154 Illinois State Laboratory of Natural History. 



leaves the vegetative threads are seldom found at a greater lat- 

 eral than vertical distance, the latter limited by the thickness 

 of the leaf; vet the whole tissue may be permeated by the my- 

 celium of different, perhaps very numerous, spores. 



In the preparation for the formation of spores, mycelium- 

 threads become densely aggregated into a parenchyma-like 

 tissue in a little area just beneath the epidermis, and from the 

 upper surface of this minute cushion the spores are produced 

 by the enlargement and modification of the end of a thread, 

 either singly, or by the formation of septa ; from two to several 

 in the letter case so as usually to form a single vertical row 

 from each fertile filament. The clusters of spores (sori), 

 usually very densely packed, are naked, or surrounded by pe- 

 culiar sterile cells (^paraphyses), produced from the mycelium 

 or entirely inclosed in a membranous envelope (pseudoperid- 

 ium), originating from the same source. By 'the growth of the 

 fungus the epidermis of the host is pushed up and finally 

 ruptured, so that the spores, mostly just at maturity, are exposed 

 to the air, in the currents of which they are light enough to 

 be carried as fine dust. 



The species of Uredinece are limited to particular host 

 species, mostly to one, or at farthest to the species of one genus 

 or closely allied genera. None are certainly known to grow 

 upon plants of different natural orders, except in the alterna- 

 tion of fruit forms. In the latter case the teleutospores pro- 

 duced upon grasses or sedges give origin, in some species, to 

 aecidia on the leaves of certain exogens. In fact, it seems to 

 be most common that when the secidium is not grown on the 

 same host with the uredo and teleutoforms, very wide diver- 

 gence in this respect is made. Wheat and the barberry bush, 

 oats and the buckthorn, red cedars and apple trees, are three 

 examples of this remarkable peculiarity, the teleutoform in 

 each case being found on the first named, the aecidium 011 the 

 second. 



Following the descriptions of species in this paper, refer- 

 ences are given for each species to the host plants, the locali- 

 ties by counties (of Illinois), and the date of collection. The 

 numbers in Arabic figures are those of the herbarium speci- 

 mens, corresponding with those of the collector's notes; the 



