524 DR. F. W. JONES ON GROWTH-FORMS [Juiie 18, 



may be seen, at many points of most colonies, that certain groups 

 of zooids are more active than their fellows. 



The special activity of these zooids may be due to purely local 

 causes ; it may be called forth merely by irritation of a localised 

 portion of the surface of the colony ; or it may be a real alteration 

 of veo-etative habit. It is an outstanding fact about all the 

 colonies observed for long intervals, that the growth tends to 

 become irregular — at different times diffei-ent portions of the 

 colony have sudden phases of active growth, and these may be 

 due to no observable cause. Again, some zooids of the colony 

 may have special advanta,ges due to the environment, and these 

 zooids will grow and divide excessively, in such a manner that 

 they dominate the growth of the whole colony, and consequently 

 modify its form. The conditions of the environment may call 

 forth special reproductive activity in any portion of the growth, 

 and so in a colony, all of whose units are of equal importance, 

 some part will be found to be growing onwards, whilst all the 

 remainder of the zooids are piuctically at rest. In this way the 

 typical hemisphere of such colonies may become modified as a 

 creeping plate, an encrusting layer, a pseudo-branching form, or 

 a mass of mere irregular nodules. 



In those coiuls whose zooids are naturally of equal i-epro- 

 ductive importance therefore, forms may arise, from physiological 

 need, that simulate exactly those forms whose zooids are naturally 

 of differing reproductive value. 



Turning now to the corals that constitute the second class, and 

 have some of their units specialised as active agents of growth, 

 it is at once seen that the possibilities of variation of normal 

 vegetative habit are greatly increased. All the elaborate 

 branching forms, plates, and leaf -like growths belong to this 

 class ; and all are evolved by special peculiarities of the gi-owing 

 point. The zooids that constitute the growing point may take 

 vai'ious forms : they may be arranged as a cluster, as a creeping 

 edo'e or as many varieties of terminal shoots of branches. 



In the first instance, it is necessary to draw very sharp dis- 

 tinctions between two subdivisions of this group. In Group 1 

 come all those forms like Montipora, whose distal zooids are the 

 newest formed members of the colony (text-fig. 152); and m 

 Group 2 are included the Madrepores, whose distal zooid is the most 

 ancient individual in the whole growth (text-fig. 153, p. 527). 



In dealing with Group 1 many forms have to be considered, 

 for when the youngest are the active cells their growth-cluster 

 may be very variously disposed, and on its disposition the 

 resulting vegetative form entirely depends. 



When the growing cells are arranged in linear series, a flat 

 o-rowth will result, which grows from one of its edges, or from 

 them all ; and in this way an encrusting layer or a free plate may 

 be formed. Corals that grow with a linear growing point may 

 settle dowu on a basis, and spread over it in all directions, taking 



