The effect of gravity upon the movements and aggregation of euglena viridis, ehrb., and other micro-organisms

Abstract

In the course some experiments on the effect of various physical forces upon the movements of Euglena 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 aggregation is only visible when the Euglenæ 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 Euglenæ 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 consisting 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.