DIV. ii PHYSIOLOGY 255 



chlorophyll and of sunlight, it cannot "come into consideration in sub- 

 terranean 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 plants are 

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

 termed autotrophic. 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 be best studied in saprophytic Bacteria and Fungi. These 

 organisms can be cultivated on various complex substrata, 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 

 be placed in order as regards their nutritive value for any particular 

 organism, this cannot be done generally ; there are many saprophytes 

 which are adapted to quite peculiar conditions and use in preference, 

 as a source of their carbon supply, compounds, which for the majority 

 of other plants have scarcely any nutritive value (e.g. formic acid, 

 oxalic acid). 



Even the saprophytes which succeed on very various compounds of carbon 

 (omnivorous saprophytes) are capable of distinguishing between them. Thus from 

 ordinary tartaric acid Penicillium only utilises the dextro-rotatory form, and Bacillus 

 subtilis only the laevo-rotatory form. Aspergillus growing in a mixture of glucose 

 and glycerine utilises the former first ("election" of nutritive materials). If the 

 glycerine alone is given, it is completely utilised. 



The power possessed by many Fungi of utilising such organic 

 compounds as starch, cellulose, etc., which are insoluble in water, is 

 very remarkable ; these substances can only be absorbed after a process 

 of transformation and solution. The Fungi and Bacteria in question 



