UNICELLULAR FORMS AND COLOR 325 



P. bursarla collects. The method of collection of Euglena 

 is described as being the same as that of P. bursaria. The 

 region in which they aggregate " wirkt wie eine Falle, 

 denn einmal hineingekommen, gehen die Euglenen in der 

 Regel nicht wieder aus. Sie kehren an der Grenze des 

 Dunkels immer sogleich wieder nur ins Helle." Engelmann 

 does not mention the fact that these forms orient and that 

 they may become negative, and of course did not realize 

 that owing to these reactions they can move directly 

 toward the optimum, so that the aggregation is not 

 entirely due to random movements. According to the 

 work of Engelmann, then, a reduction of light intensity or 

 a change from blue in the region of the Fraunhofer line F 

 to any other color of the spectrum stimulates Euglena and 

 causes it to turn and proceed in a different direction. 



Like Euglena, so specimens of Bacterium photometricum 

 collect in the more highly illuminated regions of their 

 environment, but in the microspectrum, unlike Euglena, 

 most of them collect in the infra-red between 800 and 

 900"^, some in the orange between 580 and 610"" and a 

 few elsewhere. In general they collect in those regions 

 where the absorption bands for the coloring matter they 

 contain are found. Their reactions to light are independ- 

 ent of oxygen pressure. The method of collection is 

 described as being similar to that of Euglena. A reduc- 

 tion of intensity or a change In color from that in which 

 they collect to any other causes the bacteria to swim sud- 

 denly backward (" zuriick schiessen ") 10 to 20 times 

 their length, after which they proceed in the ordinary 

 way again. This reaction has been designated " Schreck- 

 bewegung." An increase of intensity or movement into 

 the regions of the spectrum where they collect does not 

 produce the " Schreckbewegung," neither does a gradual 

 decrease of intensity. Engelmann found but little evi- 

 dence indicating that these organisms orient, and did not 

 ascertain whether or not they become negative in very 

 high intensity. 



