ASSIMILATION OF CARBON 6 1 



scribed, is transferred with a pipette to a Pasteur flask (capacity about 200 cc.) 

 of sterilized beer- wort (Fig. 38). After a day the contents of this flask are 

 poured into second flask (capacity about 500 cc.) also filled with sterile beer- 

 wort. 



Solid as well as liquid nutrient media are used for pure cultures of bacteria. 

 In' the case of liquid media the dilution method described above is used to 

 separate the cells. With solid media, which are very valuable for the pro- 

 duction of pure cultures, Petri dishes are used for this purpose (Fig. 39). Each 

 dish consists of two shallow glass pans (9 or 10 cm. in diameter), one being a 

 little larger than the other and forming a cover for it. A trace of the mixed 

 culture is introduced into a flask containing, for instance, a mixture of bouillon 

 and gelatine, at 3o°C, after which the flask is shaken, and the contents are then 

 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. Inoculations 

 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). F6r a streak cul- 

 ture, 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 by 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 plan s, the non-green portions of ordinary green plants, and 

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

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

 non-green cells as for cells with chlorophyh 1 .- 



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 



