<ii.->:'. t !;*).'fl I ' 



STUDIES OF SPORE-PLANTS 257 



on the bread even after sterilizing, they may have fallen in when the 

 tube was opened for inoculating, or more than one kind of spores 

 may have been on the moldy spot touched with the needle. If only 

 one kind appears, a " pure culture" has been secured. 



Take two tubes which have been sterilized twice, remove cotton 

 plugs, and blow into them some dust from top of furniture, door- 

 frame, or window-casing ; or sterilize an inoculating needle and with 

 it transfer some of the dust to the sterile bread. Record results 

 and conclusions. Why do foods mold quickly in a dusty pantry ? 



Leave the plugs out of two sterile tubes for 



several hours, and then replace. If molds de- 

 velop, consider the possible source of the spores. 



A better way for this experiment is to use a 



n * T i i r A j i. j n i FIG. 82. Section of a 



flat dish, known as a Petn dish, made for such Petri dish for grow _ 



experiments (Fig. 82). Place a slice of moist i ng molds' and bac- 

 bread in such a dish, sterilize twice, wait several teria. c, glass cover, 

 days to make sure that the bread is sterile, The dots in bottom 

 then remove the cover for several hours so as sh position of 

 ., i i_ j j_ J.T. a J.T. gelatin, bread, or 



to expose the sterile bread to the air of the other food 

 schoolroom. 



A small tumbler or wine-glass may be used instead of a Petri dish. 

 Place a piece of moist bread inside on the bottom of the tumbler, 

 cover with a circular sheet of cotton- wadding which by two inches 

 exceeds the diameter of the top of the tumbler, fold the edges of the 

 cotton down, and snap a rubber band around it so as to hold it close 

 to the top of the tumbler. Sterilize as in case of the Petri dish. 



A wide-mouthed fruit-jar might be used in the same way, sealing 

 the jar with the rubber as in canning fruit. The Petri dish will by con- 

 trast show that the rubber is not necessary to keep out spores. Also, 

 the Petri dish admits air. The secret is that the spores respond to 

 gravitation and do not fall upward as they must do in order to fall 

 into a Petri dish. A fruit-jar could be used with the cap and no 

 rubber, if the contents were thoroughly sterilized, the edge of the 

 cap kept dry and never turned upside down. But in ordinary 

 household canning of fruits the sterilization is not perfect (usually 

 only once), and the keeping out of air by the rubber tends to prevent 

 the development of molds which require air in order to grow. Also, 

 the rubber prevents hyphae from starting on the outside and growing 

 beneath the cover into the contents of the jar. This would surely 

 happen if fruit juice ran down around the edge of the cover, for the 

 juice would furnish excellent food for the growing hyphae. This is why 

 fruit in jars often remains sterile for months and even years, and then 



