MOVEMENTS OF AQUATIC MICRO-ORGANISMS 
IN RESPONSE TO EXTERNAL FORCES.* 
HAROLD WAGER, F.R.S. 
In t choosing as the subject of my address the ‘ Responses of 
Micro-organisms to External Forces,’ I am fully aware that we 
are still far from a complete understanding of the physiology 
of even the most simply constituted organism. But anyone 
who has observed, under the microscope, the varied movements 
exhibited by microscopic organisms, must have been led to 
reflect upon the causes to which they are due and to what 
extent their movements may be the result of conscious or 
unconscious efforts. All such living organisms require food 
and oxygen. Have they any power of judgment, or any 
special sense organs by means of which they may be directed 
to places where food and oxygen are to be found ? In other 
words are their movements purposeful, or are they merely 
mechanical responses to such external forces as light, heat, 
gravity and the like ? 
I remember on one occasion stopping to look at a large 
shallow pond which was of a bright green colour owing to a 
multitude of chlorophyll containing organisms. When I first 
looked at it the water appeared to be a dense and uniform 
green liquid, but a minute or two later it was seen to change 
its character. The organisms were settling down to the bottom, 
not uniformly but in irregular threads or streaks, which before 
long rested on the mud leaving the water clear. Here they 
formed a coarse network with meshes of varying size on the 
surface of the mud. This sinking of the green particles in- 
variably took place during the passage of a cloud over the sun ; 
when the sun broke out from the cloud, they slowly rose to 
the surface again. Sometimes a flocculent appearance, at 
other times beautiful streaks of green extending in all directions 
were observed. These varied movements, the rising and falling 
of the organisms, the formation and breaking up of the net- 
works and other groupings all appeared to be regulated by the 
varying intensity of the sunlight. 
When, however, we seek to inquire into the means by which 
the organisms respond to the action of light, we are confronted 
with other factors, such as gravity and temperature, which 
must be considered before we can arrive at any satisfactory 
explanation of the movements observed. Experiments must 
be so conducted that the conditions under which the move- 
ments take place can be modified one by one. 
Among the various organisisms that have been thus 
*Presidential Address to the Yorkshire Naturalists’ Union for 1913. 
1914 J une 1, 
