12 Introduction to Fungi [CH. 
In tinning fruit the tin and its contents are heated 
and sealed up whilst still hot; hence foreign organisms, 
such as fungi, are prevented from growing on them. 
Use is also made of the fact that fungi will not grow 
in certain substances known as “preservatives”; e.g. 
boric acid is added to cream to prevent the growth 
of organisms which would turn it sour. 
These fungi are saprophytes, but a large number of 
parasites which cause plant diseases also spread them- 
selves by means of spores floating in the air. Spores 
are produced in very large numbers and only by chance 
reach a substance on which they can feed as they have 
no special guidance towards it. Some give rise to 
several kinds of spores which were regarded as 
separate forms until it was found that the spores of 
one stage were capable of producing a mycelium 
bearing the spores of another stage. Every fungus 
is distinct from every other fungus: the spore of a 
mildew never gives rise to anything but a mildew, nor 
the spore of a rust to anything but a rust fungus, no 
matter on what plant it is growing, just as an acorn 
never gives rise to anything but an oak tree, or a 
grain of wheat to anything but a wheat plant. 
Fungus spores can be divided into two groups: 
(1) Those formed directly from the mycelium, 
usually by the cutting off of portions by transverse 
walls. These are known as Conidia. 
(2) Those formed as the result of fertilization, i.e. 
the fusion of a male and female cell. 
Up to the present we have only dealt with conidia; 
these are usually produced under favourable conditions, 
such as obtain in summer, and are often called “summer” 
spores. 
