1] Introduction to Fungi 11 
of bread as before and keep it for a short time in 
boiling water. Now divide the bread into two parts 
and place on separate pieces of glass which have also 
been immersed in boiling water. Cover one piece, 
while still hot, with a small bell-jar—call this A. Allow 
the other piece (B) to cool and to remain uncovered for 
two or three hours, and then cover it also with a bell-jar. 
After a few days a whitish fungus appears on B, but 
‘not on A. Now the only difference in treatment of 
these two pieces of bread is that B has been exposed to 
the air for a short time while A has not. As A has no 
fungus growing on it we may conclude that the fungus 
has not been produced by the bread. The only 
other source from which it could have come is the air. 
Fungus spores which we have already examined are 
very light and can readily float about in the air. The 
moist bread offers suitable conditions for the germi- 
nation of any which may find their way to it; the 
hyphae take up food from the bread and produce a 
mycelium which gives rise to similar spores some of 
which in turn may fall on the bread and germinate as 
before. 
Considerable use is made of the knowledge that 
fungus spores float about free in the air, but can be 
killed by heat. In jam making the jam is heated and 
any fungus present is killed. The jam is then tied 
down whilst still hot and any spores that reach the 
surface before tying down are killed, consequently 
there is no growth of mould on the surface. If however 
the pots are allowed to cool before being tied down, any 
fungus spores that reach the surface after the tempera- 
ture has fallen too low to kill them grow and produce 
a mycelium which is commonly known as “mould.” 
