1916] 
ZELLER—PHYSIOLOGY OF LENZITES SAEPIARIA 447 
porous ones. This shows that a certain undetermined mini- 
mum of oxygen will prevent the growth of certain fungi. Ex- 
ceptions to this, however, may be numerous. For instance, 
Münch found that Armillaria mellea is not dependent on air 
in the substrate. The rhizomorphs of this organism are of 
such a structure that they conduct air into water-saturated 
tissues. Doubtless there are other forms which have the 
power to thus conduct air to parts where there is a paucity 
of air. 
Undoubtedly, high water content will inhibit the entrance of 
certain fungi, but as soon as there is a paucity of water the 
tissues are as susceptible as ever, for there is no change in 
the properties of the host tissues themselves. Thus, an in- 
creased water content as a factor in the immunization of a 
host plant against disease (as Miinch would lead one to think) 
is not compatible with present-day ideas of immunity. 
Appel (715) concurs in Münch's idea that the paucity of 
air due to high water content may be made an effective method 
in the control of certain plant diseases. He applies this to 
die-back diseases of trees due to species of Valsa and other 
fungi. He says: 
“When such diseases occur, you will find the cause in defective 
irrigation methods, which may be remedied by changing the 
irrigation system. It is of the greatest importance that the 
land be irrigated at the time the trees contain less water and 
plenty of air, and that the next irrigations be made in time 
to prevent an excessive decrease of the water in the tissues.” 
Further, he states that the same principle may be found to 
be applicable to bacterial diseases of trees, especially Bacillus 
amylovorus, and finally remarks: 
“It may be possible that not only trees, but also herbaceous 
plants, show relations between fungous growth and air con- 
tent. I think it must be so for the organisms which cause the 
wilt diseases and the rhizoctonia disease of the potato, both of 
which have a high air requirement. . . . .. hough caused 
by a fungus, the production of conditions favorable to the 
progress of the disease [Rhizoctonia] is attributable to irriga- 
* H 
The writer believes that the close application of the work 
of Minch given by Appel to such diseases as those caused by 
