226 
Journal of Agricultural Research 
Vol. XXIV. Na 3 
or sunburn, nor does it necessarily result from drying of tissue in wounds 
if these are kept uncontaminated by microorganism or chemical stimu¬ 
lants. 
Some of the results of the investigations reported in Parts I and II of this 
paper have a bearing on this subject. It was shown that gum formation 
could be induced in cuts inoculated by Pythiacystis citrophthora or Botrytis 
cvn^ea where these were covered with grafting wax or waxed paper, 
which presumably prevented undue loss of water from the surface. It 
is, of course, possible that dehydration of the cells invaded by the para¬ 
site might have taken place even in this case, without loss of water 
through the surface, by changes in internal conditions by which water 
was lost to the surrounding tissues. With Pythiacystis gummosis ex¬ 
cessive water in the soil about the trunks, rather than drying or partial 
desiccation, is the condition favorable for the invasion of the bark by 
the causal organism that is instrumental in bringing about gum 
formation. 
While partial desiccation in itself does not appear to be an important 
factor in initiating gum formation in Citrus, it is probably one of the 
factors in producing an acceleration of the process when the other neces¬ 
sary factors are already in operation. It has been noticed also that when 
a lesion is developing and gumming moderately, the coming of dry 
weather is frequently followed by increased exudation of gum for a brief 
period. If the weather is sufficiently dry, however, the enlargement 
of the invaded zone often ceases, probably due to the dying out of the 
pararite, and the rate of gum formation soon decreases to zero. It 
appears that in Pythiacystis gumm^osis, at least, the conditions of desic¬ 
cation that tend to increase the gum flow are frequently those that hinder 
or kill out the causal agent, while furnishing a temporary stimulus to 
increased gum formation. If, as seems highly probable, partial desic¬ 
cation of the cells is one of a group of factors which favor excessive gum 
formation in Citrus, it seems clear that other more important influences 
or stimuli must precede or accompany it. 
OHEMieAI, STiMUtt AND GUM FORMATION 
^any substances have been reported as inducing gum formation in 
Citrus. A study of gum formation was made by Floyd (ji) by the in¬ 
troduction of 28 organic and inorganic substances into young Citrus 
trees. Thirteen of these induced gum formation. Most of them were 
acids, alkalies, or salts of heavy metals. In general, the salts of the 
heavy metals brought about the greatest amount of gumming. The 
gum formation was coincident with the injury from the chemicals; the 
guin was small in amount and was formed in proximity to the region of 
insertion of the chemicals except in case of some of the salts of the 
heavy metals. The gum originated in all cases in the live embryonic 
xylem tissue in regions or zones beyond the dead area produced by the 
chemical. None of the cuts or injuries used as controls, and not inocu¬ 
lated with chemicals, produced any gum formation. The dead area 
produced by the chemical is thus seen to be directly comparable to the 
invaded zone in Pythiacystis gummosis, and the region of chemically 
induced gum formation beyond this is directly comparable to thenonin- 
vaded '‘outer gummous zone” of the disease. 
Experiments on Citrus had been made previously with a limited 
number of chemicals. Butler (ij) induced gum formation with sul- 
