PHYSIOLOGICAL GRADIENTS 99 



may produce an increase in electronegativity even there. 

 These relations may of course be altered by various 

 factors: for example, higher levels of the physiological 

 gradient may and probably often do have a lower 

 threshold or a higher maximum of excitation than lower 

 levels, consequently the decrement may be different 

 at different levels. In tissues which react according to 

 the u all-or-none" law transmission may occur to an 

 indefinite distance up a gradient, and if the maximal 

 excitation is higher at higher levels the energy or inten- 

 sity of the excitation may perhaps increase as it is trans- 

 mitted upward. 



Differences in transmission up and down an axial 

 gradient appear in many functional relations. It has 

 been noted by various observers that when flatworms 

 or annelids are cut in two, at least in the more anterior 

 body regions, the portion anterior to the cut is much 

 less affected than the posterior portion (Norman, 1900). 

 Coelenterate tentacles show distinct physiological gra- 

 dients, and conductivity down the gradient, that is, 

 basipetally in the tentacle, is much better than in the 

 opposite direction. Various other similar cases might 

 be mentioned. 



This relation of dominance and subordination, how- 

 ever, is not simply a functional relation in fully developed 

 organisms, but is concerned in development and is 

 quite independent of the presence of a nervous system. 

 The existence of such a relation in plants with the 

 growing tip or, in certain cases, some other region of 

 high activity as the dominant region has long been 

 known. This relation in the higher plants has been 

 interpreted by Sachs, Loeb, and others in terms of 



