INTRODUCTION 
THE Uredinales form a group of Fungi which is also spoken 
of as the Uredinee or the Rusts. An accurate acquaintance 
with their nature is of great importance to the gardener, the 
forester, or the agriculturist, on account of the enormous loss 
which is caused by them every year and which can, at least in 
part, be avoided by a fuller knowledge. 
All the species are parasitic, growing upon or in a living 
plant, which is called the host. The majority of the species of 
Uredinales have more than one stage of growth, distinguished 
by the form and arrangement of the spores which they produce ; 
the number of distinct kinds of spores which a single species 
can possess varies from one to five. If the various spore-forms 
are all borne upon one host, the species is called autoecious. 
But it is a remarkable fact that a large number of the 
Uredinales pass their existence alternately upon two hosté, 
certain of the spore-forms being always produced upon. the 
one, and the remainder upon the other. Such species are 
called heteroecious or metcecious. Many of those which grow 
upon grasses or sedges are probably hetercecious, though this 
has not been shown in every case, and there are a few proved 
exceptions. In order to convey a notion of the complex nature 
of the Uredinales, one of the hetercecious species will be taken 
as the type, and its various stages will be described. 
