Il6 C. M. CHILD. 



of reproduction decrease basally in the colony and its branches, 

 the more basal hydranths and regions undergo senescence more 

 rapidly. 



The whole complex of physiological relations in the colony 

 represented by the differences in rate of growth, reproduction 

 and susceptibility in different regions results from the order of 

 development. The colony begins as a single axis terminated 

 by a hydranth. When the stem attains a certain length a bud 

 appears at a certain distance below the terminal hydranth and 

 is probably inhibited to some extent by it. As growth of the 

 main stem continues, a second bud arises above the first and 

 now the earlier bud or hydranth is subjected to the inhibiting 

 action of two apical regions, and so on as other buds develop. 

 The same relations arise in each branch as it develops. It 

 follows that in any axial complex the degree of inhibition will 

 increase basipetally. 



In Obelia and other sympodial forms the situation differs in 

 one respect from that in the monopodial Pennaria. The apical 

 hydranth .of the colony and of each branch in Obelia is the latest 

 developed, the youngest both in point of time and physiologically 

 of the axial complex which it terminates. In well developed 

 colonies of Obelia colonies of 0. borealis 10-14 cm. in length 

 and very symmetrical in general form are common at Friday 

 Harbor a general apico-basal gradation in susceptibility of these 

 young apical hydranths or buds of different primary branches 

 appears, those of more apical branches of the colony being 

 somewhat more susceptible than those of more basal branches, 

 like the terminal hydranths of Pennaria. Similar differences 

 appear to some degree in the secondary branches of a primary 

 branch. Moreover, the same differences in rate of growth and 

 reproduction as in Pennaria appear in different branches from 

 apex to base of the colony or in the secondary branches of any 

 branch, in other w r ords the same relations of dominance and 

 subordination exist in the two growth forms as wholes. 



The chief difference between the two forms consists in the 

 fact that in the monopodial Pennaria the terminal hydranth 

 maintains its dominance over later buds and therefore remains 

 terminal, while in the sympodial Obelia the latest bud becomes 



