4 INTRODUCTION 



been found, and the green colour seen in some fungi is due 

 to another pigment. Thus aU 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 



