THE AXIAL GRADIENTS IX HYDROZOA. 87 



different levels in which the diameter of the ccenosarc is approxi- 

 mately the same. Repeated observations of this sort leave no 

 doubt that real axial differences in the total amount, as well as 

 in the rate of reduction exist in stems and branches. 



The Stolon Gradient. Many hydroid species give rise under 

 slightly depressing conditions to stolon outgrowths, not merely 

 from basal or even from apical cut ends, but by transformation 

 into stolons of hydranth buds, in campanularians, of growing tips 

 or even apical ends or stems after degeneration of hydranths. 

 Experimental data concerning such transformations will be 

 presented in another paper. The stolon is readily distinguish- 

 able from the stem by indefinite direction of growth, by attach- 

 ment to the substratum or other solid objects and in the cam- 

 panularians by absence of the annulations characteristic of stems. 

 The stolon, like other parts of the colony, shows a gradient 

 both in rate and amount of reduction, decreasing from the tip. 

 This gradient is less steep than that of the hydranth or the grow- 

 ing tip, but steeper than that of stems. Reduction of perman- 

 ganate in the growing 'stolon is more rapid than in the stem, 

 but less rapid than in hydranth buds and growing tips. 



The Gradient of the Colony as a Whole. The reduction gradient 

 appears, not merely in the individual zooicls and parts of the 

 hydroid colony but there is a general gradient characteristic of 

 the whole colony. The gradient of stem and branches already 

 described above is of course a part of this general gradient, but 

 this gradient also appears, like the susceptibility gradient (Child, 

 '196) in the differences between the hydranths and growing tips 

 of different regions. 



The Bougainvillea colony is monopodial like Pennaria (Child, 

 '196) and the primary hydranth of the colony or branch remains 

 permanently the apical hydranth. Budding occurs subapically 

 at a certain distance below the apical hydranth, and the new 

 axes developing from the buds become lateral branches of the 

 colony or branch each bearing its primary apical hydranth. 

 In any complex colonial axis, the lateral branches grow less 

 rapidly than the main axis and the physiological relations between 

 the different axes are apparently similar to those existing in 



