60 Practical Plant Biology. 



A temperature of 140 C. is sufficient. By use of the autoclave 

 a single heating suffices to effect sterilisation. It is suitable for 

 the sterilisation of any organic substances which might be charred 

 or injured by a high dry temperature. 



The autoclave may also be used to sterilise vessels and instru- 

 ments used in the isolation experiments spoken of at the be- 

 ginning of the lecture : but most usually these are sterilised by 

 some form of hot air oven a metal oven which may be heated 

 by the application of gas burners to a temperature of i5o-2oo 

 C., and constructed so as conveniently to hold the vessels and 

 instruments required. 



Supposing by means of the processes just described we are pro- 

 vided with a few sterilised test-tubes containing culture medium 

 (meat extract and gelatine) plugged with cotton-wool and a petri 

 dish (to be described later) we may at once address ourselves to 

 attempt the isolation of some of the bacteria in the mixed growth 

 in the seaweed, and proceed as follows : 



Thoroughly liquefy the gelatine medium in a plugged and 

 sterilised test-tube by immersing it in water at about 50 C. for 

 some minutes. 



Loosen the adhesion of the plug of cotton-wool to the inside 

 of the neck of one of the test-tubes, this may easily be done by 

 holding the plug and gently rotating the tube. Set fire to the 

 projecting part of the plug and when the outer surface with any 

 adherent bacteria is burned, extinguish the flame. Take a piece 

 of fine brass wire (24 B.W.G. will do well) somewhat longer than 

 the length of the test-tube. Make a little eye at one end about 

 3 mm. in diameter. Sterilise this loop and the adjoining few 

 centimetres by heating it in a Bunsen flame ; do not let it get red 

 hot. Allow the wire to cool without letting the looped end touch 

 any contaminated surface. When it is quite cool dip it into the 

 mixed growth in the salt water. Still holding the wire between 

 the forefinger and thumb of the right hand, take up the test-tube 

 in the left hand. Pinch the projecting end of the cotton-wool 

 plug between the third and fourth finger of the right hand and by 

 rotating and pulling on the test-tube remove the plug from its 

 neck. Now, still retaining the plug between the third and fourth 

 fingers thrust the wire loop containing the drop of the mixed 

 growth into the culture medium. Withdraw the loop and replace 

 the plug in the tube. Again heat the wire to destroy the ad- 

 hering bacteria. 



If everything has gone well we have some mixed culture en- 

 closed now in the tube and cut off from subsequent contamination. 



