212 Wager : Movements of Aquatic Micro-Organisms. 
on the part of the organisms which enables them to respond 
to the gravitational stimulus. Thus it has been found that 
they may under certain conditions respond to this stimulus by 
rising to the surface of the liquid ( negative geotaxis) or by 
sinking to the bottom ( positive geotaxis). These terms cannot 
however be properly applied to the rising and sinking of 
Euglena, for both these movements can be accounted for by 
the purely mechanical action of gravity in which nothing of 
the nature of a geotactic response is involved. The down- 
ward pull of gravity operates only when the organisms are 
crowded together and their freedom of movement is in con- 
sequence impeded. Under these conditions they sink in the 
water in a vertical or slightly slanting position, the heavier 
posterior end being downwards and the fore end, which goes 
first in voluntary movement, being turned upwards. Euglena 
killed in the expanded condition by hot water or osmic acid 
take the same position when sinking in water. 
The upward movement is also brought about by the 
mechanical action of gravity. If, for example. Euglena 
which are swimming in a horizontal position towards the light 
are suddenly shaded, the forward movement is slowed and they 
immediately swerve, under the influence of gravity, into a 
vertical or nearly vertical position with their posterior ends 
downwards. In this position they will be obliged to move 
upwards, unless their freedom of movement is impeded by 
overcrowding. So long as Euglena are able to move freely 
in the water, the action of gravity has very little effect upon 
them, but if their movements are interfered with in any way, 
gravity at once begins to act. 
Combined movements due to light and gravity can be 
frequently observed. If a shallow vessel containing Euglena* 
is placed in an oblique light, the Euglena rise through the 
water towards the source of light and form a green film on the 
surface of the water. This film is at first homogeneous, but 
in a very short time it becomes broken up into ripple-like 
aggregations which move slowly across the surface of the 
water towards the light. These ripples are due to the action 
of gravity. The Euglena at the beginning of the experiment 
were more or less evenly distributed in the water, but as the 
light attracts them more and more towards the side of the 
dish, they become more and more crowded until at last they 
become so densely aggregated that their freedom of movement 
is impeded and gravity at once begins to act, the surface 
film becomes broken up and streams of Euglena sinking 
downwards may be observed. As the cells become clustered 
on the side of the vessel nearest the light, they form a dense 
layer in the form of a green line at the edge of the water. 
From this green layer streams of cells may be seen descending 
Naturalist , 
