8 
The photogrowth-reaction: The space of time in which experi- 
ments with Pellia could be made, being very limited, I could make 
only few observations on photogrowth-reaction. They were performed 
with an energy of light of 75 M.C., the light falling from above 
through 4 mirrors arranged at an angle of inclination of + 45°. It 
was directed horizontally at 4 sides of the object, which was placed 
exactly in the centre of the 4 pencils of rays. In order to watch 
the growing object with the horizontal microscope, we could illuminate 
the field of vision by a fifth mirror arranged in the axis of the 
microscope behind the object, also at an angle of inclination of 45°. 
The observations were always made with red light, so that no 
curvature could be effected when the various sides were differently 
illuminated. The black upper rim of the sporogonium was clearly 
distinguishable; unfortunately nutations most often caused difficulties 
again, because then crooked growth made the measurements un- 
reliable. It appeared to be impossible to continue our observations 
longer than an hour. 
The illumination lasted 5 or 10 minutes (Energy 22500 and 
45000 M.C.S.). We chose this energy because it produced with uni- 
lateral illumination a strong positive curvature. 
I will at once mention that with this quantity of energy Pelha 
yields a distinct photogrowth-reaction, provided the objects have 
been standing in the dark for some time. This also accounts 
for the phenomenon “disposition” (German “Stimmung’’), that with 
unilateral illumination of 45000 M.C.S. objects, which have not been 
previously placed in the dark, or only for a short interval, do not 
yield a distinct curvature or yield only a very slight one, whereas 
the more sensitive objects (vide supra) react very strongly on it. 
Arisz |.c. was the first to conceive this phenomenon of “disposition” 
as a reaction-process and afterwards v. D. SANDER BAKHUYZEN assumed 
the “disposition” to be a difference in the slope of the growth- 
retardation curves, manifesting itself in every process in which the 
reaction does not increase rectilinearly with the stimulus. When the 
object was lighted omnilaterally with 2800 M.C.S. for six minutes 
(on the klinostat), and was subsequently exposed for 20 seconds to 
a unilateral after-illumination of the same energy, no reaction ensued, 
while a strong reaction does ensue, when a unilateral illumination 
with 56000 M.C.S. is applied, in which there is an equal difference 
of energy between the anterior-, and the posterior side. 
Reverting to the photogrowth-reaction:we observe after the addition 
of 4 X 22500 or 4 > 45000 M.C.S. in 5 or 10 minutes u distinct 
retardation of growth, appearing mostly during the illumination, 
