20 
take place here at 16° C., with man at 37° C. Yet we are of 
opinion that we have every reason to see similarity between the way 
of reacting of the plant cell to dark and the appearance of a positive 
after-image, followed by the appearance of negative after-images in 
definite circumstances in the eye. There too after the fall of the 
dark a fluctuation of the impression of brightness, by which the im- 
pression of brightness or rather of darkness, with the negative after- 
image, may fall in the beginning below the normal darkness (or so- 
called intrinsic light of the retina) of the eye adapted to the dark. 
In this way the after-images are to be taken as a disturbance of 
equilibrium of the sight-apparatus adapted to light through the coming 
of the dark (=the stoppage of the energy-supply). Especially these 
negative after-images, which appear in our eye 2-—4 minutes after 
strong prolonged illumination should be noted. 
III]. What is the process of this dark-growth-response, when the 
cell has not been adapted to 64 M. C. but to slighter intensity ? 
Experiments. Experiments were made in 8—1—*/,—*/,, and ’/,,, 
M.C. In 8 M.C. the average minimum of growth was 67 °/,. It 
further appears, that the growth also after slighter intensities of 1 
and '/, M.C. decreases to about the same value, viz. to + 75 °/,. 
To be sure the reaction — just as with the common light-growth- 
response after weaker stimuli — appears later. The maximal 
decrease is from 8'/,—11 min. after 64 M.C. and 8 M.C., shifted 
to 11—14 min. after '/, M.C. After still slighter intensity of '/,, M.C., 
the growth decreases only to + 85°/,, after '/,,, M.C. to + 89 °/,. 
Therefore only after these slight intensities the dark-growth-response 
becomes clearly smaller, while the minimum remains just as after 
1, M.C. at 11—14 min. after the beginning of the darkening. 
It is evident in these and other experiments as before, that the 
moments at which maxima and minima occur in the experiments 
with the various individuals are exceptionally constant. Especially 
with threshold-determinations when we can hardly say with 
certainty whether from a greater or smaller number an increase 
resp. decrease of growth may be inferred, the constancy of the points 
of time, at which the phenomena occur is a great aid in stating 
the appearance or non-appearance of an actual response. We want 
to point out in this connection, that Grinpere (1913) in his study 
on negative after-images was also struck with the uncommon con- 
stancy of the moments at which the after-images appear. 
IV. When the cell after adaptation to 64 M.C. is not permanently 
