CHAPTER I. 



INTRODUCTION. 



. The Uredineae, or rusts, constitute one of the most important groups 

 of parasitic fungi, and their ravages are known wherever plants are 

 cultivated. The cereals and grasses of our fields, the fruit trees of our 

 orchards, even the ornamental plants of our gardens, and many of our 

 forest trees are attacked by members of this family, and their study is not 

 only interesting from the point of view of the scientist, but from that 

 of every grower of plants for pleasure or for profit. Rusts are usually 

 so conspicuous that they attract the attention of even the ordinary observer, 

 and they have been known and recognised even from the earliest times, 

 particularly from their blighting effects on the wheat and corn crops. 



Although so long known as regards their naked-eye characters, and 

 the effects they produce, their structure and life-history have only been 

 understood within comparatively recent times, and even now there are 

 many points concerning them which await investigation. 



Their structure essentially consists of an inconspicuous mycelium 

 bearing the usually conspicuous spores, and while this vegetative mycelium 

 is generally similar throughout the group, the spores produced by it are 

 very dissimilar. The general study of this group will therefore mainly 

 resolve itself into a knowledge of "the different spore forms, and their 

 relation to each other, either on the same plant or on different plants. The 

 finishing spore or teleutospore may be regarded as the ultimate stage of 

 the Uredineae, and which, after usually resting for a period, long or short, 

 germinates by putting forth a germ-tube, which bears, in turn, another kind 

 of spore. The germ-tube is known as the pro-mycelium, and the spore as 

 the promycelial spore, or sporidiolum, so that if the latter is regarded as the 

 starting point, the teleutospore will constitute the finish. 



Between these two forms there may be various intermediate stages, and 

 the series may consist of the following: 



1. The sporidiolum, when it produces its germ-tube, enters the 



tissues of the host-plant, and may either produce from its 

 myoelium teleutospores similar ,to those from which it 

 originated ; or 



2. It may give rise to uredospores at first, and subsequently 



teleutospore; or 



3. It may produce aecidio spores, uredospores, and teleuto spores in 



succession, the aecidio spores being generally preceded or 

 accompanied by a peculiar form of spore, known as a 

 spermatium. 



Hence the complete series of spore-forms will be spermatium, aecidiospore. 

 uredospore, teleutospore, and sporidiolum, although between the initial 

 sporidiolum and the final teleutospore, one or more of the above may be 

 suppressed or omitted in the life-cycle. 



