84 PLANT PHYSIOLOGY 



others will be found capable of oxidizing other substances com- 

 monly found in waters and soils. 



The Purple Bacteria. — This group of exceptional sulphur bac- 

 teria, in addition to a green pigment resembling chlorophyll (bac- 

 teriochlorin), contains also a reddish-purple pigment called bac- 

 teriopurpurin. A third pigment (bacterioerythrin) has been 

 described by some workers, but may have been confused with 

 bacteriopurpurin. At any rate, by the aid of these pigments, 

 according to Engelmann, these bacteria can manufacture their 

 own organic food in much the same manner as the green plants. 

 If this were their regular method of obtaining food and energy, 

 this group should be removed from the fungi and placed among 

 the algae. But these forms seem also to be able to derive their 

 energy from H 2 S in the same autotrophic, chemosynthetic manner 

 as the other sulphur bacteria. In fact, they seem to be able to 

 use the oxygen liberated in photosynthesis to oxidize H 2 S. This 

 permits them to tolerate very high concentrations of this gas and 

 to thrive where other organisms would perish. These forms seem 

 to be facultative photosynthesizers or chemosynthesizers as the 

 case may be. They, therefore, occupy a very special position and 

 possibly represent (Lubimenko, 1926) transitional evolutionary 

 forms between the chemosynthetic bacteria which lack pigments 

 and the blue-green algae with pigments, which, although auto- 

 trophic, are easily able to use organic food and are thus facultative 

 saprophytes. The facts that bacteriochlorin seems to be an evo- 

 lutionary step in the development of chlorophyll and that bacterio- 

 purpurin is related to the phycocyanin of the blue-green algae 

 support this hypothesis. 



Summary. — In the previous pages it has been shown that there 

 are several methods of procuring food which may be classified as 

 follows : 



I. Energy from photosynthesis. 



a. autotrophic. 



1. from chlorophyll. 



2. from bacteriopurpurin and bacteriochlorin. 



b. heterotrophic — parasites and saprophytes. 

 II. Energy from chemosynthesis. 



The chemosynthetic forms have been shown to be of extreme 

 importance in some cases in the organic cycles. They have also 

 been shown in others, e. g., the iron bacteria, to be extremely 

 important in the production of insoluble iron deposits, thus re- 



