McLEAN: STOMATA OF TWO SPECIES OF CITRUS 105 
Canker is caused by a bacterium which is motile in water, 
but entirely passive in air. It is believed that it cannot attack 
or even by its own activity traverse dry cutinized or waxy cell 
walls of Citrus. The outer walls in both of the species studied 
are cutinized and normally dry, except when moistened by rain or 
dew. This is true also of the walls of the outer chambers of 
the stomata, which are likewise cutinized, as shown by the cross 
sections of both species studied (Fics. 1, B, and 1, E). Granting 
the above to be true, the bacteria can only penetrate to the un- 
cutinized cells of the air spaces inside the leaves in continuous 
filmsof water. In intact Citrus leaves these can only form through 
the stomatal openings. 
If a Citrus leaf of either of the sorts studied is immersed in 
water and studied under the microscope, air bubbles are found in 
the stomatal openings. In cross sections these bubbles are seen 
to extend to the ridge of entrance of the stoma. Thus when a 
water film is formed over a Citrus leaf, this film is held outside of 
the stomatal openings by the ridges of entrance. With the sway- 
ing of the leaves in the wind and with changes in temperature of 
the air inside the leaves, there are variations in the pressure against 
the water films covering the stomata, such that there will be a 
tendency for the water covering the outer surface of the leaf to 
be drawn into the intercellular spaces. 
It will require less pressure to drive the water film inward 
through a wide aperture with nearly parallel walls, such as form 
the sides of the outer portion of the outer chamber of the Florida 
seedling grapefruit, than will be required to drive a water film 
past a narrow opening, along receding walls, such as form the 
outer portion of the outer chamber of the stomata of the Szinkum 
mandarin. Therefore, assuming that a certain minimum pressure 
is necessary to drive water into the stomata of grapefruit and 
thus establish a passageway for the entrance of bacteria, then a 
much greater pressure will be required to accomplsh the same 
result in the case of Szinkum mandarin. Once water has passed 
the widest part of the outer chamber, it will then contract its 
air-water film as it approaches the pore, and surface tension will 
then accelerate instead of retard the process. When water has 
passed through the pore, it is then in contact with moist, un- 
