20, 3 Lee: Citrus Tissues and Citrus Canker 339 
It is of less immediate practical importance, and much more 
difficult, to obtain exact data on the increase in resistance with 
advance in maturity of foliage tissues. The difficulty is, in the 
main, in obtaining a criterion of the degree of maturity of a leaf. 
The data obtained have been from Washington navel leaves 
classed as (a) young, actively growing; (b) size fully developed 
but leaf still glossy, and the color only slightly deepened; and 
(c) fully matured and hardened leaves. The experimental 
results from similar infusions with identica] technic and main- 
tenance of inoculations indicate that, as the leaves become fully 
developed in size, the amounts of infection obtained, both with 
needle punctures and as stomatal infections, are very much 
lessened. Leaves become entirely resistant when they reach the 
size of maturity. 
DISCUSSION OF RESULTS 
The results obtained from the foregoing experiments appar- 
ently would warrant the statement, in conclusion, that the 
Susceptibility of fruit and foliage tissues decreases with their 
advance toward maturity. 
These results and conclusions are very intimately connected 
with field practices in preventing citrus canker. As the writer 
has pointed out in a paper now in press the problem of canker 
control on the moderately susceptible hosts, from the growers’ 
viewpoint, may be narrowed to the prevention of fruit infec- 
tions. The fruits of the Washington navel orange form in late 
May or June in western Japan. The period of susceptibility for 
such fruits in this district, as shown in Table 3, extends over 
possibly eighty-five days, during which stomatal infections are 
Probable. After this age the fruits are but slightly susceptible 
to stomatal infection, although infections at wounds and injuries 
may take place in a large percentage of the chances until one 
hundred ten to one hundred twenty days. Thereafter, in this 
district, fruits of this variety are, for all practical purposes, 
immune. It would follow that preventive methods may largely 
be confined to the period of June, July, and August in the 
district where the results here reported were obtained. 
The data on the susceptibility of the Unshiu orange are of 
less interest in local canker prevention because lesions upon 
this fruit are small, scarcely noticeable and, as observed in the 
seasons of 1918 and 1919, not at all common in Japan. Canker 
upon fruits of this variety, therefore, is almost negligible from 
the Japanese growers’ viewpoint. 
