THE AXIAL GRADIENTS IN HYDROZOA. 1 95 



The experiments show, however, that stolon formation is not 

 necessarily limited to basal regions of the stock, but may occur 

 anywhere, even at apical ends, under inhibiting external condi- 

 tions. In consequence of the differential susceptibility of differ- 

 ent levels of the axis, the effect of such conditions is to decrease 

 the steepness of the gradient. This change induces disintegra- 

 tion or resorption of hydranths, and new buds, instead of develop- 

 ing into hydranths, give rise to stolons. Moreover, a greater or 

 less degree of physiological isolation of stem regions results from 

 the disappearance of the dominant hydranths and in some species, 

 as in many plants "adventitious" buds, i.e., buds not localized 

 in conformity to the usual order, arise (Figs, n, 13). But under 

 the inhibiting conditions the buds develop as stolons, not as 

 hydranths. Such adventitious stolons have been seen most 

 frequently in the basal halves or thirds of Bougainvillea and 

 Gonothyrcea stocks, but apparently may occur anywhere. And 

 finally, the inhibiting conditions alter the steeper hydranth-stem 

 gradients of terminal regions into the less steep stolon gradients 

 and stolons therefore appear in place of hydranths. 



Often more or less acclimation to the inhibiting conditions 

 occurs in the course of a few days, and new hydranths begin to 

 develop either from terminal regions which have not formed 

 stolons (Fig. 7), from new buds, or by the transformation of 

 stolon tips (Fig. 4). Such hydranth development retards or 

 completely inhibits further growth of subterminal stolons near 

 the hydranth, but the growth of terminal stolons may continue 

 indefinitely (Figs. 10, 14, 15, 18) unless conditions are so altered 

 as to induce their transformation into hydranth-stem gradients. 

 In the case shown in Fig. 10, for example, each of the two chief 

 stolon outgrowths gave rise at one point in its growth to a hy- 

 dranth bud (h, A), but these two buds were unable to develop 

 further than the stage shown in the figures and later underwent 

 resorption. 



SEPARATION OF STOLONS FROM THE STOCK. 



In all species investigated continued growth of the stolon 

 leads sooner or later to loss of continuity between its ccenosarc 

 and that of the parent stock. In Bougainvillea (Fig. 10) and 

 Gonothyraea (Fig. 14) such separation of stolon and stock usually 



