256 
AMERICAN JOURNAL OF BOTANY 
[Vol. io. 
as an enzym. Brown (4) has shown that the germ tube of Botrytis cinerea 
is unable to affect chemically the cuticle of the host, nor does it secrete any 
toxic substance which can pass through the cuticle and bring about the 
death of the underlying cells. He found that the fungus is unable to affect 
the underlying tissue until the obstacle offered by the cuticle has been 
removed. Penetration of the cuticle must take place in a purely mechanical 
way. In this connection, in another paper (5), he says: 
Once penetration of the cuticle has taken place the problem becomes simply an enzy- 
mological one. Further, in the case of the so-called wound parasites the problem presented 
is much simpler as the problems which arise antecedent to the penetration of the cuticle 
do not come into consideration. 
Gortner (13), in similar studies, found that Sclerotinia cinerea when 
grown on prune- and apple-juice media elaborated a very active pectase. 
When the fungus penetrates the host tissue it dissolves the middle lamellae, 
forming a product which, instead of being assimilated as food, is precipitated 
as a certain compound of calcium pectate. Rhizopus is one of those organ¬ 
isms that are unable to make their way into plant tissue which has not 
previously been disintegrated by enzymic action. In view of this fact, the 
fungus evidently depends upon the secretion of the enzym antecedent to 
infection. When exposed to sufficient moisture and to the proper tempera¬ 
ture, the spores germinate. Although the spores may germinate on a fresh 
wound, they seldom make sufficient growth to infect. The results seem to 
indicate that infection must start where there are dead cells or tissue on 
which the fungus can grow. In these dead cells the growth of the hyphae 
is accompanied by the secretion of the middle-lamella-dissolving enzym. 
This enzym secreted by the growing mycelium, when once it comes in 
contact with the healthy host cells beneath, brings about a disintegration 
of the tissue which is later invaded by the hyphae. 
Summary 
1. Rhizopus can not infect sweet potatoes through the unbroken skin. 
Spores and hyphae smeared on a freshly cut surface will produce infection 
only rarely. However, when the fungus is given a saprophytic start by 
growing on dead rootlets, in synthetic agar solidified on the cut surface of 
the potato, or in dead cells killed by charring over a Bunsen burner, infection 
takes place readily. Furthermore, infection can be brought about readily 
by growing the organism for one or two days in sweet-potato decoction, 
if the decoction and mycelium are poured into a “well” made in the potato 
and then sealed over with a cover glass to prevent evaporation. Infection 
is accomplished only after the dissolution of the middle lamellae by means 
of an enzym (pectinase) secreted by the growing hyphae. In practically 
all cases infection takes place in wounds where there is some dead tissue 
upon which the fungus can get a saprophytic start. During the growth 
of the mycelium in these dead cells, the enzym is produced which, when it 
