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actively downwards under the influence of the gravitation. 
The fact that in strong light, we get the impression of a 
passive motion, is caused by the negative phototaxis. By 
this the objects always remain near the glass wall. Under 
the influence of the phototaxis, the geotaxis and of the 
impediment of the glass, the direction of the movements 
becomes rather irregular. 
$ 4 Electrolytes can prevent the reaction to gravity. 
Ï have been astonished at the fact, that I did not find 
anywhere a word about this positive geotaxis of Chla- 
mydomonas and think this is caused by the circumstance, 
that this property only crops up, when the fluid is neutral 
and there is not too much salt in the solution. In distilled 
water the phenomenon has a splendid course. AÀt last the 
organisms form green grounds on the bottom of the tube 
(see fig. 1 C. page 34). I used for these experiments 
narrow glass tubes, which were tapering at the bottom 
and had been closed by melting. So the grounds could 
be seen very distinctly, it made the impression of a settled 
precipitation (see fig. 1 C. page 34). With the microscope 
we could notice that the movement had not at all stopped, 
the algae continued very movable and swarmed in the 
point of the tube. If we put the tube with the settled 
algae in the dark and if we let this stand for half an 
hour or an hour, then we perceive, that the sediment has 
been enlarged, in course of time the algae spread diffusely 
in the lower part of the tube. 
Led on by statements in literature, Î thought, that this 
diffusely spreading might have something to do with want 
of oxygen or with aërotaxis. In stead of common distilled 
water, [| now used boiled out, distilled water. If the want 
of oxygen played a part, the moving downwards should, 
