106 McLEAN: STOMATA OF TWO SPECIES OF CITRUS 
cutinized cell-walls, and if the bacteria penetrate by means of con- 
tinuous water from the surface of the leaf to this point, they 
appear then to be able to persist and develop in most Citrus species, - 
as shown by the inoculation experiments of Lee. 
The differences in stomatal structure observed in the two 
kinds of Citrus here studied thus are of such a character as to 
account satisfactorily for the observed difference in their resistance 
to canker. Field observations and inoculation experiments with 
Citrus canker show that many of the other Citrus species and varie- 
ties possess resistance to canker of the same general character as 
that of the Szinkum mandarin but differing in degree. It may 
be that these other resistant and partially resistant sorts may 
show structural characters also resembling the Szinkum mandarin. 
Further study of stomatal structure and canker resistance among 
the Rutaceae is therefore desirable. 
The writer wishes to express his thanks to Dr. N. L. Britton 
and his associates at the New York. Botanical Garden, partic- 
ularly Dr. A. B. Stout, for their kindness in furnishing laboratory 
facilities and valuable suggestions during this study. 
SUMMARY 
I. Szinkum mandarin; which is resistant to Citrus canker, 
and Florida seedling grapefruit, which is susceptible, are compared, 
and from a review of previous studies, their difference in resistance 
is believed to be due to a difference in the character of the stomata. 
2. Both sorts have stomata of about the same size and type, 
differing mainly in the ridge of entrance, which is broad in the 
mandarin variety, overarching the outer chamber and forming a 
narrow external opening. In the grapefruit variety it is narrow, 
making the upper part of the walls of the outer chamber nearly 
parallel and affording a large opening. 
3. These differences are such as to practically exclude water 
from the stomata of the mandarin, whereas it can more readily 
enter those of the grapefruit. 
4. The exclusion of water is sufficient reason to account for 
the resistance of Citrus varieties to canker, since the canker 
bacteria are motile when in water but not when dry. 
UNIVERSITY OF THE PHILIPPINES 
