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 "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 
