SECT. II PHYSIOLOGY 221 



carbon compounds which at any moment exist on the earth owes its 

 origin to these Bacteria. The fact of their carbon-assimilation remains 

 none the less interesting, especially since it takes place in an essentially 

 different manner to the assimilation of the green plant. Some other 

 source of energy must take the place that sunlight does in assimilation 

 in a green plant to build up the organic substance ; this energy is 

 obtained by oxidation of ammonia or of nitrites (p. 245). We may 

 therefore term the formation of organic material in green plants 

 PHOTOSYNTHESIS and in the Nitro-bacteria a chemosynthesis. 



3. The Gain in Carbon in Hetepotrophie Plants 



While the gain of carbon from carbon dioxide is to be considered 

 as the typical carbon-assimilation of plants, it is by no means the only 

 method found in the vegetable kingdom. Since it depends — leaving 

 the Nitro-bacteria out of account — on the presence of chlorophyll and 

 of sunlight, it cannot come into consideration in subterranean parts 

 of plants, in all plants that are not green and in the case of all 

 animals. All these are in fact dependent on organically combined 

 carbon which has been derived directly or indirectly by the 

 assimilatory activity of green parts of plants. All organisms which 

 in their nutrition are dependent on the activity of green j^lants are 

 termed heterotrophic ; the green plants and also the Nitro-bacteria 

 are termed autotrophic. The word " auto "-trophic must not be taken 

 too literally, for autotrophic plants also depend on other organisms. 

 It will be seen that life is only continuously maintained on the earth 

 by the changes in substances effected in one direction by particular 

 organisms being balanced by the activity of other organisms. Hetero- 

 trophic organisms show by their mode of life, and especially by the 

 situations in which they live, that they make other demands on food- 

 material than do autotrophic plants. They occur either as parasites 

 on living plants and animals, or they live as saprophytes on dead 

 organisms or substances derived from organisms. 



The demands which heterotrophic plants make on a source of carbon 

 can l)e best studied in saprophytic Bacteria and Fungi. These 

 organisms can be cultivated on various complex su])strata and con- 

 clusions can be drawn from their growth as to the nutritive value of 

 the compounds supplied as food. The nutrient solution must as a 

 rule contain, in addition to the indispensable mineral substances and 

 a source of nitrogen (usually a salt of ammonia), sugar as a source 

 of carbon. It should have a slightly acid reaction for mould-fungi and 

 be weakly alkaline or neutral for bacteria, and is often converted into 

 a solid medium by mixture with gelatine or agar-agar. The sugar 

 can, in many cases, be more or less suitably replaced by other organic 

 substances such as other carbohydrates, fats, albumen and derived 

 substances, organic acids, etc. While these sources of carbon can 



