i8o 
E. B. MAINS 
obligative relation and make it seemingly impossible for the fungus 
to grow elsewhere than upon its host or hosts. Much work has been 
done and a number of theories developed with reference to the first 
condition; but concerning the second only a little work has been at- 
tempted, and but few theories advanced. Some authors in an en- 
deavor to explain such parasitism have expressed the idea that the 
solution of the problem might be sought in the requirement of the 
fungus for some special nutrient which only its own particular host 
would be able to supply. What this nutrient might be, if such is the 
case, would be of extreme importance. Failing to determine this, it 
would be of not much less importance that some idea of its nature be 
obtained. Since the obligate parasites are distinguished by the abso- 
lute need of a living host for their food supply, it is from the host that 
the evidence for the solution of such a problem must be sought, and 
it is through the control of the various physiological activities of the 
host that one may hope to do this. It was to this end that this work 
was undertaken with the object of obtaining more data regarding the 
factors which control the obligate condition and determining, if pos- 
sible, the substances or class of substances which are necessary for 
parasitism of this kind. 
The work was carried on in the Cryptogamic Laboratory of the 
University of Michigan during the years 1914, 1915, and 1916 at the 
suggestion and under the direction of Dr. C. H. Kauffman to whom I 
am under deep obligations for many helpful suggestions and stimu- 
lating criticism. 
II. HISTORICAL 
The early history of the parasitism of the rusts has been well sum- 
ixxdrized by de Bary (1853), who was the first to study the rusts and 
smuts with scientific accuracy. According to de Bary, early naturalists 
such as Pliny, Theophrastus, Malpighi, Duhamel, Tillet, Tessier, and 
Plenk considered rusts not as the cause but as the result of a diseased 
condition brought about by atmospheric conditions. In the course 
of time, they were looked upon as foreign material which was partly 
the cause and partly the result of the disease. Later, the rusts were 
recognized as fungi by Linnaeus and Persoon, but they were still con- 
sidered as the product of a diseased condition due to an injury such as 
the sting of an insect, etc. Unger (1834) believed that the rusts were 
produced by disarrangements in the respiratory organs of the plant 
due to which sap exuded into the intercellular spaces and there coagu- 
