62 PHYSIOLOGY OF NUTRITION 



poured into the dish and the latter is covered. After some time each bacterial 

 cell builds a colony around itself, which can be seen by the unaided eye or with 

 a simple magnifying glass. 



When a pure culture of a certain microorganism is finally obtained, then 

 any number of pure cultures of that form may be readily prepared. Inocula- 

 tions of liquid nutrient media are effected by means of a glass rod, a platinum 

 wire or a pipette, with all the requisite precautions. Inoculations of solid media 

 may take the form of either stab or streak cultures. To make a stab culture a 

 platinum needle is dipped in the original culture and is then thrust upward into 

 the solid medium held in an inverted test-tube (Fig. 40). For a streak culture, 

 a test-tube of solid medium with a slanting surface is prepared, and the point 

 of the inoculating needle is drawn across this surface. 



Summary 



1. General. — Plants without chlorophyll cannot form carbohydrates from carbon 

 dioxide and water by means of the energy of sunlight. They derive energy, as well 

 as material, from chemical compounds. Such plants may be divided into two groups: 

 those of one group get energy from organic compounds alone (these compounds having 

 been previously made directly by green plants or by other organisms out of organic 

 materials derived from green plants), those of the other group derive energy from 

 inorganic substances. Cells with chlorophyll utilize sunlight energy to form carbo- 

 hydrates (and oxygen) out of carbon dioxide and water, while cells without chlorophyll 

 either get carbohydrates (or related organic compounds) ready-made from their 

 surroundings, being unable to utilize either sunlight energy or carbon dioxide, or else 

 they derive energy from inorganic compounds and thereby form their carbohydrates 

 and related compounds out of carbonates or carbon dioxide and water. 



2. Non-green Plants That Derive Energy Only from Organic Compounds. — Yeasts, 

 fungi, non-green seed plants, the non-green portions of ordinary green plants, and 

 most bacteria, derive their energy supply exclusively from ready-made organic com- 

 pounds. These compounds also supply carbon, which is of course as essential for 

 non-green cells as for cells with chlorophyll. 



The microorganisms of this group are very important in nature, being largely 

 responsible for decay and putrefaction. They live by decomposing the organic sub- 

 stances produced by other organisms, including green plants. They may be dis- 

 tinguished from one another by the nature of the substances required for their growth, 

 and they may be grown in artificial nutrient media, such as Pasteur's culture solution 

 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, 



