JOURNAL OF AMdiUM, RESEARCH 
You. XXI Washington, D. C., August i, 1921 No. 9 
STUDIES IN THE PHYSIOLOGY OF PARASITISM WITH 
SPECIAL REFERENCE TO THE SECRETION OF PECTI- 
NASE BY RHIZOPUS TRITICI 
By L. L. Harter, Pathologist , and J. L. WEIMER, Assistant Pathologist, Office of 
Cotton, Truck, and Forage Crop Disease Investigations, Bureau of Plant Industry, 
United States Department of Agriculture 
INTRODUCTION 
As a result of investigations of the relationship existing between a para¬ 
site and its host some knowledge has been obtained regarding the manner 
in which the [infection takes place, as well as the way in which the host re¬ 
sponds to the attack of the parasite. That some hosts are resistant to the 
attack of certain fungi or bacteria and not to others is well known, but 
why they are resistant is a subject on which there is altogether too little 
information. Furthermore the number of different hosts which may be 
parasitized by some of the so-called specialized parasites is at most 
only a few. The mode of infection is fairly well understood, but why a 
particular fungus is restricted to one or a few different hosts is not clear. 
On passing to parasites of a lower order the number of hosts which a 
particular fungus may attack is, in some cases at least, much larger. 
It is a well-known fact that some fungi can produce disintegration of the 
host tissues only after a wound has been made through which they can 
enter, while others have the power to penetrate the epidermis either 
mechanically or by the dissolution of the epidermal cell walls by enzymic 
action. There are, therefore, all degrees of resistance and susceptibility 
which apparently are dependent upon certain properties peculiar to 
either the host or the fungus. In other words, if a fungus can overcome 
the resistance of one host and not that of another it must be that the 
one possesses properties which the other does not. However, the experi¬ 
mental data at hand are altogether too limited to justify any sweeping 
generalizations as to what constitutes parasitism. 
Rhizopus tritici Saito belongs to a large physiological group of fungi 
which are characterized by their ability to destroy the host quickly and 
to “act in advance” of their growth. It is assumed, and the evidence 
of the writers and of others seems to justify the assumption, that this 
“action in advance” is due to an enzym secreted by the fungus which 
Journal of Agricultural Research, Vol. XXI, No. 9 
Washington, D. C. Aug. 1, i 9 « 
Key No. G-240 
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