820 The Philippine Journal of Science 
SUMMARY 
1. A method is outlined of applying measurable pressure to 
Citrus leaves and determining by this means the pressures ne- 
cessary to cause penetration of such tissues by water. 
2. Leaves of Szinkom mandarin orange, Washington navel 
_ orange, seedling East Indian pummelo, and Pernambuco grape- 
fruit were tested by this method to determine their comparative 
injection pressure with water. 
‘Tests of the injection pressures of Citrus leaves gave the 
following results: The average pressure for Pernambuco grape- 
fruit was 19.5 centimeters of mercury; seedling East Indian 
pummelo, 19.6; Washington navel orange, 20.8; and Szinkom 
mandarin orange, 33.6. 
8. The average injection pressures of the above four varieties 
are directly proportional to their canker resistance, as shown 
by field observations. . 
4, Leaves of Szinkom mandarin orange, a resistant variety 
of Citrus, and seedling grapefruit and pummelo trees, both very 
susceptible, were tested for their resistance to the entrance of 
canker organisms applied in water under pressure. Szinkom 
mandarin orange leaves were resistant to canker infection by 
immersion, and up to pressures of 10 centimeters of the mercury 
column, With high pressures numerous cankers developed in 
leaves of this variety. Grapefruit and pummelo leaves devel- 
oped canker readily by immersion without added pressure. 
The pressures necessary to cause canker infection were thus 
in agreement with the degree of observed field resistance of 
the sorts tested. 
5. The results obtained strongly substantiate the theory pre- 
viously advanced that structural differences in the stomata con- 
stitute one cause for the differences in susceptibility of the 
mandarin orange and the grapefruit and pummelo varieties. 
In the mandarin orange apparently the structure of the sto- 
mata prevents the ingress of surface water; in the grapefruit 
the stomatal structure is such as to allow the ingress of surface 
water which thus affords a medium of entrance for the canker 
bacteria. ,; 
6. The results definitely indicate that the resistance of the 
mandarin orange is due to mechanical structural differences. 
