4 INTRODUCTION 
been found, and the green colour seen in some fungi is due 
to another pigment. Thus all fungi have to live either 
parasitically or saprophytically, and their mode of life has 
become entirely specialized to assist them in this kind of 
existence. Though the distinction between parasites and 
saprophytes is not always easy to draw, there is a very 
marked difference between the two modes of life, at any 
‘rate in the extreme cases, and in pathology, which is the 
science of disease, we shall naturally be chiefly interested 
in parasites. But, as will be shown later, saprophytes must 
also claim our attention for the reason that they can some- 
times live on the dead parts of a living tree, such as the 
heart-wood of the trunk, and thereby not only destroy the 
timber but weaken the tree and render it more easily blown 
by the wind. 
Parasites, again, are not all of the same kind. Some can 
thrive only on living organisms, and are called obligate 
parasites, and others can grow either on living organisms 
or on dead ones, and are therefore called facultative parasites. 
An example of the former is any one of the rusts such as 
Peridermium and Coeoma on larch needles, and of the latter 
the canker fungus or the heart-rot fungus which are to be 
described. The common mushroom will serve as an instance 
of a saprophyte, as it lives on the organic remains in the 
soil. 
Life-history of a fungus. The part of the fungus which 
absorbs food from whatever it is growing on is called the 
mycelium, and is composed of a more or less felted mass of 
fine threads or hyphae. Each hypha grows at its extremity 
and may also branch, giving rise to numerous other hyphae 
as shown in fig. 21. It contains protoplasm and nuclei, and 
certain spaces in the protoplasm, known as vacuoles, filled 
with an aqueous solution, besides drops of oil and other 
food reserves. The mycelium is formed of these hyphae 
growing and becoming intermingled, and, though a single 
hypha is too fine to be seen with the naked eye, the mycelium 
as a whole may be very conspicuous, and is often seen as 
a white felt-like mass on pulling away the bark of a rotten 
