FLOWERLESS PLANTS 459 



experiment we must be sure that we have killed all the 

 germs that may be in the culture medium. Can any of 

 the children suggest a way to do this ? How is it done 

 in their homes in the canning of fruit ? 



A convenient way is to plug the bottles with a wad of cotton 

 batting and let them stand in a tightly covered steamer over boiling 

 water for half an hour. This kills growing mould plants and bac- 

 teria, but not all the spores that may be present in the liquid. The 

 heat will be likely to start any such spores into growth, so that if they 

 are steamed again on the following day, or before the spores have 

 had time to germinate and form spores again, we may be reasonably 

 sure that no germ remains alive in our cultures. Two or three of 

 these vials should be set aside to compare with others that are 

 planted with dust or with the spores of different moulds ; and, if all 

 the germs have been killed and the cotton is not removed, the cul- 

 tures will remain clear, and no growth of any kind will appear in 

 them. This is known as " sterilizing." With a number of the cul- 

 tures thus sterilized we may plant them with dust or the spores of any 

 mould we wish to study. To do this, pick up a little of the dust or 

 spores with the point of a clean needle and apply to the culture. 



After a clay or two, if there are any mould spores in 

 the cultures, we should see a fine woolly growth spreading 

 over the surface and sending its delicate threads down 

 into the liquid. This is the mycelium, and the threads 

 are known as hyphae. The function of the mycelium, as 

 in the mushrooms, is to absorb nutriment. Next we 

 observe that a number of the hyphae near the center of 

 the mycelium are growing up into the air, and the forms 

 they assume are characteristic of different species of 

 mould. Four of these typical forms are shown in Fig. 190, 

 but it is not intended to go farther into any details that 

 require the use of the microscope. A number of the 

 larger moulds, however, have fruiting hyphae an inch or 



