NUTRITION IN THE LOWER PLANTS 81 



with some antiseptic substance such as tar which hinders the en- 

 trance and growth of fungi. 



Chemosynthesis. — All the forms of plants thus far mentioned, 

 including autotrophic forms, saprophytes, and parasites, have 

 obtained their food from organic compounds built up by means of 

 energy derived from sunlight through the agency of chlorophyll. 

 For all practical purposes this is the only way in which food can 

 be manufactured. There is, however, a small group of fungi 

 which, although lacking chlorophyll or an analogous pigment, 

 are nevertheless able to build up their organic food in an entirely 

 different manner quite independently of sunlight. These plants 

 use the energy derived from the oxidation of inorganic compounds 

 to build up organic foods, thus exhibiting a physiology of a very 

 different type, which up to the present time has not been thor- 

 oughly investigated. This construction of food by means of energy 

 derived from the oxidation of inorganic compounds or by other 

 similar chemical reactions has been called chemosynthesis. 



The most important of these forms are the nitrifying bacteria 

 (Nitrosomonas) of the soil which are able to oxidize ammonia into 

 nitric acid and construct the foods for their own bodies with the 

 energy derived. 



NH 3 +20 2 = HN0 3 +H 2 0+energy. 



Ammonia gives 5.3 calories of heat per gram while starch gives 

 only 4 ; so that it would not be surprising if the plant not only used 

 the oxidation energy produced (in place of light) for building a 

 part of the necessary food, but also used the ammonia as food. 

 This is a phase of the activity of these bacteria which needs thor- 

 ough investigation. To how great a degree is the ammonia used 

 as food and to what extent is the energy derived used as a substi- 

 tute for sunlight? Is this oxidation a substitute for photosynthesis 

 or is the ammonia a substitute for organic food? Winogradski 

 found that when grown in a nutrient medium containing no 

 organic food of any kind the bacteria grew and formed an ap- 

 preciable amount of organic substance, indicating that we are 

 dealing with a genuine chemosynthesis. Godlewsky (1896) further 

 showed that these same bacteria obtained their carbon from 

 carbon dioxide of the air and not from carbonates in the nutrient 

 medium. As will be emphasized later (Chap. IX) these bacteria 

 play an important part in the nitrogen cycle. 



