lithothamnion. Obser- 

 vations show that a coral once attacked by the nullipore will remain 

 stunted, and thus the amount of addition to the reef depends on 

 the period of growth reached by the coral when attacked by the 

 Lithothamnion. 



On the Great Barrier Reef it was found that reef forming was 

 less a function of the species of coral than of the temperature, for 

 the same species forming solid reefs in the warmer portions of the 



