102 McLEAN: STOMATA OF TWO SPECIES OF CITRUS 
inoculated have internal tissues susceptible to canker, but into 
which canker cannot ordinarily penetrate. Their resistance is 
believed to be due to differences in the structure of the stomata.* 
The present paper gives a comparison of the stomatal struc- 
tures of two kinds of Citrus; one resistant to canker in the manner 
mentioned above and the other susceptible. For this purpose 
the Szinkum mandarin (Citrus nobilis var. Szinkum) was chosen 
as a highly resistant sort, and a seedling of the Florida grapefruit 
(C. grandis) was chosen as an example of a susceptible sort. 
These sorts belong to closely related species, they differ compara- 
tively little in leaf morphology, and therefore such differences as 
are noted in the stomata are the more likely to be directly related 
to their canker resistance or susceptibility. 
PROCEDURE 
Young leaves two thirds of their mature size were gathered 
from the plantation at the College of Agriculture, University of 
Philippines, on April 10, 1920, and were preserved in alcohol of 
about 80 per cent concentration. Only the young leaves were 
used in this study, because the older leaves are no longer susceptible 
to canker infection in either species, and the old leaves are exceed- 
ingly difficult to section for microscopical study. The sectioning 
and structural study was carried out at the New York Botanical 
Garden during June, 1920. 
Thin slices were cut with a razor parallel to the upper and under 
surfaces of the leaves. Those from the upper surface showed no 
stomata. The sections from the under surface were mounted, 
partly with the cuticle uppermost and partly with the cuticle 
below. Cross sections were also made of the leaves parallel to 
their margins. Free-hand sections were found to be the most 
satisfactory. Imbedding in paraffin was also tried, but the waxy 
portion was apparently removed from the leaves or rendered 
transparent by this treatment. 
The stomata of the two species were found to be similar in size, 
general form and mechanism of opening and closing. They are 
* The opinion has been expressed that this resistance is due to the epidermis, 
and evidence in support of this view is given in a paper to be published by Lee and 
the writer on the resistance of Citrus nobilis to Citrus canker, with a suggestion for 
the production of resistant varieties. 
