1920] Allen: Plankton of the San Joaquin River 123 



natural bodies of water than it is in artificial cultures. In the pres- 

 ent instance it seems possible that the decline in numbers of certain 

 zooplanktonts toward midday might be due to negative phototropism 

 caused by supersaturation of water by oxygen liberated during photo- 

 synthesis by plants. But it might be due to negative phototropism 

 due to rising temperature, or to negative phototropism or positive 

 geotropism due to increasing agitation of the water, or to various 

 other factors or combinations of factors. 



So far as green organisms are concerned, it may be readily under- 

 stood that photosynthetic and growth and reproductive processes 

 might all be accelerated by the presence of carbon dioxide with a 

 rising temperature and a considerable amount of sunlight (though 

 less than the maximum because of rough water). It is also true that 

 increase of oxygen formation might increase the buoyancy of the 

 plant cells so that larger numbers of them would be in the region of 

 water traversed by the net. This is especially probable in the pres- 

 ence of wind because any agitation of the shallow (two and one-half 

 meters) water would be an aid to buoyancy. 



There is no possibility of segregating tidal from other influences 

 on our present information. So far as the currents, wind and tide, 

 and oscillations of the water are concerned, taken all together, they 

 might induce negative phototropism, or positive geotropism and thig- 

 motropism in the animals. They might also affect the green organisms 

 by aiding buoyancy and by increasing the circulation of the water, 

 thus bringing more carbon dioxide to the absorbing surfaces and 

 rapidly removing oxygen and other waste products from such surfaces. 



Perhaps enough has already been said about temperature, but it 

 will do no harm to recall that it affects flotation of organisms through 

 changes in viscosity, that it probably plays an important part in 

 reversal of tropisms under natural conditions, and that any change 

 either accelerates or retards all of the activities of the organisms. 



For this series the influence of light cannot be segregated from 

 that of temperature since both are dependent on the sun's rays. It 

 may be said, however, that light plays more or less part in reversal, 

 or intensity of reversal, of tropisms of organisms, and that it is of 

 major importance in photosynthesis and thus in effect on the gas 

 content of the water. 



So far as the other factors listed above are concerned, it is not 

 seen that they could be of appreciable influence in this period under 

 the conditions of variability recorded for the organisms and for the 

 general factors involved. 



