298 



THE BIOLOGY OF HYDRA : 1961 



PATTERNS OF COLONIAL GROWTH 



This brief report cannot cover the extensive hterature on pat- 

 terns of growth. By 1914 Kiihn (10) had provided a comprehen- 

 sive review and his figures have been used and recopied ever since. 

 Recently Berrill has clarified many points concerning hydroid 

 morphogenesis, and his recent book (2) provides us with both an 

 excellent survey and a bibliography. 



The pattern of colonial growth of a typical athecate hydroid is 

 shown in Figure 1 A. The oldest hydranth, terminal in position, is 

 designated as 1, the next oldest, 2, etc. There is a zone of growth 

 just proximal to each hydranth. Each such zone contributes to furth- 

 er increase in the size of the colony in two ways: it lengthens the 

 pedicel or stem in which it lies, and it gives off laterally at regular 

 intervals a new hydranth bud with its own distinct growth zone. A 

 newly produced hydranth initially has few tentacles and is small. 

 Tentacles are gradually added as the hydranth grows in size. It is 

 easy to determine the relative ages of the hydranths of a col- 

 ony with this growth pattern, on the basis of both the position and 



7 /^6 





H 



2 3 



Fig. 1. Diagrams to show the growth pattern of colonial hydroids. A. The 

 pattern typical of most colonial athecate species. B. The pattern of many 

 thecate species, e.g. Campanularia, Obelia. The black regions are zones of 

 growth. The numbers show relative age of hydranths. From Kiihn (10). 



