MOVEMENTS OF AQUATIC MICRO-ORGANISMS 309 



not take place but the effect of gravity is, first of all, to cause 

 an orientation of the cells in a vertical direction, with their 

 anterior ends upwards. Their direction of movement is thus 

 definitely determined and as they are not sufficiently crowded 

 for the downward pull of gravity to be effective, they begin to 

 move more or less directly upwards. As this takes place 

 simultaneously in all parts of the dish, they soon become 

 congested as they reach the upper la}^ers ; they lose more and 

 more their freedom of movement and now the conditions are 

 such that the downward pull of gravity becomes operative and 

 the Euglenae begin to sink. At the same moment the cohesive 

 force comes into play and a network-like cohesion figure is 

 formed. 



" If the cell is a shallow one, the aggregation is more regular 

 at the beginning and takes place more quickly than in a deeper 

 one. This is explained by the fact that the upward movement 

 brings them more quickly to the upper surface and that they 

 form here, before cohesion has had time to take effect, a more 

 continuous layer than is possible in a deeper cell, where the 

 upward movement is more irregular and cohesion begins to take 

 effect before they have formed a homogeneous layer." 1 



Our answer to the second question on p. 299 is therefore 

 that Euglena networks are formed by molecular attraction or 

 cohesion, set up, just as in a film of inert particles in water, 

 whenever the Euglenae become massed into a tolerably homo- 

 geneous layer, upon which gravity can act. 



We may now attempt to answer our third question, as to 

 the benefit which the organisms derive from their movements 

 and aggregations. 



Euglena is nourished in two different ways : (1) through its 

 chlorophyll, as in a green plant ; and (2) saprophytically or 

 possibly in animal fashion by the absorption of organic food. 

 Obviously, frequent insolation at the surface of the water 

 would promote carbon fixation but the clustering of the motile 

 cells at the surface must not be dense nor long continued. If 

 it were, the organisms would obstruct one another, assimilation 

 and respiration would be hindered and the organic food in the 

 lower depths of the pool would be insufficiently utilised. The 

 action of gravity obviates such disadvantages by causing densely 

 clustered cells to sink and setting up a continuous circulation 

 of the organisms, which are induced to move steadily from the 

 depths to the well-lit surface and then from the surface to the 



1 Wager, loc. tit., pp. 378-9. 



