6 
The light from the source of 0.1 C.p. was dimmed by paper. 
Control-experiments were made with a continuous illumination of 
85 M.C., in which within an hour (55 or 60 minutes) a positive 
reaction was achieved. Then we ascertained the effect of several 
quantities of light-energy. The results are shown in the annexed 
table; they have been arranged according to the rising quantities 
of energy. The experimental reaction-time is expressed in minutes, 
the reaction is indicated by the symbols: 0 == no reaction ; ? = doubt- 
ful reaction, curvature in fewer than 10 out of the 20; +? = faint 
positive reaction; + = distinct reaction; + + = strong reaction, 
+ + + == very strong reaction; — ? = doubtful negative. reaction. 
In the experiments designated with (a.c.o.) the rays were trans- 
mitted through a dish filled with ammoniacal cupric oxide, (the walls 
of the dish running parallel) in the experiments designated with (w) 
the rays went through a dish filled with water. 
As appears from the table the experimental threshold-value') lies 
in the neighbourhood of 400 M.C.S.; this value might be determined 
with greater accuracy if the material were more homogeneous, and 
in that case it would perhaps prove to be lower. At any rate the 
sensitivity is here much smaller than in Avena. 
The table also shows us the “product rule” for the threshold 
value’), further that the quantity of energy is invariably the decisive 
factor. Likewise it substantiates Arisz’s conclusion that to every 
quantity of energy belongs a maximal curvature of definite magnitude 
and shape. 
The large decrease of the experimental reaction-time with increasing 
energy is very striking here; for the threshold-value it is + 150 
minutes; for the highest quantities of energy it is only 15 min. 
(+ 15 million M.C.S.). With long stems and unilateral clear daylight 
the time is +10 min, in the sunlight even less, viz. 5 min. The 
decrease of the exp. reaction-time is pretty regular, but in the 
neighbourhood of the weak reactions the duration is difficult of 
determination, or in Pellia this time might be broadly considered 
as an index of the strength of the stimulus. Whereas in Avena the 
shortest experimental reaction-time appears already between 50 and 
100 M.C.S., it does not manifest itself here under 15 million M.C.S. 
Pronounced negative reactions were not noticeable in Pellia. We 
did observe, however, a retrogression of an appearing positive reac- 
tion (not past the O-position, however), viz. at 3—7 million M.C.S. 
Between the strong positive reactions at 40000 and 15 million 
1) Arisz le. and Rec. d. trav. bot. neerl. Vol. XIII 1914—1915. 
2) Bhaauw A. H., Die Perzeption des Lichtes Rec. d. trav. bot. neerl. 1909. 
