104 McLEAN: STOMATA OF TWO SPECIES OF CITRUS 
mandarin were 6 w in length and 0.6 y in width. These dimen- 
sions were very uniform, the extreme lengths being 5 and 7 un, 
and the extreme widths 0.5 and 1.5y. Therefore twenty-five 
measurements were deemed sufficient to give a satisfactory aver- 
age. Sixty corresponding measurements of Florida seedling 
grapefruit stomata gave an average length of the opening of 9.8 u, 
and a width of 6.64. The extreme lengths were 7 u and 15 pn, 
and the extreme widths 5 wand 1iy. Thus the narrowest opening 
in the case of Florida seedling grapefruit (5 4) was more than 
three times as wide as the widest in Szinkum mandarin (1.5 u). 
By focusing downward, the outlines of the guard cells become 
visible and are shown at d in both Figs. 1, A, and 1, D. In 
addition, the outline of the wall of the outer chamber is shown at 
o in 1, A, and the outline of the pore, p, is shown in 1, D. 
The parts shown in the median cfoss sections (Fics. 1, B, and 
1, EZ) are labelled to correspond to those described above. The 
positions of the views shown in 1, A and 1, D, are indicated by the 
horizontal lines AA and DD, and the positions of the Fics. 1, C, 
' and 1, F, are shown in a similar manner. The shaded portions of 
these drawings show the portions of the cell walls which are 
cutinized. The most prominent differences between the two 
species is again seen to be the ridge of entrance, 7, which is elon- 
gated, projecting over the outer chamber in the case of Szinkum 
mandarin. In the Florida seeding grapefruit the ridge of entrance 
is so short that its inner walls are nearly perpendicular, even in 
the closed stoma, and assume a more spread position in the open 
stoma. Another feature of interest, though common to both, 
is the extension of the cutinized tissue along the vertical walls 
of the guard cells down to the pores. 
The under views (Fics. 1, C, and 1, F) show clearly that the 
size of the pore, , is approximately the same in the closed stomata 
of both species. 
The main differences in the two species are, then, in the size 
of the opening in the cuticle, and in the shape of the ridge of 
entrance, which bounds this opening. The opening is much larger 
in the grapefruit than in the mandarin, and the ridge of entrance 
has its inner walls more nearly perpendicular to the leaf surface. 
The bearing of these differences upon the resistance to citrus 
ae canker will now be considered. , 
