10 
very slow rise, then proceed horizontally over some length, (may be 
with slight falls), sueceeded by an ascending portion. For the present 
it is impossible to give an accurate description *), which requires a 
detailed examination, with omnilateral fore-illuminations, succeeded 
by unilateral ones. [ purpose to examine this more closely later on. 
Perhaps then also negative reactions will become evident. 
Geotropism: Only few experiments were made regarding geotro- 
prism; in nature it is negative in the stems. When the excitation 
was prolonged with vertical position of the thallus (stem horizontal), 
the experimental reaction time was 80—90 min. So we see that 
the difference in the rate of photo- and geotropic reaction is not so 
great as in Avena. 
The experimental threshold-value for geotropism was + 10 min., 
the experimental reaction-time 150 min. 
By experiments with the centrifuge I tried to ascertain whether 
in geotropism the experimental reaction-time was equally dependent 
on the quantity of energy. The object appeared however unsuitable 
for this purpose, the flaccid stems are bent too much out of shape. 
Also the dishes proved impractical for this object. In experiments, 
on an inclined plane of 45° 1 found however, a much longer reac- 
tion-time. 
Finally I wish to add a few remarks about the mechanical side ~ 
of the curvature-process and the photogrowth-reaction in objects 
like Pellia, in which there is no question about conduction of exci- 
tation, and its progress, therefore, can be simpler than in Avena. 
When we try to find an explanation for the growth and the curvature 
of multicellular plant-parts, two factors may be considered; viz. 
change of the turgor-pressure or of the cell-wall. At one time 
the first was believed to be the chief factor, at present the 
second’). The turgor-pressure is, indeed, a necessary condition for 
the growth, but the changes of the cell-walls must be looked upon 
as the principal factors. This is borne out by the fact that during 
the winter-rest the osmotic pressure of the stem-cells is high in Pellia 
and still no growth reveals itself. On the contrary it begins and 
reaches its culmination point on the apex of the great period, when 
the osmotic pressure is regularly decreasing. It thus appeared that 
in comparing the convex and the concave sides of stems, which had 
just been curved, there was no difference in osmotic pressure worth 
mentioning. Nor does a distinct curvature brought about by unilateral 
1) The slope of the growth-retardation curves is sure to decrease slowly, since 
with fore illumination the threshold value becomes higher. 
4) W. PreFFER Physiology, II. Cap. XIII. 
