30 
When the growth of Phycomyces-cells has been 
adapted to intensities of '/,, to 8 M.C., the quantity of 
light still ealling forth a response of growth (the thres- 
hold of stimulation) rises proportionally to that 
intensity. 
In verifying the law of Weser two stimuli are compared with 
each other, which are unequal but applied in the same way, and 
the proportion is determined which is still observed as different. 
Here however the cell has been adapted to an intensity and we 
simply determine the quantity of stimulus which — applied in a 
short time — is threshold of stimulation with that adaptation. The 
cell therefore has been adapted to one stimulus, while the other 
stimulus is quickly applied as a portion. In fact this is another 
and more elementary experiment than the comparison of two inten- 
sities. As however the rule obtained so much resembles the law 
of Wersrr for the comparison of two intensities, it is very probable 
that in point of principle we have to deal with the same phenomenon. 
Not in that sense that with the sensitiveness to light of this single 
cell there would. be question of any psychieal condition or power 
of discrimination, but conversely that these psycho-physical rules 
for the human perceptive faculty are at bottom based on simpler 
reactions in the individual cells to which those rules are already 
applicable. 
It further deserves attention, also with a view to experiments 
and placing in so-called weak light, that in a so slight intensity as 
bera meter-candle these cells are already + 15 times less sensitive 
than in the dark. 
After these quantitative measurings of the adaptation or tone- 
change we have still to emphasize what follows. While the growth 
e.g. in 64 M.C. is only 6 or 7°/, more than in the dark, so that for 
the rest such a cell at its growth cannot be distinguished at all 
from a cell in the dark, there is inwardly after adaptation to light 
quite a different condition, a different ‘‘tone’. For that appears 
directly from the quantity of light that is wanted to induce such a 
cell adapted to light to a light-growth-response. The tone (condition 
of adaptation or degree of sensitiveness) is quite different and espe- 
cially in this response of growth the phenomenon of adaptation 
appears in much purer form than in phototropical movements. The 
phenomena of tone already appearing in phototropical reactions and 
the subject of much study [see e.g. BLaauw (1909), Princsneim (1909), 
