192 
The H-ion concentration, which the algae seek by 
themselves (the optimum-H-ion concentration or the 
optimum acid concentration), and the H-ion concentration, 
in which the transition of positive to negative phototaxis 
takes place, lie, just like the concentration, where the 
fastening to glass is most clearly seen, in the most acid 
solutions. The possibility exists, that just like the H-ion 
concentration, where the fastening is very clear, the two 
other H-ion concentrations too are observed too large 
under the influence of the charge of the glass. To be sure, 
in all cases, where the motility is great, the algae either 
by the reaction on the light or by that to gravity come 
continually into contact with the glass side. It is very 
probable that they vield charge or take up charge; the 
result can be, that many phenomena take place in a more 
acid solution, than when the influence of the glass were 
linked out. We will not discuss all the consequences of 
this. Only one of them is too important not to be dis- 
cussed; the positive chemotaxis of Chlamydomonas with 
regard to acids is only caused perhaps under the influence 
of a negative charge of the glass. The optimum-H-ion 
concentration might even be expected in the isoëlectric 
point, that is in weaklv alkaline fluids, by which the 
positive chemotaxis would change with regard to acids 
into a positive chemotaxis with regard to bases. 
The charge of the glass might also have had an influ- 
ence on the values which we found for the limiting con- 
centrations. By this might be caused a removal of the 
curves to a greater H-ion concentration. However this 
removal cannot have been very great, since the motility 
of Chlamydomonas in the solutions with the limiting 
concentrations was not great, by which collisions with 
the glass side did not occur at all or only in a small number. 
