ig6 SULPHUR BACTERIA 



The following is a summary of his conclusions : — 



1. Within certain limits, a direct correlation exists between 

 the velocity of the organism and the intensity of light. The 

 dependence of one factor on the other is more marked at 

 low oxygen pressures. 



2. The movement completely stops when the organism is 

 left in the dark beyond a certain period, and if kept in the dark 

 beyond a further period it will not resume movement when 

 again exposed to light. 



3. Different species show responses which differ both in 

 kind and in degree. An increase of intensity produces different 

 rates of increase of velocity in different species. In some 

 species an increase of intensity may even result in a decrease of 

 velocity. 



4. The change of velocity is influenced by the amount of 

 HgS present. 



5. Exposure to light of constant intensity ultimately 

 effects a stoppage of movement. When a quiescent organism, 

 kept in the dark, is exposed anew to light, movement does not 

 begin until a certain interval elapses. Engelmann called this 

 delay in response Photokinetic Induction. The converse pro- 

 cess, namely, the cessation of movement as a result of the 

 removal of light also takes place after a slight time interval. 

 This was named Photokinetic After-effect. 



These delays in response to a given stimulus are comparable 

 to the " presentation time " which elapses in roots of higher 

 plants before geotropic curvature takes place. 



Shock Movements (Schrenk Bewegungen). 



The curious effect was observed in many of the sulphur 

 bacteria that the sudden removal of light resulted in first a 

 stoppage, then a slight backward movement, and finally a 

 forward movement once more, but with diminished velocity. 

 The organisms react as though they had received a slight shock 

 from the sudden change. The shock becomes weaker on repe- 

 tition, and is much less pronounced when it takes place in 

 a medium rich in oxygen. The extent of the reaction differed 



