ASSIMILATION OF CARBON 63 



but their source of energy is very different. Another example of this group is furnished 

 by the sulphur bacteria (as Beggiatoa), which oxidize hydrogen sulphide to sulphur 

 and water, thus securing an energy supply. The sulphur produced is finally oxidized 

 into sulphates, such as calcium sulphate. The sulphur bacteria grow in the presence of 

 organic material. Some hydrogen bacteria (Hydrogenomonas) can form organic 

 material from hydrogen, oxygen, and carbon dioxide, in the absence of organic 

 compounds. Hydrogen is oxidized, thus supplying energy. In the presence of 

 organic compounds hydrogen is not oxidized, and these bacteria are then to be 

 considered as belonging to the preceding group. 



This whole matter of the carbon nutrition of plants may be stated as follows: 

 Apparently all organic compounds in plants are formed, directly or indirectly, from 

 carbohydrates fsuch as sugars). (1) The carbohydrates used may be formed in cells 

 with chlorophyll, out of carbon dioxide and water, and by means of sunlight energy. 

 (2) The carbohydrates used may be formed in cells without chlorophyll, out of carbon 

 dioxide (or carbonates) and water, by means of energy obtained through the oxidation 

 of inorganic substances such as ammonia, sulphur dioxide, hydrogen, etc. (3) The 

 carbohydrates used may be derived from the surroundings, either ready-made or else 

 by the decomposition of other organic compounds that are themselves supplied ready- 

 made in the surroundings. These other organic compounds may also be used directly, 

 without the preliminary step of forming carbohydrates. There are just two general 

 sources of energy for plant activities, (a) sunlight and (b) energy derived from the 

 oxidation of substances; and the substances oxidized may be either organic or inorganic. 



4. Microorganisms in Nature. — Since Spallanzani's time it has been known that all 

 organisms are formed by the reproduction of other organisms, and that the micro- 

 organisms found everywhere in nature arise in this way. On the basis of this principle 

 Appert originated the art of preserving foods by sterilization. If all organisms in a 

 preparation are killed at the start, and if no more are allowed to enter from without, 

 there will be no living ones in the preparation. Fermentation and the decay of foods 

 are generally caused by microorganisms, and these substances may therefore be pre- 

 served by sterilizing and then hermetically sealing them. This whole proposition was 

 finally clearly worked out by Pasteur, who showed, among many other things, that the 

 microorganisms that cause fermentation in foods, etc., originate from individuals of 

 the same forms, which fall in from the air, etc. The air generally contains large num- 

 bers and many kinds of microorganisms as do also soil, water, the human alimentary 

 tract, etc. 



5. Sterilization and Disinfection. — To obtain objects or material absolutely free 

 from living microorganisms sterilization is necessary. In many cases this is accom- 

 plished by dry heat. In other cases steam is used, especially in a closed chamber, 

 such as the autoclave. The heat must be applied for an adequate period, and the 

 temperature must be sufficiently high. Liquids are frequently sterilized by passing 

 them through a suitable filter (such as the Chamberland), which retains the bacteria, 

 etc. Sterilization may also be accomplished by the use of antiseptics or disinfectants, 

 such as mercuric bichloride, phenol, etc. These simply poison the microorganisms. 



6. Pure Cultures. — Pure cultures of any given kind of microorganism may be 

 obtained by inoculating a suitable medium with a single cell of the form desired, 

 and allowing this to develop without the entrance of any other cells. Unless obtained 

 in this way, a culture cannot be surely considered as pure. Single-spore inoculation is 

 generally accomplished by repeated dilution of a liquid medium that contains the 

 particular form desired. For this sort of work special technique has been devised. 



