16 A TEXT-BOOK OF BOTANY. 



may be a million organisms representing various groups of bac- 

 teria. In trying to solve the problem of their separation it 

 occurred to Koch that if he could secure a medium which was 

 solid at the ordinary temperature and liquid at a slightly higher 

 temperature, he could mix a certain quantity of liquid containing 

 micro-organisms with the medium in a sterile condition, and then 

 by solidifying the mixture the organisms would be fixed, and 

 thus from each organism a colony would be developed which 

 could be isolated and further studied. We are indebted to Koch 

 for the use of solid culture media like nutrient gelatin and 

 nutrient agar in the study of these organisms. 



IV. The application of stains for differentiating the various 

 organisms was introduced by Weigert in 1877. Staining is of use 

 in the determination of the number of flagella of certain organisms, 

 in the study of spores, and the identification of certain pathogenic 

 organisms, which occur in mucus and pus, as tubercle bacilli, 

 etc. Gram's method of staining is of great use in differentiating 

 many pathogenic as well as non-pathogenic organisms, and is of 

 importance in classifying bacteria. 



Characteristics. Algae are characterized by their habit of 

 living in water or in moist places. They vary from simple, i -celled 

 microscopic forms to those of great size like the sea-weeds. In 

 the various types, however, the cells show little variation in shape. 

 All the Algae contain more or less of a green coloring matter, even 

 though it may be concealed by other pigments of a blue (as in 

 Schizophyta), brown, or reddish color. The possession of this 

 green cell-content or chlorophyll enables the Algae, in the presence 

 of sunlight, to manufacture food substances from simple materials 

 like carbon dioxide and water. 



The occurrence of chlorophyll can be readily demonstrated by 

 extracting it with 95 per cent, alcohol. Even in the most delicate 

 of the red Algae it can be shown by placing the fresh material in 

 a strong solution of common salt and afterwards extracting the 

 chlorophyll with alcohol, the other pigments being unaffected. 



Algae are sometimes grouped as Fresh- Water Algae, includ- 



