148 BEHAVIOR OF THE LOWER ORGANISMS 



reason for swimming toward the source of light is not the progression 

 into a lighter region. But they do not indicate in the least that the 

 reactions are not due to changes in intensity of illumination. So long 

 as turning the sensitive anterior end away from the source of light 

 causes a greater decrease in its illumination than does movement into 

 the slightly less illuminated region, the organism will move toward the 

 source of light. If the difference in intensity of light in different parts 

 of the drop were increased till the change in illumination due to pro- 

 gression is greater than the change due to swinging the anterior end away 

 from the source of light, then the positive organisms would gather in 

 the more illuminated regions. This is the condition of affairs in the 

 experiment shown in Fig. 98. 



In the swarm spores, as in Euglena, the positive reaction usually 

 changes to a negative one when the light is much increased. We can 

 thus distinguish an optimum intensity of light, to which the organisms 

 may be said to be attuned. Either increase or decrease from the op- 

 timum causes the avoiding reaction. Often the organisms are positive 

 when placed at some distance from a window, but become negative 

 when brought nearer. There is much variation among different species, 

 and even among different individuals of the same species, as to the 

 amount of light that causes this change from positive to negative. Some- 

 times, with a given intensity of light, half the individuals of Ulothrix 

 are found to be positive, the other half negative (Strasburger, I.e. p. 17). 

 The same individual is seen at times to be at first positive, later negative. 

 Some of the influences which modify the reaction to light are known. 

 Certain swarm spores are attuned to a stronger light in the early stages 

 of development than in the later stages. Specimens grown in shaded 

 regions seem attuned to less intense light than those living in well- 

 lighted cultures. That is, the organisms are attuned more nearly to 

 the light to which they are accustomed. But subjection to darkness 

 sometimes causes negative organisms to become for a short time positive. 

 Haematococcus is negative in a certain intensity of light, gathering at 

 the negative side of the drop. Now the preparation is covered and 

 left in the dark for a few minutes, then the cover is removed. At once 

 the Haematococci leave the negative side and swim toward the light 

 for a short distance. But this lasts only a moment. After reaching 

 the middle of the drop, they swim back again to the negative side. An 

 increase of temperature increases the tendency to a positive reaction to 

 strong light ; a decrease of temperature has the opposite effect. Lack 

 of oxygen increases the tendency to a positive reaction. This is ac- 

 counted for by the fact that the green organisms produce oxygen in the 

 light. 



