THE ACTION OF GRAVITY UPON THE 

 MOVEMENTS OF AQUATIC MICRO- 

 ORGANISMS 



By HAROLD WAGER, F.R.S. 



Last July, while taking a country walk, I stopped to look 

 at a large shallow pond which took a bright green colour 

 from a multitude of living particles charged with chlorophyll. 

 The pond was about six yards across, perhaps a foot deep at 

 the centre and very shallow towards the margin. The sun 

 shone but was frequently obscured by small clouds. On step- 

 ping close to the brink, the water seemed to be a dense and 

 uniform green liquid but a minute or two later it was seen 

 to change its character. A green sediment seemed to be 

 settling down to the bottom, not uniformly but in irregular 

 threads or streaks, which before long rested on the mud, 

 leaving the water clear and colourless. The sediment was not 

 spread out equally but formed a coarse network, with meshes 

 three or four inches in diameter, much smaller, however, in 

 the shallow parts of the pond. This sinking of the green 

 particles invariably took place during the passage of a cloud 

 over the sun ; when the sun broke out from the cloud, they 

 slowly rose to the surface again. Descending particles were 

 instantly arrested by the reappearance of the sun and began 

 to move towards the surface. The meshes were most apparent 

 during sinking or after settling to the bottom. In a faint light 

 the green streaks showed a tendency to extend themselves 

 transversely but in a bright light they took a direction more 

 nearly parallel to the rays. I spent an hour or so by the 

 pond, watching the rising and falling of the particles, as well 

 as the alternate formation and dissolution of the network and 

 satisfied myself that the changes constantly taking place were 

 regulated by the varying intensity of the sunlight. The green 

 organism present in this pond is known as Chlamydomonas; 

 it is closely allied to the Volvox familiar to naturalists ever 



since the time of Leeuwenhoek. 



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