VII. On the effect of gravity upon the movements and aggregation of Euglena viridis, Ehrb., and other micro-organisms

Abstract In the course of an investigation upon the effect of various physical forces upon the movements of Euglena viridis, I found that under certain conditions a peculiar aggregation into networks or separate groups occurs in which the Euglenæ remain in an actively motile condition. Nägeli in 1848 had observed groupings of a similar kind in other organisms, and Sachs in 1876 found that certain green swarm cells exhibited a peculiar net-like pattern, which he ascribed to currents set up in the water by the unequal distribution of heat. The aggregations described in the present paper are, however, as I shall endeavour to show, not due to heat, but are in large measure the mechanical result of the action of gravity combined with cohesive forces acting upon organisms heavier than water. In the first part of the paper a description is given of the movements of Euglena and the various experiments made to determine the conditions under which the aggregations of this and similar organisms occur. The second part contains a general discussion of the facts observed and the conclusions drawn from them.