122 GRAVITY AND PROTOPLASM [Ch. V 



almost without exception with the broader flagellate pole down- 

 wards. Both from the fact that it can be easily demonstrated 

 that when a body heavier than water falls in that medium its 

 larger end will precede, and from the fact that Jensen was 

 especially careful that the killed organism should not be 

 deformed, his results must be considered the best established. 

 Now, since the dead Euglena tends to sink with flagellum 

 downwards whereas the active Euglena stands flagellum 

 upwards, we must conclude that the orientation of Euglena 

 and probably other Protista is not passive but due to their 

 activity and must be regarded as a response more or less directly 

 due to gravity. 



But just how does gravity act as a stimulus to determine the 

 direction of orientation of the body? We have two principal 

 theories to examine. First, that of Jensen, that gravity acts 

 indirectly on the organisnl by directly causing a difference in 

 pressure in the water at different levels. This difference in 

 water pressure, at various levels, affects directly the two poles 

 of the organisms, which stand at different levels, and the 

 organism responds to this difference in pressure. The second 

 theory, which I adopt, is that the organism, owing to its specific 

 gravity being greater than the medium, experiences greater 

 resistance (friction + weight) in going upwards even to the 

 slightest extent than in going downwards (friction — weight). 

 Another stimulus, which is probably associated with this, de- 

 pends upon the fact that an unsymmetrical body, heavier than 

 water, tends to fall with its larger end down. Those nega- 

 tively geotactic organisms, which stand with their larger end 

 up, will be consequently in a condition of unstable equilibrium; 

 those organisms which stand with their larger end down will 

 be in stable equilibrium. In the first case a deviation from 

 verticality would be accompanied by relatively diminished 

 resistance on one side ; in the second by relatively increased 

 resistance on one side. In either case, the distribution of the 

 mass of the animal may give the organism the means of deter- 

 mining, but not in a mechanical way, the position of its axis. 



The evidence for the first theory Jensen finds especially in 

 a fact which he believes opposes the second. Negatively 

 geotactic organisms, placed in an inclined tube, move towards 



