X.] PROTOCOCCUS. 395 



[B. Physiology. 



1. Get some water that is quite green from containing a 

 large quantity of Protococcus; introduce some of it 

 into two tubes filled with and inverted over mercury, 

 and pass a small quantity of carbonic anhydride into 

 each : keep one tube in the dark and place the other 

 in bright sunlight for some hours. Then measure the 

 gas in each tube and afterwards introduce a fragment 

 of caustic potash into each ; the gas from the specimen 

 kept in the dark will be more or less completely ab- 

 sorbed (= carbonic anhydride), that from the other 

 will not be absorbed by the potash alone, but will be 

 absorbed on the further introduction of a few drops of 

 solution of pyrogallic acid (= oxygen). Protococcus, 

 therefore, in the sunlight, takes up carbonic anhydride 

 and evolves oxygen. A comparative experiment may 

 be made with a third tube containing water but no 

 Protococcus. 



2. Place some water containing numerous zoospores of 

 Protococcus near a window, exposed to bright sun- 

 light. Observe that the majority of the zoospores 

 accumulate on the side away from the source of 

 light. 



Next remove them to some distance from the window, 

 or otherwise diminish the intensity of the light acting 

 on them. Observe that now the zoospores accumulate 

 on the side towards the light. 



The zoospores of Protococcus are thus seen to be 

 photometric^ i.e. they seek or avoid the light according 

 to the degree of its intensity.] 



