SUSCEPTIBILITY IN HYDROIDS. 115 



hydranths of branches differ in susceptibility as noted above, 

 according to their position in the colony. These differences in 

 physiological condition indicate that for some reason the ter- 

 minal hydranths of the colony and many branches undergo 

 physiological senescence less rapidly than others. In this con- 

 nection it may be pointed out that the terminal hydranth is 

 more closely' associated than any other with the process of 

 hydranth reproduction, since the new hydranth buds arise a 

 short distance below it (Fig. 9). Moreover, growth and the 

 development of new buds occurs most rapidly in the apical region 

 of the main axis of the colony, and the rate of growth and budding 

 decreases basipetally in the colony as a whole and in each branch. 

 Apparently, as in those plants with similar growth form, there 

 is a relation of dominance and subordination in the colony and in 

 each branch, the apical region being dominant and inhibiting 

 growth and reproduction in other axes to a greater or less degree. 

 Since new buds arise a short distance below the apical hy- 

 dranth of any axis (Fig. 9), they probably produce some effect 

 upon the hydranth, either in the way of reorganization or through 

 the demand for nutrition which they create. The nutrition for 

 the bud must be derived largely from the reserves or the newly 

 digested food of the apical hydranth, consequently the nutritive 

 condition of this hydranth may be altered to some degree in the 

 direction of starvation, but when the new hydranth becomes able 

 to obtain food for itself the apical hydranth may recover its 

 former nutritive condition. The effect of starvation in pro- 

 ducing regression to a physiologically younger condition in the 

 lower invertebrates has been clearly demonstrated (Child, '14, 

 '150, Chap. VII, '190) and it seems probable that alternating 

 changes of this sort associated with the development of new buds 

 are concerned in the retardation of senescence in the apical 

 hydranths. In a monopodial colony such as Pennaria the 

 hydranth in which such changes occur most frequently, i.e., 

 the apical hydranth of the colony, will, according to this hypo- 

 thesis, undergo senescence least rapidly, and the relative rate 

 of senescence in the apical hydranth of any axis will vary in 

 general inversely as rate of growth and frequency of reproduction 

 in that axis. Consequently, since rate of growth and frequency 



