30 4 SCIENCE PROGRESS 



gravity causes them to take the vertical position and they begin 

 to sink. If they are less crowded, they may retain the power of 

 movement but being compelled by the pull of gravity to assume 

 the vertical position, they will be obliged to move upwards. 



If a drop of water crowded with Euglenae be introduced by 

 a pipette into a vessel of water, the Euglenae at once begin to 

 sink ; if they are thinly distributed through the drop, no down- 

 ward movement will be observed and the organisms will swim 

 about in all directions. 



Or we may take a long tube containing water slightly tinged 

 with Euglenae and place it upright in a dark place ; downward 

 streams of Euglenae will be set up here and there ; they descend 

 for a certain distance and then gradually disappear by the 

 spreading out or diffusion of the organisms in the water. This 

 is due to the gathering of the Euglenae into heaps upon which 

 the pull of gravity is effective. If a large number of Euglenae 

 are present the downward streaming is very pronounced and 

 may carry most of them to the bottom, where they remain 

 moving up and down within a distance of from three-quarters 

 of an inch to one and a half inch. 



In the dark or in a faint light the movements of Euglenae 

 are apparently controlled by gravity alone ; in a bright light the 

 action of gravity is negligible and the organisms swim upwards, 

 i.e. towards the light, even when crowded together and this in 

 deep water, such as that in a pond. If the light be cut off or 

 much diminished, gravity acts once more and they sink. This 

 explains the sudden clearing of a pond when a cloud passes over 

 the sun. 



" The upward movement which always regularly follows the 

 downward one is caused by the active movements of the organ- 

 isms themselves. As soon as the congested streaming mass of 

 cells approaches the bottom of the vessel, it begins to spread out 

 and the Euglenae become diffused in the water. The crowding 

 becomes diminished and the result is that the Euglenae are now 

 once more free to move. But they are still too crowded for entire 

 freedom of motion ; their axial orientation is still more or less 

 vertical with the anterior ends upwards ; consequently they are 

 compelled to move upwards. That they are not able to move 

 in any other direction is due to the action of gravity. Freedom 

 of motion in all directions is only possible when the Euglenae 

 are few in number and more or less isolated from each other. 

 The force of gravity always tends to bring them into a vertical 

 position and anything which impedes, even to a slight degree, 



