44 BOTANY AND PHARMACOGNOSY. 



323). The time required for producing a sterile solution, that is 

 one free from micro-organisms or their spores, can however be 

 much reduced by increasing the temperature, or pressure, or 

 both. By use of the autoclave in which the pressure can be 

 increased from 10 to 20 pounds, sterilization can be accomplished 

 in 30 minutes by using a temperature of 110 C. 



As already indicated one of the greatest difficulties is to 

 isolate the organisms. In a cubic centimetre of water there 

 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. 



The application of stains for differentiating the various organ- 

 isms 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. 



ARCHEGONIATES. 



The two main features which distinguish the Archegoniates from 

 the Thallophytes are the structure of the sexual organs and the dis- 

 tinct manner in which the peculiar phases known as alternation of 

 generations is shown. The antheridium or male sexual organ is a 

 well differentiated multicellular body which is either sunk in the 

 adjacent tissues of the plant or is provided with a stalk. Within 

 it are organized the sperms or spermatozoids, which are ciliate 

 and swim freely in water. Corresponding to the oogonium of 

 the Thallophytes is the archegonium or female sexual organ 



