ti2 BOTANICAL GAZETTE [AUGUST 
action of external factors. After it has once arisen it becomes, as 
I have endeavored to show, the chief factor in determining the 
metabolic rate in the other regions of the axis to which it gives 
rise, so far as they are within the range of its influence. Since the 
changes transmitted from it to other regions undergo a decrement 
in energy or effectiveness in the course of transmission, a more or 
less definite gradient in metabolic activity and therefore in behavior 
of the cells results. The more active cells divide more frequently 
and differentiate and grow old less rapidly than the less active cells. 
The apical region itself, as the most active region of all, may remain 
embryonic and physiologically young indefinitely if the processes 
of senescence and rejuvenescence balance each other in it; but if 
this is not the case it may gradually grow old, though senescence 
is less rapid in it than in other parts because of the higher metabolic 
rate. According to this conception, therefore, an apical region 
arises wherever the metabolic activity of a cell or a cell group, or 
in unicellular forms of a part of the cell protoplasm, becomes high 
enough to enable it to dominate or control to some degree the meta- 
bolic activity of other cells within a certain range. When the apical 
region has arisen, the development of the plant or animal individual 
proceeds as the specific constitution of the protoplasm and the 
conditions determine. 
As soon as differences between different parts along the gradient 
arise, chemical correlation, that is, the influence of substances 
produced in one part upon another, must begin to play a part, 
and this is unquestionably a factor of great importance in determin- 
ing the further course of events along the axis. It is evident, how- 
ever, that chemical .correlation cannot occur in an orderly way 
unless orderly differences already exist at different points or levels, 
and I have endeavored to show how these differences may find their 
origin in the axial metabolic or irritability gradient (12). If my 
conclusions are correct, the physiological axes in both animals and 
plants consist in their simplest form of such a gradient, and it is 
the interrelation or correlation in this gradient which determines 
the orderly behavior in relation to this axis. 
