May, 1923] HARTER AND WEIMER — PECTINASE AND INFECTION 249 
dissolving agency precedes the penetration of the cells. Miyoshi was able 
to show that B. cinerea could penetrate mechanically a membrane such as 
gold leaf. The work of De Bary (9), Nordhausen (29), Smith (32), and 
others has been referred to in this connection in other papers and will be 
reviewed only briefly here. De Bary noted that the expressed juice of 
certain plant organs which had been decayed by Sclerotinia libertiana 
Fuckel if heated was no longer active, and therefore concluded that the 
cell-wall-dissolving action is due to an enzym. A toxic action was also 
noted, but to just what this action is due he was uncertain. Nordhausen 
as well as Smith claimed the production of a toxic substance which penetrates 
the cuticle and kills the underlying cells. 
Some interesting results in this connection are those of Brooks (3), who 
attempted to infect lettuce by placing spores of Botrytis cinerea from a 
grape-extract medium, dry and in drops of water, on the leaves of plants 
kept uncovered in a greenhouse. Although some of the spores germinated, 
no infection was obtained, even when the plants were confined under a 
bell jar. If, however, young mycelia were placed in drops of the grape- 
extract medium upon healthy leaves, infection took place. Wounding 
insured infection. He says: 
In such cases the juices which exude from the wounded areas provide saprophytic 
nourishment for the further development of the germ tube. Infection could be brought 
about on leaves which had only just begun to turn yellow, but not on normal green leaves. 
Botrytis can not be considered a true parasite since it kills tissue in advance of the growth. 
The results of the investigations cited above show that some organisms 
can infect although there is no apparent injury of the outer layer of cells 
of the host, while others require a wound. That Rhizopus belongs to the 
latter class has been demonstrated repeatedly in the following manner. 
Sweet potatoes of the Yellow Jersey variety, one of the most susceptible 
varieties, which had been cured in the usual way in the storage house, were 
used in these experiments. Sound potatoes were carefully washed, and a 
glass ring about one centimeter deep and one and one half centimeters in 
diameter was sealed on an uninjured spot on the surface by means of a 
wax made of beeswax and vaseline. A 48-hour-old culture of Rhizopus 
tritici grown in about 2 cc. of sweet-potato decoction was poured into the 
glass cell, which was then covered with a cover slip held in place by vaseline. 
In no case did infection take place. This seems to prove that this fungus 
is unable either to penetrate mechanically the unwounded skin of the 
potato or to secrete an enzym which will dissolve it. 
A mature sweet potato has no true epidermis. Instead of an epidermis, 
which probably sloughed off early in the growth of the potato, there is a 
layer of cork two to four cells deep. This layer, which will be designated 
hereafter as the skin, is able to withstand the attack of the fungus itself or 
digestion by any of its secretions. A sweet potato inoculated with Rhizopus 
tritici will entirely decay with the exception of the skin, which the hyphae 
