INTRODUCTION 



The Uredinales form a group of Fungi which is also spoken 

 of as the Uredinese 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 hosts, 

 certain of the spore-forms being always produced upon the 

 one, and the remainder upon the other. Such species are 

 called heteroeciotis 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. 



