366 SIXPEBIMENTS WITH PLANTS 



small tumblers and insert a cotton plug as in Fig. 212) 

 and sterilize them all for half an hour. After they 

 have cooled down again, remove the cotton from one 

 (a), expose to the air for a minute and replace the 

 cotton : to another (&), transfer a minute quantity of 

 bacteria from the liquid by means of a needle.^ 

 For this purpose we pass the needle back and forth 

 several times through the flame, and when cool dip it 

 into the liquid. We now lift the cotton at the edge 

 and draw the needle once across the surface of the 

 potato and at once replace the cotton. Leave the 

 third tumbler (c) intact, as a control. 



Put all the tumblers away in a warm place in the 

 dark and observe them daily. The bacteria in («) 

 come from the air ; many disease -producing bacteria 

 (e. g., those of tuberculosis) are wafted about in the 

 air; for this reason consumptives should take special 

 precautioias to avoid infecting the air. Do the spots of 

 decay appear in (&) anywhere except along the needle 

 track: do any appear in (c)? Does the potato beneath 

 the spots alter in color, consistency, etc.? Scrape a bit 

 of the potato from one of these spots, mount the scrap- 

 ing in a drop of water, and examine under the micro- 

 scope. The large, glistening bodies are starch grains, 

 while the bacteria appear as minute bodies floating 



1 This is prepared by lieatinfr the end of a glass rod in a flame until it fuses 

 and then forcing into it >■ piece of fine steel (or platinum, if obtainable) wire 

 about two inches long. 



