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 beon 

 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 



B 2 



