OBSERVATIONS ON UNICELLULAR FORMS 1 27 



each of the two beams was then alternately intercepted, and 

 thus the direction of the rays was changed. It was found 

 that in the process of orientation under such conditions, 

 the swarm-spores always turn away from the source of 

 light, never toward it, but in this turning they merely 

 swerve farther in their spiral path every time that the course 

 in the spiral is directed away from the source of light, and 

 not so far when it is directed toward it. Since the same 

 side is always directed out in the spiral it is evident that 

 they do not turn directly from the source of light. They 

 turn from the source of light only when a given side faces 

 the light, not when the opposite side faces it. 



The orientation therefore takes place in these swarm- 

 spores just as it usually does in Euglena and Stentor when 

 the ray direction is changed without a change of intensity 

 of light. This shows that the organisms are more sensitive 

 when one side is illuminated than they are when the op- 

 posite side is exposed, just as was shown to be true in 

 asymmetrical forms. They are however not very readily 

 stimulated by changes of intensity; it was impossible to 

 induce the avoiding reaction in this way. If mounted on a 

 slide containing a bright area in a dark field, the collection 

 in the brightarea is not definite, as it is in the case of Euglena 

 under similar conditions; the swarm-spores usually pass out 

 and in without any apparent response. If the light Inten- 

 sity Is only moderately changed after they are oriented 

 there Is but a slight increase in the width of the spiral path. 

 If it Is changed much the spores immediately turn and 

 swim up, since they are strongly negative In their reactions 

 to gravity. 



Orientation in these symmetrical forms Is then governed 

 by the same factors as it is in the asymmetrical forms. It 

 Is due to changes of light intensity on the organism. These 

 changes are produced in a field of constant intensity by the 

 rotation on the axis. There is no evidence that light acting 

 constantly as a stimulus has any effect on orientation. The 

 swarm-spores are more sensitive when one side is exposed 



