1920] Allen: Plankton of the San Joaquin River 121 



neighboring organisms of any type. It may be similarly affected by 

 the gaseous content of the water, which in turn is determined to some 

 extent by temperature, light and currents in water and air, and so 

 on with an indefinite number of combinations. It is necessary to limit 

 our present discussion to those combinations which are most probably 

 determinative within the twelve-hour period. Continuing the refer- 

 ence to locomotion with this limitation in view, we have to note that 

 the more typically animal organisms may have been less numerous in 

 afternoon catches because of migration to bottom layers of water, the 

 lowest of which the net did not reach. Such a migration might be 

 due to the influence of several other factors. Kising temperature 

 might induce it directly by discomfort or indirectly by lessened vis- 

 cosity of water, relatively increasing the influence of gravity, or by 

 reducing the gaseous content, probably not greatly effective, or by 

 changing the gaseous content through more oxygen excretion and 

 carbon dioxide absorption by green organisms, or by increase of dis- 

 agreeable excretions from the surface organisms as they became more 

 active under higher temperature. 



Locomotion might also be affected by the wind, directly by dis- 

 comfort due to surface agitation, indirectly by interference with food 

 taking near the surface through rapid oscillations of surface layers 

 of water. Leeward drive of the wind is not considered because it 

 would affect green organisms as nuieh or more than the locomotor 

 organisms. 



Light might affect locomotion directly by discomfort and indirectly 

 by the increase of starch manufacture, in green plants, with the larger 

 amount of oxygen liberation and carbon dioxide consumption, thus 

 shifting the region of the mean gas content to which motile forms 

 are accustomed, to some distance below the surface. 



Locomotion might affect non-motile surface organisms through re- 

 moval of predatory organisms by migration, thus letting growth and 

 multiplication go on unchecked. It would not be necessary for an 

 organism to pass a whole life cycle in order to show this effect. Un- 

 doubtedly there are at any given moment in a plankton population 

 many very young individuals, many just maturing, and many just 

 ready to divide. Unfavorable conditions would arrest or deter devel- 

 opmental processes which would be again accelerated by favorable 

 conditions. At a favorable time, then, many new organisms, which 

 had been restrained during an adverse period, might be liberated and 

 allowed to grow. 



