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AMERICAN JOURNAL OF BOTANY 
[Vol. 8 
levels of the gradient. The question how the primarily quantitative 
differences in such a gradient can give rise to the qualitative differences 
characteristic of differentiation of cells presents no fundamental difficulties. 
Differences in the relation between available nutritive substance and the 
rate of oxidation at different levels of a gradient undoubtedly determine the 
appearance of certain substances in the cells at one level and their absence 
at another. Differences in concentration of certain substances at different 
levels may also determine the formation of different products, and various 
other factors in the complex protoplasmic system doubtless play a part in 
determining the origin of qualitative differences from the quantitative 
differences of the gradient. But whatever the local factors involved in 
each particular case, the physiological gradient constitutes the primary 
factor in determining localization and differentiation of parts along an axis. 
The important point for present purposes is that in such a gradient a 
relation of dominance and subordination exists, the high end, the most 
active region, of the gradient being the dominant region and determining to 
a greater or less extent conditions at other levels within a certain distance, 
which differs with the stage of development, the condition and differen- 
tiation of the protoplasm, and the degree of activity of the dominant region. 
Since this dominance is effective only within a certain range or distance, 
the possibility of physiological isolation exists, that is, either in consequence 
of increase in length of the organism, of decrease in activity of the dominant 
region, or through a blocking in some way of the passage of the controlling 
influence, certain parts may become isolated from the action of the dominant 
region, even though still in physical continuity with it. In the simpler 
'animals and plants such physiological isolation results, like physical isola- 
tion, in dedifferentiation and development of new axes, or parts already 
present but previously inhibited, such as latent buds in plants, become 
active and develop. 
The question of the nature and origin of physiological gradients is 
obviously of fundamental importance. The gradients, so far as can be 
determined, represent primarily quantitative rather than qualitative 
differences, and if this is true, as all the evidence indicates, the relation of 
dominance and subordination cannot be fundamentally a matter of chemical 
or transportative correlation, that is, of mass transportation and action 
upon one part of specific substances or hormones produced in another. 
In order that such correlation may exist the parts concerned must already 
be qualitatively different, and the evidence indicates that dominance and 
subordination exist in the absence of such differences. Unquestionably 
chemical correlation is of great importance as soon as qualitative differen- 
tiation begins, but it cannot be the primary factor in correlation and in 
determining such differentiation in the organism. 
The only other possibility appears to be the transmission of dynamic 
change of some sort, that is, of excitation. We must therefore inquire 
