290 
AMERICAN JOURNAL OF BOTANY 
[Vol. 8 
experimental investigation R. S. Lillie has developed an electro-chemical 
conception of excitation and transmission which seems to interpret and 
account for the various phenomena more satisfactorily than others pre- 
viously advanced. The unexcited surface layer of the cell behaves as if 
more permeable to positive than to negative, or to certain negative, ions, and 
is therefore electrically polarized. Excitation increases its permeability 
to the negative ions and depolarization results, with an increase in electro- 
negativity of the external surface. In this change a chemical reaction, an 
oxidation, is involved, whether as the primary or as a secondary factor is 
not at present known. The electric current arising at any point of excita- 
tion becomes the factor determining depolarization and excitation at all 
points within a certain distance, beyond which it is too weak to be effective, 
and each new region of excitation becomes the source of current which may, 
if strong enough, excite further points. At the same time the current tends 
to restore the polarization at the point of original excitation and so to 
reverse the excitation process at that point. By means of this current, then, 
according to Lillie, transmission occurs. With simple inorganic models he 
has been able to demonstrate the occurrence of transmission in this way, 
both with and without decrement and at different speeds, as well as the 
development of fixed gradients. The speed of transmission has no relation 
to the speed of electrical transmission, but depends on the velocity of the 
changes at each point of excitation which give rise to the current. The 
development of fixed gradients occurs when conditions determine the 
persistence of the region of high potential which in turn determines a poten- 
tial gradient extending over a greater or less distance and so a gradient in 
the conditions determined by the electric current. 
Whether this theory of excitation is in all respects correct or not, it 
enables us to see how a region of excitation in undifferentiated protoplasm 
may determine the origin of a physiological gradient. The facts indicate 
that these gradients do arise in this way, and if we admit this, it follows that 
the primary factor in dominance and subordination is transmission. The 
establishment of a region of high activity must affect adjoining regions 
within a certain distance as a region of excitation affects them, and it is 
difficult to believe that the electric potential characteristic of such a region is 
not a factor and probably the primary factor in such a relation. If the 
degree or intensity of excitation at each level is in any degree proportional 
to the strength of current, a gradient must result, and if the conditions 
determining the gradient persist for a certain length of time, changes in the 
protoplasm at the different levels may determine the more or less permanent 
fixation of the gradient. In most protoplasms this relation between stim- 
ulus and excitation does exist, and a gradient results from local excitation. 
In the nerve fibers of the higher animals, however, the excitation process 
is specialized so that any stimulus above the threshold gives rise to maximal 
excitation and there is therefore theoretically no decrement in transmission. 
