PRINCIPAL GROUPS OF PLANTS. 15 



and the liquid then boiled for some time it would keep 

 indefinitely. 



II. It remained for Koch and Pasteur to show what took 

 place in the boiling of the liquid, who at the same time developed 

 the principles of sterilization in bacteriological work. These 

 authors discovered that micro-organisms have two stages of de- 

 velopment, one of which is active and the other resting, the latter 

 being known as the egg or spore condition. They found that the 

 organisms in the active condition were completely destroyed on 

 heating the solution containing them for 30 minutes at 100 C. 

 If this solution was allowed to stand for 24 hours or longer 

 there would be evidences of decomposition, which was due to the 

 fact that the spores representing the resting stage of the organ- 

 isms were unaffected by the first heating and developed into 

 the active stage. As a result of further experiments they found 

 that if the solution were heated on the second day for 30 

 minutes at a temperature of 100 C. the second growth of organ- 

 isms was destroyed, but it was found that the solution might still 

 undergo decomposition in the course of time, owing to the later 

 development of a few remaining spores. It was, however, found 

 that heating the liquid again on the third day was sufficient to 

 kill all of the spores as well as the organisms in the active stage. 

 By repeating these experiments the authors confirmed their 

 observations and established the process known as discontinuous 

 sterilisation, which simply means that if a solution of a putrescent 

 or fermentative substance is heated on three consecutive days for 

 30 minutes at a temperature of 100 C., the flask or bottle being 

 stoppered with absorbent cotton, it will keep indefinitely. Instead 

 of using a plug of absorbent cotton the neck of the flask can be 

 drawn out into a narrow tube and directed downwards (see Re- 

 agents). 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. 



III. As already indicated, one of the greatest difficulties is to 

 isolate the organisms. In a cubic centimetre of water there 



