2 Introduction to Fungi [CH. 
equally susceptible to the disease. The plots which 
have received the largest quantities of nitrogenous 
manures but no potash suffer most. Except for the 
manuring all the plots are under similar conditions. 
Variations in food supply therefore seem to be important 
in that they cause varying susceptibility to disease. 
The fungi belong to the lowest group of the plant 
kingdom and, as we shall see later on, cause a large 
number of plant diseases. They differ from ordinary 
green plants in that they do not contain any of the 
green colouring matter so characteristic of the latter. 
Itis this substance, known to botanists as “chlorophyll,” 
which enables green plants to take from the air certain 
food materials that they are unable to obtain from the 
soluble substances in the soil. A fungus is unable 
to live in this way on the constituents of air and soil, 
but requires its food to be manufactured for it, and so 
it takes advantage of the food materials made by 
other plants. Some live on the decaying remains of 
plants and animals, e.g. toadstools live on leaf mould 
and also on manure heaps; others can only take their 
food from living plants, or in a few cases from animals. 
Those living on dead materials are known as Saprophytes 
and those on living plants as Parasites. There is no 
definite boundary line between these two classes: 
some members are capable of living as parasites at 
one period of their life and as saprophytes at other 
periods when the living food supply is not at hand. 
Among cultivated plants are forms which differ very 
considerably in appearance and this naturally is found 
in other groups of plants. It is difficult to see any 
resemblance between the mushroom which we eat and 
the smut that blackens corn, or the yeast used by 
