60 
material, rapidly grows and branches to form a dense felt 
such as we saw on the moist bread. : : 
We have seen that suitable food material for this 
fungus is provided by bread, dung, etc.; that is to say by 
dead or decaying plant or animal matter, just as the 
mushroom or toadstool lives on similar dead organic 
remains in the ground. Fungi, which in this way draw 
all their nutriment from the rotting remains of plants or 
animals, are known as saprophytes, and although they 
are unable to injure living plants they readily feed on 
their remains after death. 
Fungi, unlike green plants,.possess no chlorophyll 
and are therefore unable to construct their own carbon 
compounds such as starch and sugar from the carbonic 
acid of the air. They, however, take in such complex 
organic substances ready-made from the remains of plants 
which have previously manufactured them, and saprophy- 
tic fungi play an important part in Nature in living upon 
and decomposing the dead organic remains of plants and 
animals. The saprophytes as a rule cannot attack living 
plants, and therefore do not give rise to plant diseases. 
A large number of ‘fungi, however, are unable to live 
even upon decaying plant remains, and derive nutriment 
from the cells of living plants. Such fungi are sarasites, 
and not only do they require to take in the carbon com- 
pounds of their food material ready-made, but they can 
only take their food substances from living cells. Now 
it is clear that in obtaining substances forcibly from plants 
while still alive, such parasitic fungi rob the plants 
attacked of materials which otherwise would have been 
used for their own life and growth, and may harm them 
more directly in doing so. A plant which harbours a 
parasitic fungus is spoken of as the host plant, and in 
most cases the host suffers injury not only because it is 
robbed of substance by the fungus, but also because the 
work of the particular parts of the plant infected are 
seriously interfered with. We shall see for example how 
the stores of food material in roots like turnips, or tubers 
like potatoes, are raided by fungi, and on the other hand 
how mildews and rusts prevent leaves from manufacturing 
food supplies. 
Most of the larger and more prominent fungi are sapro- 
phytes and live, as we have seen, on decaying organic 
matter. Some of them, however, are parasites. The 
