June, 1921] 
CHILD — PHYSIOLOGICAL CORRELATION 
farther away from the buds to be isolated it is less effective. Moreover, 
with temperatures which are not too low, a long cooled zone is more effective 
as a block than a short one, but with sufficiently low temperatures the 
short zone is effective. In these respects the correlative factor apparently 
behaves to some extent like the nerve impulse. 
Growth of the buds isolated from the tip by low temperature is usually 
evident within one to two days. If the temperature is near the upper limit 
of effectiveness, the growth of the buds below the zone usually ceases 
after a few days in spite of the presence of the cooled zone. This is un- 
doubtedly due to the occurrence of some degree of acclimation in the cooled 
zone with consequently more effective passage of the block by the inhibiting 
factor. If the buds are allowed to grow for ten days or more before the 
removal of the low temperature, they usually continue to grow more or 
less rapidly afterward, and in this way plants with three or more stems can 
be produced. Earlier removal of the low temperature usually results in 
renewed inhibition. 
' In my experiments with the saxifrage, the low temperature is applied 
to a zone of a runner which has not attained its full length, with the result 
that the runner soon ceases to elongate and begins to develop a new plant 
at the tip, even when suspended in air. Here also water and salts reach 
the runner tip only by passing the cooled zone, and the rapid development 
of the new plant shows that this flow is not seriously affected. According 
to Loeb's hypothesis, it seems that substances inhibiting the development 
of the new plant at the runner-tip must be transported by this current, 
but as a matter of fact the low temperature isolates the tip without stopping 
the current. 
Whatever the nature of the correlative factor may prove to be, these 
experiments, particularly those on the bean seedling, seem to me to offer 
difficulties to the hypothesis that this factor consists of an inhibiting sub- 
stance or substances transported in mass through the plant." Since the 
correlative factor can be blocked by a zone of low temperature, we must 
assume, if it consists of a substance or substances, either that it is trans- 
ported through the living protoplasm and that its passage is dependent upon 
the physiological condition of the cells, or that it is of such a nature that it is 
precipitated out of, or otherwise removed from, the fluids of the plant as 
they pass the cooled zone. 
On the basis of the first assumption, we should expect a substance 
which inhibits the growth of vegetative tips to inhibit the growth of the 
cells through which it passes. Below the chief growing tip, for example, 
such a ^substance must pass through the region of most active growth in 
the axis, but it does not inhibit this region. In fact, if such a substance 
passes through the living cells of the plant, most complex and remarkable 
relations of immunity and susceptibility must exist. Each growing tip, 
for example, or any other part producing such a substance, must be immune 
