318 The Philippine Journal of Science 1922 
than in the case of the Szinkom mandarin orange. Infections 
were numerous at pressures indicated by 10, 7, and 5 centimeters 
of mercury, and even with only the almost negligible hydro- 
static pressure of the liquid in which the leaves were submerged. 
In Table 4 results on pummelos are recorded. 
The results of the tests recorded in Table 4 are uniformly 
positive, with the single exception of leaf 1, twig 2. It is evi- 
dent that at very low mercury pressures, and even with simple 
immersion, the citrus-canker organism gains ready access to the 
tissues of the leaves of this host. 
The observed injection pressures shown in Tables 1, 2, 3, and 
4 are much higher than the pressures which cause canker infec- 
tion, thus indicating that water and canker organisms are drawn 
into the leaf tissues at lower pressures than those causing visible 
injection. The pressure causing visible injection thus appears 
to be roughly proportionate to that necessary for infection, but 
not identical with it. 
DISCUSSION OF RESULTS 
It is apparent from the foregoing results that canker will de- 
velop in the leaves of the mandarin orange, even in the absence 
of injury to the tissues, once the canker bacteria have gained 
entrance into the leaf. 
It has been concluded from the results presented in the papers 
previously listed, that the resistance to citrus canker of the man- 
darin orange varieties is due to mechanical peculiarities of struc- 
ture, and that such mechanical peculiarities apparently exist in 
the epidermis. It was also shown that the character of the 
stomata of the mandarin orange was such as to prevent the 
ready ingress of water; the results just presented, moreover, 
show that in the mandarin orange pressure is required to draw 
the water into the stomata. On the other hand, it has been 
shown that the stomata in the grapefruit are of such a structure 
as to fill readily with water, and the foregoing experiments 
indicate that simple immersion is sufficient to fill the stomata 
with water and cause stomatal infections. 
The present results, therefore, rather definitely support the 
theory previously advanced that the resistance of the mandarin 
orange is dependent upon its stomatal structure and that the 
structural differences in the stomata of the mandarin orange 
and grapefruit and pummelo constitute at least one cause for 
their differences in susceptibility to citrus canker. The struc- 
