FLOWERLESS PLANTS 469 



with a sterilized knife (a knife that has been held in a 

 flame for an instant or in boiling water for a few seconds) 

 makes a good field on which to sow dust and observe the 

 growth of bacteria. The best culture medium, however, 

 is gelatine,^ which may be used in the pupils' vials after 

 the hay-infusion experiment. Dust sown on its trans- 

 parent surface will leave little doubt in their minds as 

 to the reality of microbes. 



Some of the gelatine poured while warm upon a small 

 pane of clean glass may be made to yield a most instruct- 

 ive demonstration. Touch the hand, soiled and dusty as it 

 is, to the solidified gelatine for a few seconds ; then wash 

 the hands with soap and apply to a fresh surface of the 

 gelatine ; keep protected from dust for two or three days. 

 The soiled hand will be outlined by colonies of bacteria, 

 while possibly none or few will grow where the washed 

 hand touched the gelatine. I know of no experiment 

 which demonstrates so forcibly the reason for washing 

 the hands before breaking^ bread. 



Pear Blight. — The leaves and bark on branches of pear 

 trees sometimes turn black. This is due to the work of 

 bacteria which gain access to the tree through the blos- 

 soms or throuo:h some wound in the bark. It was thou^rht 

 that bees were largely to blame for carrying the germs of 

 blight from blossom to blossom, and the experiment has 

 been tried of removing them entirely from the neighbor- 

 hood of pear orchards during the blossoming period. This 



1 Gelatine for study of bacteria is prepared by dissolving in 200 cubic 

 centimeters of water i gram each of Liebig's beef extract and common salt 

 and 2 grams of peptone, adding 20 grams of gelatine. Dissolve and neu- 

 tralize, or make slightly alkaline with sodium carbonate. Stopper the bottle 

 with cotton batting and boil for fifteen minutes on three successive days. 



