iS) 
64 M.C., grow some percentages quicker than in the dark; when 
left again in the dark, they constantly grow 4 — 10 °/, more slowly 
after dark-adaptation. 
Il. Does the dark call forth a response of growth in a cell adapted 
to light, v.0.w. does the dark work as a stimulus, or is this decrease 
of growth of 4—10°/, in the dark quite gradual? 
Experiments. When the cell adapted to 64 M.C. is made perma- 
nently dark a decrease of growth sets in after 3'/,—5'/, min., so 
that after 8—11'/, min. the rate has decreased to + 73°/,; next 
there follows an increase, by which after 15'/,—18'/, min. the rate 
is about recovered (98'/, °/,) in the light to decrease again a little 
and become gradually constant at + 93°/, of the rate of growth in 
64 M.C. (See fig. 1, curve 8). 
So a disturbance of equilibrium takes place in consequence of the 
stoppage of the energy-supply, so tbat a typical response to stimulus 
ensues, which is contrary to the response of growth to light. If 
after the darkening the growth had gradually decreased to its dark- 
value, we could hardly have spoken of a response to stimulus. Now 
that there always occurs a reaction-time of 3'/,—5'/, min., just as 
with the response of growth to light and in consequence of the 
dark-fall an evident disturbance of equilibrium takes place, showing 
itself in fluctuation of growth, we may talk here of a typical response 
to stimulus. 
For a cell adapted to constant light the dark (sudden stoppage of 
light-supply) works as a stimulus. For some minutes the rate of 
growth in the light vs maintained in the dark, then a sudden reaction 
follows, contrary to that which light causes. 
Responses to dark and to decrease of light have already been 
ascertained by Simrp for Avena. The response of growth to light of 
Avena is mainly a retardation of growth; the reactions observed 
by SierP on dark- and light-decrease were accelerations of growth. 
It seems suitable to me to use the name of dark-growth-respunse 
for this phenomenon, as SterP proposes with some reserve, provided 
an ample meaning is attached to the conception light- and dark- 
growth-response, viz. a response of growth to increase, resp. decrease 
of light. 
Man states the result of the light-energy on the retina by himself 
much quicker through his impressions of brightness, than we can 
read the result of the light-stimulus on the metabolism in that cell 
in consequence of its change of growth. Moreover those processes 
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