May, 1923] HARTER AND WEIMER — PECTINASE AND INFECTION 
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principle would naturally depend upon the number of spores per unit 
volume of solution. 
The second flask was taken off at the end of 7 hours, the solution was 
filtered, and raw sweet-potato disks from the same source and of the same 
size were suspended in it. Maceration of the tissue in this case was much 
more rapid, being quite advanced in 24 hours and complete in 45 hours. 
The spores were mostly germinated, and the amount of hyphal growth was 
far more abundant than in the flask removed an hour earlier. A duplicate 
experiment with R. nigricans gave similar results, although the rate of 
maceration was somewhat slower than in the case of R. tritici. It would 
seem from these results that a substance capable of dissolving the middle 
lamellae is secreted early in the process of germination, and that this 
macerating principle very rapidly increases in amount at least in the early 
period of growth. The maximum is reached in about 2 days in the solution 
and in about 3 days in the mycelium (17). 
It has been shown that this enzym is produced early in the germination 
of the spores (6 hours). More recent researches (not published) have 
shown that the spores themselves independently of their germination 
contain an enzym which, when extracted with water, will bring about the 
disintegration of raw sweet-potato disks. 
The data presented thus far show fairly conclusively that Rhizopus is 
unable to penetrate mechanically the unbroken skin of the sweet potato. 
It also shows that, even though the spores and hyphae alone are placed on 
a fresh wound, infection usually does not take place, although what might 
be regarded as favorable temperature and humidity are provided. It was, 
however, shown by a number of experiments that if the fungus is grown for 
a day or two on a decoction made of sweet potatoes, and this, together 
with the mycelial growth, is poured into a “well” in the potato, infection 
will almost always take place. The writers showed elsewhere (17) that if 
raw sweet-potato blocks were immersed in the substratum on which the 
fungus had grown, after it had been freed of the fungus, or in a watery 
extract of the dead mycelium, a rapid dissolution of the middle lamellae 
took place. This action on the living tissue is in every respect identical 
with that produced when the fungus itself is decaying the potato. Atten¬ 
tion was also called to the fact that in decaying sweet potatoes there is a 
zone of disintegrated tissue adjacent to the sound tissue which is sterile of 
the fungus. These and other results show that for this fungus at least a 
saprophytic start is nearly if not always required before infection takes 
place. That enzyms play a part in the decay of various plant organs is 
pretty generally agreed, and that they play an important role in infection 
has been suggested. De Bary, Ward, Nordhausen, Smith, and others noted 
what undoubtedly was enzymic action. Although the evidence as presented 
by them does not unqualifiedly prove the action of an enzym, there can 
be little doubt that they were dealing with what the writers have regarded 
