86 CORAL AND ATOLLS 



plates whicli bear zooids below are but variant forms of growth 

 of other types : for such corals as have no growth-form except 

 that of a flat horizontal plate do liot as a rule bear zooids 

 below. 



Some further remarks must be made about plate-like 

 growths ; but here is a convenient place to refer to a rather 

 strange fact to which Darwin called attention, that vertical 

 plates grow so that they offer their flat surfaces to the 

 currents. The same thing is true of growths that are branch- 

 ing forms, for then the plane of greatest branching is at right 

 angles to the line of current. At first sight this seems a 

 strange thing, for plates growing in rough water become so 

 much more exposed to damage ; and yet it is doubtless for the 

 exposure of a larger surface of zooids to the food-bringing 

 currents that the plate spreads in this direction. This phe- 

 nomenon is especially notable in the Millciwra, the broad 

 laminae or fan-shaped growths of which are always opposed 

 to the line of the waves, even when the colony happens to 

 dwell in very rough water. 



Two general rules may therefore be laid down which apply 

 to all the forms of growth that have been described : the first, 

 that all corals having symbiotic alga3 tend to grow upwards ; 

 the second, that all tend to offer their greatest surfaces to the 

 line of currents. 



We may therefore assume that every coral embryo that 

 settles upon a site of election, and starts the foundation of a 

 colony, has three inherent tendencies : it has its inherited type 

 form of vegetative growth, and its inclinations to grow up- 

 wa,rds and to oppose its growth to currents. Noav these 

 tendencies are affected by the nature of the water in which 

 the embryo settles down ; and for nearly every coral there is 

 a modification of vegetative growth dependent on the en- 

 vironment : thus most corals have a deep-water form, a smooth- 

 Avater form, a rough-water form, and numerous variations 

 depending upon the amount of sedimentation that is taking 

 place in the water of their habitat. A coral that grows in 

 rough water is obviously exposed to injury; and those people 



