6o 



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 parasites, 

 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 



