94 
The Effect of Gravity upon the Movements and Aggregation 
of Euglena viridis, Hhrb., and other Micro-organisms. 
By Harotp WaGER, F.R.S. 
(Received August 2,—Read November 17, 1910.) 
(Abstract. ) 
In the course of some experiments on the effect of various physical forces 
upon the movements of Huglena viridis, it was found that, when placed in 
the dark in shallow vessels or narrow tubes, a peculiar aggregation of these 
organisms takes place into network-like patterns or more or less well-defined 
circular groups. The ageregation is only visible when the Euglene are 
abundant, and is more regular and pronounced in a shallow vessel than in a 
deep one, especially if it is completely filled with the liquid containing the 
organisms, and sealed up so that the upper surface is not in contact with the 
air. If a narrow tube filled with water containing sufficient Euglene to give 
it a pronounced green colour is placed horizontally in the dark or in a weak 
light, the aggregation takes the form of a series of nearly equally spaced 
groups, like green bands, crossing the tube from one side to the other, and 
extending along its whole length. 
Each group shows clearly two distinct regions, a central denser one 
consisting of cells moving downwards, and a lighter peripheral area con- 
sisting of cells moving more or less regularly upwards. There is, in fact, a 
constant cyclic movement downwards and upwards, which is kept up so long 
as the aggregation persists. Examination with a pocket lens shows that, as 
the organisms reach the bottom of the stream, they gradually separate from 
one another, and begin to move upwards. As they reach the upper surface, 
they are seen to be drawn towards the central denser region of the group, and 
again enter the downward stream. 
This aggregation, with its regular cyclic movements, may persist for 
several days, until the Euglenz die, in fact, provided they are kept in the 
dark or under red glass. In a good light the aggregation soon disappears, 
but reappears again in a few seconds when placed in the dark. So also, 
‘if the aggregation is caused todisappear by shaking the vessel in which the 
Euglene are contained, it reappears again immediately the disturbance has 
ceased. 
The ease and rapidity with which the aggregation takes place appear 
to depend, among other things, upon the activity of the Euglena. Anything 
