62 PHYSIOLOGY OF NUTRITION 



for yeast. These organisms are either saprophytic (living on dead material from other 

 organisms) or parasitic (living on tissues that are still alive). There are also a few 

 saprophytes and parasites among flowering plants. Dodder (Cuscuta) is an example 

 of a parasite of this kind. Mushrooms are examples of large saprophytic forms. 



3. Non-green Plants That Derive Energy from Inorganic Compounds. — This 

 group is composed of certain kinds of bacteria that are able to oxidize inorganic com- 

 pounds and thus secure a supply of energy. Of these, nitrifying bacteria are very 

 important. They oxidize ammonia to nitric acid. They must be grown in surround- 

 ings free from carbohydrates and other organic substances, but they require carbon 

 dioxide (or carbonates) and oxygen. They form carbohydrates and other organic com- 

 pounds out of water and carbonates or carbon dioxide, somewhat as do green plants, 

 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 (such 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 caused by microorganisms, and these substances may therefore be preserved 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 micro- 

 organisms 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 numbers 

 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 done by 

 dry heat. In other cases steam is used, especially in a closed chamber, such as the 



