6io 
Journal of Agricultural Research 
Vol. XXI, No. 9 
has the power to dissolve certain constituents of the cell walls so that 
coherence is lost. 
The fungi belonging to this group kill rapidly and by destroying the 
middle lamellae transform the tissues into a mass of more or less isolated 
cells together with a large amount of a clear, somewhat colored liquid 
inclosed by the skin. The fungus secretes a substance which diffuses out 
of the hyphae into the tissue of the host and kills often several cells in 
advance of the fungous threads. The mycelium then must derive its 
nourishment from the dead material in which it is embedded. No doubt 
these fungi, since they live on the dead material thus produced, would 
be classed by some as saprophytes; nevertheless, they differ from true 
saprophytes in that they can attack and destroy living organisms. In 
this sense they are parasites. 
The dissolution of the middle lamella has been demonstrated by several 
investigators to be due to a substance which is enzymic in nature. This 
enzyni has been considered by most investigators to possess two rather 
distinct types of action on the cells of the host: first, a killing action on 
their protoplasmic contents, and, second, a dissolution of certain of the 
constituents of their cell walls. The writers at present will deal only 
with the latter. 
The enzym which acts upon the pectic compounds of the cell walls, 
causing loss of coherence, has been designated by some investigators as 
pectinase and by others as pectosinase {17 ). 1 If it had been definitely 
proved that the middle lamella was largely composed of pectase the 
enzym which acted upon it should properly be designated as pectosinase. 
However, since the composition of the middle lamella has not been defi¬ 
nitely determined and since the term pectinase is so widely used in the 
literature, the writers have employed it to designate the enzyni secreted 
by Rhizopus tritici which dissolves the middle lamellae and produces 
what is termed maceration. 
HISTORICAL 
There is a considerable amount of data published which bear direetly 
or indirectly on the physiology of parasitism, but only those which are 
germane to this phase of the subject will be considered here. DeBary (1) 
was probably among the first to detect the activity of a fungous extract 
in dissolving the cell wall. He showed that the drops which exuded 
from the sclerotia of Sclerotinia libertia'na Fuckel., as well as the juice 
expressed from decayed plant tissues, contained a substance of the nature 
of a ferment which was thermolabile and possessed the power of dissolv¬ 
ing certain constituents of the cell walls. He observed differences in the 
activity of the boiled and unbilled extracts, but he was not able to deter¬ 
mine whether or not the entire action of the extract was due to one or 
1 Reference is made by number (italic) to “literature cited,” p. 624-625. 
