194 SULPHUR BACTERIA 



Methods of Investigation. 



Light. — The procedure in determining the influence of light 

 is simple. The organisms are exposed to light of varying 

 intensity and direction. By projecting a spectrum on to the 

 organisms their movements through the different colours 

 may be noted. In various simple ways it is also possible to 

 expand or contract the length of the component colours of the 

 spectrum, and to alter their intensity. In the examination of 

 the effect of colour elaborate apparatus has been used which 

 will be explained later. 



Chemicals. — Pfeffer's Capillary Tube Method is used for the 

 determination of the effect of chemicals on the sulphur bacteria. 

 A piece of capillary tube of approximately 0-05 mm. bore by 

 I cm. long is sealed at one end, filled with the salt solution under 

 investigation, and completely immersed in the fluid containing 

 the bacteria. If sensitive, the organisms move either towards 

 the open end of the tube, or away from it. Some of the 

 movement is due to diffusion between the two fluids, but its 

 extent can be estimated by control experiments. The move- 

 ments usually occupy one to several days before they are 

 completed, and by a system of controls it is possible to allow 

 for all other influences except that due to the fluid under 

 investigation. By using a fluid which does not contain 

 nutrient matter multiplication of the bacteria is avoided. The 

 entrance of the bacteria into the capillary tube can be easily 

 followed when viewed under the microscope. 



Influence of Light on the Coloured Sulphur Bacteria. 



The colourless sulphur bacteria are indiftcrent to light, 

 whether they are cultivated in the dark or in the light. On 

 the other hand, the coloured sulphur bacteria are extremely 

 sensitive. Light affects plants in various ways. A distinction 

 must be made between the tonic, the directive, and the photo- 

 synthetic effect of light. An organism like Volvox, for example, 

 is influenced by light in three different ways. Light is necessary 

 for its continued health, and so exerts a tonic influence ; its 

 direction of movement is determined by the line of incidence 



