^ MAY 10)917 
AMERICAN ,Mn.^ 
JOURNAL OF BOTANY 
Vol. IV April, 1917 No. 4 
THE RELATION OF SOME RUSTS TO THE PHYSIOLOGY 
OF THEIR HOSTS^^ 
E. B. Mains 
I. INTRODUCTION 
The relation of the rusts to their hosts has long occupied the at- 
tention of many workers, not only because of their economic impor- 
tance, but more especially by reason of the extremely interesting 
biological problems which they offer. Not only have the rusts afforded 
a wide field for the study of the questions of immunity, susceptibility, 
physiological varieties, heteroecism, etc., but they, together with a 
few other groups such as the Peronosporales and Erysibaceae, make 
up part of the group of fungi which de Bary has called obligate para- 
sites. This group of fungi is characterized by the requirement of a 
living host as the source of food supply. Saprophytes, on the other 
hand, obtain their food from dead organic material. Between the 
two classes are the intergrading facultative parasites and facultative 
saprophytes, determined by the degree a fungus is independent or 
dependent upon a living host. The saprophytic and facultative para- 
sitic fungi have long been studied with attention to their food relations, 
but most of the work upon the obligate parasites has been confined to 
other lines, since the parasitic condition itself puts great difficulties 
in the way of an investigation of the nutrition of the fungus. 
One must not overlook the fact that there are two conditions in 
obligate parasitism. In the first, we have the problems concerned 
with immunity and susceptibility, a condition which is common to all 
parasites whether obligative or facultative as well as to the facultative 
saprophytes. The other condition is that which goes to produce the 
^ Paper No. 156 from the Botanical Department of the University of Michigan. 
179 
