484 Mr Gardiner, The Coral Reefs of [Mar. 7 



The spirit material I brought from Rotuma is unsatisfactory, so 

 far as I have examined it. Some sections, however, of Euphyllia 

 and Prionastrwa polyps show apparently the presence of uni- 

 cellular algae in the endoderm and structureless lamella. 



Prionastrcea and Pocillopora, the two genera which gave by 

 far the best results, both live on the rim of the reef and are 

 uncovered, though never dry, at low tide. Should the giving off 

 of oxygen, as my experiments seem to indicate, be confined to 

 certain genera it would be of interest to note their distribution on 

 the reef, and in depth. That oxygen is given off, I do not believe, 

 to be due in all cases to parasitic algse ; in the genus Madrepora 

 I have completely failed, so far, to find any such, and while the 

 species of this genus only gave off small quantities of gas, all the 

 species yet gave off a certain amount. Experiments at Wakaya, 

 Fiji, on species of Pocillopora and Prionastrcea, fully confirmed 

 my results in Rotuma, though the quantity of gas seemed relatively 

 less. I do not know how far the gas in some cases may not have 

 been due to the algse in the ccelenteron, but I do not think 

 this factor is of any importance. As the species of Madrepora 

 and Pocillopora, obtained from depths off Funafuti up to 35 

 fathoms, all had their corallites distinct to the eye, like pins' heads, 

 when fresh, and of a dark greenish colour, it seems to me probable 

 this assimilation of carbon from carbonic acid gas is a very im- 

 portant factor in the growth of corals and hence in the formation 

 of coral reefs. 



SECTION IV. 



Conditions affecting the Growth of Coral Reefs. 



Most of what I have now to draw attention to under this 

 heading has been already anticipated in my descriptions of the 

 different reefs. I have, in connection with many of the reefs, 

 referred to the effects of currents, of which two kinds must be 

 distinguished, namely, tidal and ocean ; the first are usually 

 shallow currents, but the latter are both surface and deep, and 

 do not, I think, vary much in the central and western parts of the 

 Pacific with moderate winds. The tidal currents show their 

 influence mainly in the passages of reefs and inside reefs, while 

 the ocean currents affect rather the growth of the whole reef. In 

 fringing reefs the passages are generally free from corals, which 

 nevertheless sometimes grow very luxuriantly, where the tidal 

 current commences to spread out over the whole boat channel. It 

 is in this position, where the current slackens, that sediment would 



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