207 
MOVEMENTS OF AQUATIC MICRO-ORGANISMS 
IN RESPONSE TO EXTERNAL FORCES. 
HAROLD WAGER, F.R.S. 
(PLATE XIX). 
( Continued from page 178). 
I do not suggest that the Euglenae have any conscious power 
of choice, but it does seem to me that these and similar experi- 
ments with other organisms indicate clearty that the nature 
of the response to the light must depend upon the protoplasmic 
constitution of the cell or, as Jennings terms it, its ‘ physio- 
logical state.’ 
In discussing the effect of light intensity upon the move- 
ments of micro-organisms we must consider the effect of light 
from two different sources upon the direction of locomotion. 
Loeb states that if two sources of light of equal intensity are 
at an equal distance from a heliotropic organism, it will move 
in a direction at right angles to a line connecting the two 
sources of light, that is, not in the direction of the rays of 
light, but along a path which is the resultant of the attraction 
or repulsion exerted by the two sources of light. If, however, 
the two sources of light are of different intensities the organism 
is oriented by the stronger of the two. Mast, on the other 
hand finds that the organisms always move along a resultant 
path whether the sources of light are equal or unequal. Thus, 
when the two sources of light were equal (? approximately 
equal), Euglenae moved in a general way towards a point 
very nearly half way between the two sources of light. But 
when the light was unequal, they moved towards a point 
nearer the source from which the more intense light came. 
Mast argues from this that the direction of the rays of light 
does not absolutely control the direction of motion. But 
surely the exactly opposite conclusions would be the more 
correct. If the organisms do actually move along a path 
which is the resultant of the light acting at the two different 
sources, it seems to me that this would lend very strong support 
to the view that it is the direction of the rays of light and not 
the intensity that is all important. 
But my own experimental observations on the movements 
of Euglenae and of barnacle larvae under the influence of two 
sources of light placed at right angles, do not altogether 
support Mast’s results. I find that although the organisms, 
at the beginning of the experiment move more or less along a 
resultant path, they ultimately accumulate near the sources of 
light in numbers which are approximately proportional to 
the strengths of the two sources of light. Thus, if the lights 
are of approximately equal intensity they accumulate in 
1914 July 1 . 
