228 
Journal of Agricultural Research 
Vol. XXIV* No. 3 
in a mortar with sand and water. One liter was decanted off and filtered 
through a sterilized clay filter. No organisms were found in the filtrate. 
Part of this filtrate was boiled and the other part left unheated. Two 
small holes slanting downward were bored, one in each side of a large 
lemon tree, and burettes holding about 50 cc. of the liquid were inserted 
into these holes and sealed. One burette contained a portion of the boiled 
filtrate just referred to, and the other unboiled filtrate. In each case 
the solution wsis taken up by the tree in a few hours. On November 5, 
1912, a considerable quantity of gum was observed pushing up into the 
burette which had contained the unboiled filtrate. No gum was observed 
in the other burette. No further development had taken place in either 
case on Noveniber 30, 1912. 
Experiment 2.^—On April 16,1913, filtrate from a large piece of lemon 
bark containing a lesion 35 by 8 cm. produced by inoculation with 
Pythiacysiis citrophthora (PI. 3, B) was obtained as described in the pre¬ 
vious experiment. At the same time, a filtrate from a large piece of sound 
bark from a healthy tree was obtained in the same manner, as a control, 
^e filtrate from the diseased bark was placed in a separatory funnel 
and was allowed to be absorbed by a yoimg lemon tree (PI. 8) through a 
small hole in the bark. About 100 cc. were taken up in five days. The 
filtrate from the sound bark was placed so as to be taken up by another 
lemon tree of the same size. 
On May 6, 1913, thin gum was observed pushing up into the lower end 
of the funnel tube through which diseased filtrate had been introduced. 
Gum undoubtedly had begun to form internally long before this time. 
In about two weeks the gum (about 10 cc.) had accumulated in the lower 
enlarged portion to a height shown in Plate 8, after which the pressure 
resulted in this breaking through the rubber connections at the bottom. 
Very little gum was exuded after June 3, 1913, when the funnel w^ 
removed. The bark remained alive around the opening. During this 
entire time no gum appeared in the funnel through which the filtrate 
obtained from the sound bark had been introduced. 
The results of these experiments indicate that there is a substance in 
the diseased bark which is capable of passing through a fine clay filter 
and of inducing gum formation. In the experiments, however, gum 
formation soon ceased without serious injuiy to the bark, such as is 
produced by the invasion of Pythiacysiis citrophthora. It would seem, 
therefore, that the gumming is induced by some substance that^ is 
formed either by the fungus or by the interaction of the host and parasite. 
The fungus during its invasion of the host probably secretes a substance 
which stimulates either directly or indirectly the production of a gum- 
inducing enzym by the host cells themselves. This substance doubtless 
also passes out into adjacent cells, and into the conducting tissue, and 
thus brings about gum formation at long distances from the invaded zone. 
This substance passing out from the invaded regions probably accounts 
for the large outer noninvaded gummous zones which so readily recover 
when the inciting cause, the parasitic fungus, is removed. This substance, 
capable of inducing gum formation, but not capable of bringing about 
the formation of lesions characteristic of Pythiacystis gummosis, appears 
to be destroyed by boiling and may, therefore, be an enzym. Further 
investigation as to the real nature of this gum-inducing substance is 
needed. 
