3 o2 SCIENCE PROGRESS 



It seemed possible that convection currents might be con- 

 cerned but both the aggregation and the movements were 

 perfectly maintained in vessels kept at a uniform temperature 

 and close to vessels of water containing light particles of dead 

 matter which gave no sign of convection currents. Currents 

 due to evaporation were precluded by the sealing of the vessels. 



It is well known that under certain conditions micro-organ- 

 isms respond to the action of gravity. Thus it has been found 

 that they may rise to the surface of a liquid (a phenomenon 

 which has been described as negative geotaxis) or sink to the 

 bottom {positive geotaxis). The terms imply that the organisms 

 do not respond to gravity simply because they are heavier or 

 lighter than water but by reason of some kind of sensibility 

 on the part of the organisms which enables them to respond 

 to the gravitational stimulus. 



In discussing the orientation ol a motile organism under the 

 influence of gravity, it is important to consider the purely 

 mechanical possibilities due to density, structure and shape and 

 the position of the centre of gravity. If the organism be heavier 

 than the liquid in which it lives ; if the position which it takes 

 up in moving downwards be the same as that which it assumes 

 when dead ; and if this be in opposition to its normal direction 

 of movement, it is fair to conclude that the downward movement 

 is mechanical and not geotactic. 



Now, Euglena is found to exhibit all these characteristics. 

 It is distinctly heavier than water. The density was determined 

 by means of solutions of gum arabic, varying in strength, in 

 which it was found that Euglena can live for several days. At 

 first they seem to be paralysed but in a short time many recover 

 their mobility. In solutions of density below roi4 they begin 

 to sink at once; when the density is from roi4 to 1*015 they 

 sink more slowly; at roi6 very few sink at once, the majority 

 forming a distinct green layer at the surface, which after a time 

 begins to sink. At I'oiy and roi8 the majority do not sink 

 at all; and at roiq. all remain floating for an indefinite time. 

 The density of Euglena viridis may therefore be set down as 

 between roi3 and roi9; the average density as about roi6. 



Live Euglenae, when sinking in water, take a vertical or 

 slightly slanting position, the fore end, which goes first in 

 voluntary movement, being turned upwards. Euglenae killed 

 by hot water or osmic acid take the same position when sinking 



