LIFE AND DEATH IN CONCURRENT PROGRESS. 95 



yps gradually disappear, and even their cells become superfi- 

 cial and fade out. Trees of Madrepores may also have their 

 limits — all below a certain distance from the summit being 



o 



dead ; and this distance will differ for different species. But 

 this is not a limit to the existence of the zoothome, even 



CAUIiASTK^CA FUKCATA. 



though a slender tree or shrub, or of its flourishing state ; for the 

 dead coral below is firm rock itself, often stronger than ordinary 

 limestone or marble, and serves as an ever-rising basement 

 for the still expanding and rising zoophyte. 



But this death is not in progress alone at the base of the 

 column or branch. Generally the whole interior of a corallum 

 is dead, a result of the same process with that just explained. 

 Thus, a Madrepora, although the branch may be an inch in 

 diameter, is alive only to the depth of a line or two, the grow- 

 ing polyps of the surface having progressively died at the low- 

 er or inner extremity as they increased outward. 



The large domes of Astraeas, which have been stated to 

 attain sometimes a diameter of ten or fifteen feet, and are 



