The Reversal of Tropisms 99 



darkness swim toward strong light for a short time 

 before they show the negative reaction. The 

 stronger the light the quicker the negative reaction 

 occurs. 



In addition to these changes in reaction we have 

 the slower adjustment of many organisms to the 

 continued influence of a particular intensity of light. 

 Adaptations of organisms to stimuli to which they 

 are habitually exposed are common, and the cases 

 of so-called light attunement may be regarded as 

 particular instances of this general process. The 

 diatom Navicula brevis is positive only in the weak- 

 est light. Verworn exposed a culture of this spe- 

 cies to a window for two weeks, after which the dia- 

 toms became positive in light to which at first they 

 gave the negative reaction. According to Stras- 

 burger the swarm spores of algae, if kept exposed 

 for a considerable time to relatively strong light, 

 will react positively to a much stronger intensity of 

 light than they did previously. Similar phenomena 

 have been noted by Oltmanns and by Mast in Vol- 

 vox. Ordinarily the exposure of Volvox to strong 

 light, if not prolonged, has the effect of making the 

 organism negative to light in which it had previously 

 been positive. However, a longer exposure to strong 

 light will have the reverse effect of raising the opti- 

 mum, the organisms retaining the positive reaction 

 in light which would otherwise have produced a 

 negative response. 



From the foregoing facts we may conclude that 



