:i20 DR. F. W. JONES ON GROWTH-FORMS [June 18, 



Speaking quite broadly, a colony may grow according to five 

 diflerent types of vegetative growth. It may grow as ( 1 ) a spherical 

 mass, (2) an encrusting layer, (3) a free plate, (4) a branching 

 tree-like growth, or (5) a mere amorphous lump ; and though 

 a definite inherent growth tendency is sti'ongly implanted in the 

 embryo, still the demands of the environment may call forth any 

 type of vegetative growth. 



The growth-forms are purely the results of I'epeated divisions 

 of the zooids, and so it will be seen that the relative value, from 

 a reproductive point of view, of the zooids in a colony, is a thing 

 of the greatest importance. This consideration brings in its train 

 the division of all the colonial corals into two groups of normal 

 growth-forms ; for all the zooids may take an equal share in the 

 asexual reproduction, or again, some maybe of greater importance 

 than others, and the asexual reproductive functions may be lodged 

 in a very few individuals only. These two great divisions must 

 be considered sepaiutely, for the rules that maybe applied to their 

 respective methods of growth are widely different. 



Taking first the class in which every unit is of equal value, and 

 going back to the earliest origin of the colony, it is easily seen 

 that a zooid " A " settled on a nucleus will divide into zooids 

 " B " and " C," and " B " will further divide into " D " and " E," 

 and "C" into " F" and " G," and so on; each newly-divided 

 individual taking its equal share in future divisions. The natural 

 outcome of this state of things is that, if the site of election of 

 growth be a prominence, or, as is not uncommon, a small isolated 

 fragment, then the equal divisions will tend to form a spherical 

 mass. The rapidly growing colony will tend to surround the 

 nucleus on all sides, and in this manner are formed those rounded 

 masses of Porites and Astrceojjora that are commonly to be found 

 lying free in sandy pools, and which, when broken across, are 

 seen to be formed around a central nucleus, that generally consists 

 of a fragment of dead and altered coral. 



It is of course but natural that the true spherical form cannot 

 long survive in very large colonies, for the zooids growing below 

 are of necessity killed by pressure. The mass will therefore 

 become a hemisphere, and continue its growth as a rounded 

 boulder. This boulder-form is a very common type of vegetative 

 ijrowth among the reef-bviildins' corals ; for if the s^rowth starts 

 as a perfect sphere, it will ultimately assume this form, and if the 

 colony starts its growth on a basis that is not an isolated fragment, 

 it will start this mode of growth from its first beginning. 



There are many difi'erent ways in which the asexual reproduction 

 of the zooids is carried out, for the budding and division may 

 proceed in various fashions. The principal types of budding vary 

 from each othei- in the actual site of origin of the daughter zooid 

 from the parent, and in the degree of the final separation of the 

 two zooids. These types, I think, do not merit individual de- 

 scription here, for what was said previously is true for them all. 



