RELATION OF SOME RUSTS TO THEIR HOSTS l8l 
lated, thus forming the rust. The next great step in the direction of a 
true understanding of the nature of the rusts was the recognition that 
they were the cause of the disease. The behevers in this theory first 
concerned themselves with the study of the rusts as fungi and their 
entrance into their hosts. Leveille (1839) showed that the rusts did 
not differ from saprophytic fungi in their development of mycelium and 
fruiting bodies except that they were within the living host. Prevost, 
according to the Tulasne brothers (1847), first observed the germina- 
tion of rust spores. The Tulasne brothers and de Bary (1853, 1863) 
showed that the germ-tubes of the rusts enter through the stomata of 
their hosts and in some cases (the germ tubes of basidiospores) through 
the cell wall. 
It was de Bary (1853), however, who finally definitely established 
that the rusts were parasites and that they were the cause and not the 
result of the disease. He concluded that the " Brandpilze," in which 
group he included both smuts and rusts, are to be considered as para- 
sitic growths, since they arise from spores whose germ-tubes penetrate 
the host, develop a mycelium within the host's tissue, form spores^ 
and finally break through the epidermis and infect other plants. De 
Bary in Die Brandpilze (1853, p. 109) defined a parasite as, "solche 
Pflanzen oder Thiere, welche auf lebenden Geschopfen existiren, und 
ohne diese nicht bestehen konnen, welche durch den Reiz, den sie 
verursachen, durch die Nahrung, die sie dem Wohnorganismus ent- 
ziehen, Storungen in dessen Organfunctionen hervorrufen; diese 
schwinden, sobald der Parasit entfernt oder getodtet wird." In ""iew 
of this definition, de Bary's work on the "Brandpilze" was hardlv 
sufficient to establish the rusts as parasites, since he did not show th 
they could not exist outside of living organisms. 
A rather exhaustive search of the literature of this period does not 
reveal that any attempts were made to grow the rusts saprophytically. 
The general opinion which is now held appears to have arisen from 
the earlier idea that the rusts were diseased products of the host, first 
non-living and finally living products. In part this assumption of the 
obligate character of the rusts is due to the fact that they, unlike the 
facultative parasites, are never found in nature growing on other than 
living plants. 
Among the later workers upon the obligate parasitism of the rusts 
is Brefeld (1883, 1908), who believes that the growing of rusts sapro- 
phytically is merely a matter of technique. He was able to obtain 
