May, 1923] HARTER AND WEIMER-PECTINASE AND INFECTION 
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writers’ observations go, the hyphae pass between the cells but do not 
penetrate, at least during the earlier stages of decay. 
Investigations by the writers have demonstrated that neither Rhizopus 
nigricans nor R. tritici produces cytase, which may account for the fact 
that they do not penetrate the cell wall. The absence of a cellulose¬ 
dissolving enzym was demonstrated in several ways. Some preliminary 
and somewhat crude experiments were conducted in which it was attempted 
to grow Rhizopus on a nutrient solution with Whatman chemically prepared 
filter paper as the only source of carbon. A modification of Czapek’s 
nutrient solution in which ammonium sulphate was substituted for sodium 
sulphate served as a medium. Rhizopus makes a rapid and profuse growth 
on this medium with glucose or even starch paste as a source of carbon. 
When the filter paper was substituted for glucose, the fungus made practi¬ 
cally no growth. The filter paper appeared to be unaltered, and no reducing 
sugars could be detected according to the method of Clark (8). On the 
other hand, when starch paste, for example, is used as a source of carbon, 
reducing sugars are formed in advance of the needs of the fungus (18). 
A second series of experiments were carried out in which the action of 
Rhizopus on cellulose was studied. A cellulose agar was prepared according 
to the method of McBeth and Scales (26) from a good grade of filter paper 
as prepared by Scales (31). A uniform distribution of the flocculent 
cellulose imparted a milky appearance to the agar. Kellerman (25) showed 
that if this kind of agar in test tubes is inoculated on the surface with 
Penicillium pinophilum Hedg., the enzym cytase is excreted which eventu¬ 
ally clears the medium by the dissolution of the cellulose. Furthermore, 
he found that, if discs of the clarified agar were transferred sterile to cellulose 
agar in plates, the cellulose in the latter would likewise be dissolved, thus 
proving conclusively that cytase was produced. The writers duplicated 
the experiment of Kellerman with Rhizopus nigricans and R. tritici but no 
dissolution of the cellulose took place, which would seem to indicate that 
the enzym is not produced. 
It was suspected that perhaps in the absence of available carbohydrates 
these organisms were unable to make sufficient growth to produce the enzym, 
and that, if they were cultivated on a medium on which they would grow 
independently of the cellulose, they might digest the cellulose. With this 
possibility in mind the cellulose was added to beef agar and the same test 
as before was applied, with negative results. The results of these experi¬ 
ments seem to prove the absence of cytase production in the species nigricans 
and tritici. 
Mode of Infection 
The results of different investigators have shown that fungi may enter 
the host tissue, first, by way of the stomata; second, by penetrating mechan¬ 
ically the unbroken epidermis; third, through both the stomata and the 
epidermal cells; fourth, by means of an enzym secreted by the fungus which 
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