6 INTRODUCTION 
precede their formation ; but as nuclear phenomena will not 
be dealt with in this book, the application of the terms will 
have to be taken on trust. At the same time the word 
conidium is very useful as a term for an extra reproductive 
cell which is generally not an essential part of the life-cycle 
of the fungus: the qualification is used advisedly, for in 
a large number of fungi no typical fructifications have ever 
been found, so that they must reproduce themselves solely 
by conidia. The discovery of the formation of conidia in 
the life-history of any fungus is manifestly of the first 
importance to the pathologist, for infection may be caused 
by conidia just as well as by spores. For instance, a whole 
plantation of larch may be heart-rotted by Fomes annosus 
without a single fructification being produced, and this is 
probably due solely to infection by conidia. (For conidia 
of this fungus, see fig. 43.) 
The morphology of the larch. The study of disease 
thus resolves itself into a study of the relationship of the 
parasite and host. The complete life-history of the parasite 
must be known as well as the structure and mode of growth 
of the host. In this book many different parasites are 
dealt, with, but the host is always the same; so a descrip- 
tion of those parts of the host with which we shall be chiefly 
concerned will be given at the outset. The account will be 
made as simple as is consistent with accuracy, and only 
those parts will be described which are necessary to an 
understanding of the more important diseases. 
Long and dwarf shoots. ‘When growing under forest con- 
ditions the leading shoot of a larch grows rapidly and 
maintains its vertical direction until maturity, while the 
lateral branches are mainly horizontal. The apical bud of 
each shoot, and those other buds which develop into branches, 
elongate in the spring and become what are called long 
shoots, i.e. shoots in which the internodes, or portions of 
the stem between the leaf insertions, are lengthened. The 
young stems are at first green, but soon put on a layer of 
bark and then appear yellowish brown. The leaf bases are 
decurrent, so that any transverse section shows five swellings 
