INTRODUCTION 3 
less to be feared in mixtures than in pure stands. So far 
is this true that in Germany larch now scarcely appears at 
all, except in mixed forest. 
The relation of fungus to host. What is the cause of 
disease ? Why, when the disease has once appeared, does 
it spread and become more and more general? It is the 
object of this book to answer these questions ; but before 
going into the details of various forms of disease, it will add 
to the clearness of subsequent descriptions to explain a few 
of the fundamental ideas on which all our knowledge of 
tree diseases is based ; and as all the diseases described in 
this book are caused by fungi, the peculiar nature of fungal 
nutrition and the parasitism which it often involves should 
be made clear at the outset. 
The normal green plant is self-supporting in a sense which 
is not applicable to other organisms, even to the higher 
animals. That is to say, it can obtain all its food without 
the intervention of any other living being. Water and 
mineral salts it obtains from the soil through its roots, 
and carbon-compounds it manufactures from the carbon 
dioxide of the air which diffuses in through the pores of 
the leaves. Combining the carbon dioxide with water and 
giving out part of the oxygen, the plant forms carbohydrates, 
and subsequently more complex organic compounds. But 
this process of carbon assimilation can only be performed 
by those parts of plants which contain the green colouring 
matter, chlorophyll, and, even so, only in the light. Such 
plants are called autotrophic, or self-feeding. 
In the course of evolution some plants have taken to 
living on carbon compounds which have already been 
elaborated ; that is to say, these plants have become either 
parasitic on other living plants or animals, or else sapro- 
phytic on the remains of such plants or animals. Such 
plants always tend to lose their chlorophyll, and are called 
heterotrophic. Some of the higher plants are heterotrophic, 
such as the bird’s-nest orchis, but the phenomenon is most 
commonly seen in the large group of lower plants known 
as the Fungi. In no member of this group has chlorophyll 
B2 
