6 INTRODUCTION 



precede their formation ; but as nuclear phenomena wiU 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 



