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demarcation of negatively phototactical to unsensible to 
light and of positively geotactical to unsensible to gravity. 
Besides the fact that the individuals of Chlamydomonas 
stick to the glass side with the cilia, it can also.occur, 
that the cilia of various individuals are attached to one 
another. Under the microscope we often see algae that 
stick to each other in this way. With the experiments 
on the reaction to gravity this mutual sticking together 
was often so general, that it became with the naked eye 
visible. In the green stripe which was formed under the 
influence of the light on the shady side of the tubes, we 
saw then dense green spots, which moved down much 
more quickly than the rest of the stripe. With the micros- 
cope it could be seen, that the green spots consisted of 
aggregations of the algae. 
$ 7. The Chemotaxis and Aërotaxis of Chlamydomonas. 
With the help of the capillary-method I found the 
Alga positively chemotactical for all sorts of acids 
(inorganic acids, carbonic acid, acetic acid, malic acid, 
phosphoric acid), and positively chemotactical for acid 
and alkaline phosfates; not for malates, To alkaline 
solutions the algae reacted exclusively negatively. 
Again and again we find in literature statements about 
complications with the reactions of Chlamydomonas which 
are ascribed to positive chemotaxis to the oxygen of the 
air (aërotaxis). [ tried to make out, whether we may 
ascribe to Chlamydomonas aërotactical sensibleness. For 
this we must try the method whether we can get an 
»OXygen-line” under the cover-glass, according to Engel- 
mans data (1881). Since however, we have to do here 
with a green Alga, that assimilates carbonic acid in the 
