IRRITABILITY : INFLUENCE OF LIGHT 205 



1. The indispensability of light for the growth of the 



purple bacteria, and for the development of the 

 colouring matter. 



2. The fact that carbon dioxide is the source of carbon of 



these organisms. 



3. The fact that the part of the spectrum which absorbs 



the greatest amount of energy is also the part at which 

 it is highly probable oxygen is evolved. 



4. The known use of the light in this manner in green plants. 

 Molisch (3) has advanced the opinion that light is used 



to break up the organic matter which he claims is absorbed by 

 the purple bacteria. He brought no experimental evidence 

 in support, and the opinion is evidently unsound, for the 

 purple bacteria do thrive in the complete absence of organic 

 matter, as has been shown by the investigations of Skene, 

 Bavendamm, and others, and the necessity of light is as great 

 in the absence as in the presence of organic matter. 



If photosynthesis does occur in the purple bacteria it is 

 of special interest, for whilst in the green plants the red and 

 the green constituents of light are the most effective, in these 

 bacteria the most active rays are in the infra-red. It is of 

 interest that di|Terent organisms use light of different wave- 

 lengths and suggests that light of other wave-lengths may 

 be utilized by other organisms of whose physiology we have at 

 present little knowledge. 



The Directive Effect of Light. 



Engelmann stated that light exerted no directive influence 

 on the purple bacteria, but Winogradsky maintained that 

 there was such an influence, because the purple bacteria 

 collected on the side of the culture vessel which was turned 

 towards the light. This statement is, however, incomplete, 

 for it is a matter of common observation that in intense light 

 they collect with equal readiness on the side removed from the 

 direction of light. Also it may be pointed out that the 

 appearance of colour does not alone indicate movement, 

 for it may appear as a result of the development of pigment 



