145 
or psychological process, at any rate not when the con- 
nection only exists in some degree. 
Also the meaning of the inversion of the reaction is 
entirely unknown to us. It is sufficiently clear that the 
term stimulation is very vague. 
The terms physical process, chemical process, physio- 
logical process and stimulation-process are not suitable to 
keep the various phenomena of apart. 
$ 3 The Geotaxis of Chlamydomonas. 
There is not much literature, treating of geotaxis. The 
negative geotaxis of Paramaecium is well-known, owing 
to fennings’ book. It is easy for every-body to observe, 
that Paramaecium swims upwards in a tube with pure 
water, often many decimeters without getting disturbed 
or to rest for a little while. Of the geotaxis of Chlamy- 
domonas pulvisculus we can read in Schwartz (1884, 
page 51). What is told there is not very convincing. The 
greater part of the experiments are done in wet sand. 
In water the phenomena are not very distinct. If we mix 
wet sand with water, which holds Chlamydomonas and 
let this stand for a time (for instance during six hours) 
in the dark, so we see that many Algae have come above 
the sand. Schwartz concludes that we have to do with 
negative geotaxis and bases his conclusion upon the fact 
that under the influence of slight and avarage centrifugal 
forces, Chlamydomonas pulvisculus behaves itself as towards 
the gravitation, and that when the acceleration of the 
centrifugal force becomes greater than 8!/, X g., the object, 
under the influence of that force turns its motion. 
Then we find something about Chlamydomonas pulvis- 
culus in Massart (1891, page 148). He, too, concludes 
that Chlamydomonas pulvisculus is negative geotactical 
Recueil des trav. bot. néerl. Vol. XVII. 1920. 10 
