82 CORAL AND ATOLLS 



on the one hand, and into the complicated branching forms on 

 the other. 



In the many highly branched forms of vegetative growth, 

 the growing zooids form a cluster, and this cluster divides as it 

 grows upwards, with the development of many growing points 

 at intervals on the parent stem. The cluster may be of 

 various shapes, and its form determines that of the stem which 

 results from it. The stem may be rounded or flattened, may 

 be thin or thick, and it may branch at frequent intervals, or 

 it may scarcely branch at all, so that straight rods of uniform 

 thickness may be characteristic of the vegetative growth of 

 the coral. 



Another modification of this method of growth is that in 

 which practically the whole colony represents the growing 

 point, and then the entire mass grows upwards as a solid 

 column; or the growing point may be confused and irregularly 

 distributed, and then an irregular, lumpy amorphous mass 

 results. 



Whatever the vegetative growth may be, it is in these 

 cases the product of a mass of growing zooids, and these zooids 

 are being perpetually renewed, so that the growing point always 

 contains the youngest individuals in the colon3^ 



In Grou/p 2, however, this state of things is entirely altered, 

 for there one zooid, which is situated at the extremity of the 

 stem, and which I shall call throughout the " dominant apical 

 zooid," constitutes the growing point ; and this zooid is the 

 parent of the entire colony. 



The zooid that settles down to establish a Madrc/pora 

 colony has the peculiar innate property of perpetual growth 

 and perpetual youth ; and this original zooid grows up and up, 

 budding new zooids from its sides, until destruction overtakes 

 it. As a matter of fact, a Madrepora colony usually starts as 

 a flat growth which spreads from its edges, but this method of 

 growth lasts for only a very short time ; and all the charac- 

 teristics of the " dominant zooid" — which is here the central 

 zooid — are even then well marked. 



Besides possessing these jjcculiar physiological distinctions, 



