THE FUNGI 165 
Although the fungi, as a class, are dependent 
for their nourishment on substances which 
have originated from other living things, 
they differ a good deal among themselves as 
to the kind of food material they can use. 
Some are dependent on the living bodies of 
animals or plants, and then we call them 
parasites. Others live on dead or decaying 
remains, and are commonly termed sapro- 
phytes. 'The saprophytes are a very extensive 
class, and include many species that can live 
on relatively simple organic residues such as 
sugar, organic acids, and so forth. But these 
simpler feeders are connected with more 
obviously saprophytic types by all con- 
ceivable intermediate forms, and even the 
distinction between saprophytes and parasites 
is far from absolute, for many saprophytes 
can become parasites, and vice versa. 
Now the utilisation of all food, regarded as 
a means to an end, is connected with changes 
in the states of energy. Complex organic 
substances possess a considerable amount of 
energy in a locked-up, or potential form, 
When the food substances undergo oxidation 
and are broken down into simpler ones, the 
potential energy is set free as kinetic energy, 
just as happens when a piece of coal is 
oxidised or burnt. This kinetic energy is 
directly available for doing work, and may 
be utilised to boil a kettle or to build up the 
body of a fungus. In the former case, the 
kinetic energy is used to alter the physical 
