C. M. CHILD. 



plants of similar growth-form, i.e., the chief apical region inhibits 

 in some way and to some extent the lateral branches. It has 

 been shown (Child, '19^) that these complex axes possess a general 

 susceptibility gradient, the apical hydranth of the main axis 

 being in general more susceptible than the apical hydranths of 

 lateral branches and the apical hydranths of the more apical 

 lateral branches, more susceptible than those of more basal 

 branches. In Bougainvillea this general gradient is less distinct 

 than in some of the campanularians with more definite growth- 

 form. 



In Obelia and Gonothyrcea a similar general gradient appears 

 in the whole colony and in the complex axial systems represented 

 by its branches. In these forms, however, the growth form is 

 sympodial (Child, '196) and instead of a permanent apical 

 hydranth each growing tip and the hydranth bud developing from 

 it is temporarily apical, but is displaced by the next bud. The 

 general colony gradient appears in the rate of reduction of the 

 different growing tips and hydranth buds of an axial complex, the 

 most apical showing in general the most rapid reduction. 



In vigorous, rapidly growing colonies of Obelia borealis 5 cm. 

 or more in length, taken from piles and floats, the general gradient 

 is usually clearly visible to the naked eye by reflected light after 

 reduction is completed or has progressed so far that the tissues are 

 opaque in water. On removal to water and washing the depth 

 of brown color of the tissues of such a colony, as seen by reflected 

 light, decreases in general from apex to base in the colony as a 

 whole and in each axial complex. The most apical growing tips 

 and buds are very deep brown, almost black, the growing tips 

 and buds of more basal regions distinctly lighter in color and 

 even in the developed hydranths similar differences appear to 

 some extent. The color gradient in the stem also appears in 

 spite of the fact that the thicker perisarc of the more basal 

 regions is more deeply stained by permanganate than that of more 

 apical regions. 



It is perhaps necessary to emphasize the point that these 

 general gradients are most distinct in vigorous, growing colonies. 

 The observations recorded above were made on colonies which 



