UNICELLULAR FORMS AND COLOR 323 



a. Diatoms and oscillaria with different species of Na- 

 vicula and Pinnularia as types (1882, p. 390). — These 

 organisms are active in dayhght or in the yellow, orange 

 and red of the spectrum, but become quiet if the Hght 

 intensity is decreased or if they are transferred to the green, 

 blue or violet regions. In these colors they may however 

 become active again if the intensity is sufficiently in- 

 creased. It is evident that owing to these reactions they 

 would tend to collect in the spectrum toward the violet 

 end, or at least leave the opposite end, since they become 

 quiet whenever they chance to get out of the yellow, 

 orange or red. This region however has the greatest 

 stimulating efficiency. Engelmann thinks that the reac- 

 tions of these organisms are dependent upon the liberation 

 of oxygen, and that they react primarily to the longer 

 light waves because of the liberation of oxygen when they 

 are exposed to them. This conclusion is based upon the 

 fact that a reduction of oxygen pressure without a change 

 of light intensity or color causes cessation of movement. 

 It appears then that these organisms do not respond to 

 sudden changes in light intensity and that the reaction to 

 light is dependent upon the absolute intensity rather than 

 upon change of Intensity. 



b. Ciliates which have chlorophyll with Paramecium 

 bursaria as a type (1882, p. 393). — Engelmann found that 

 these organisms do not react to light at all, unless the 

 oxygen pressure is reduced below the normal. If the air 

 surrounding the preparation Is replaced with hydrogen 

 they become very active (" sehr unruhig ") and aggregate 

 in the region of highest light intensity. Under such con- 

 ditions they also collect in the red of the gas or solar micro- 

 spectrum between the lines B and C at 650-700''''. 

 Engelmann describes the process of aggregation as fol- 

 lows: " Ueberschreiten sie z. B. zufalllg die Granze von 

 Licht und Dunkel, oder tauchen sie auch nur mit der vor- 

 deren Halfte Ihres Leibes eine Strecke welt in das Dunkel 

 ein, so kehren sie sofort um nach dem Licht, wie wenn das 



