19,1 McLean: Permeability of Citrus Leaves 119 
ment of the leaves had a great effect upon the injection pressure 
observed. Therefore, several different treatments were tried, 
and the results of these are presented in Tables 1, 2, 3, and 4. 
All of these tables are arranged in the same manner. An ar- 
bitrary number is given to each test for convenience in designa- 
tion. Pressure, causing penetration of water, was measured 
by means of the height, in centimeters, of the mercury column 
in the pressure tube. The averages of the width of the ridge 
of entrance of the stomata on the injected parts of the leaf are 
based on few measurements — usually ten and sometimes fewer — 
because the injected areas of the leaves were themselves small. 
The stomata varied considerably in size and only the larger ones 
were measured, as they were the ones through which the water 
would pass the most readily. 
RESULTS 
The earlier tests were made on leaves which had been kept 
in the comparatively dim light of the laboratory for varying 
lengths of time before testing. The results of these tests are 
assembled in Tables 1 and 2, those of Table 1 including those 
which had been in the laboratory for one-half day or less, and 
those of Table 2 for a longer time. 
Table 1 . — Tests of the pressure required to inject water into freshly 
gathered Citrus leaves, detached from twigs and kept in water for a 
short time. 
Test 
No. 
Date. 
Hour. 
Kind of leaf. 
Pressure. 
Width of ridge of | 
entrance of stoma. 
Maxi- 
mum. 
Average. 
a. m. p. m. 
cm. Hg. 
u 
u 
i 
April 3 
4.00 
Young 1 Pernambuco grape- 
fruit. 
6.5 
8.3 
7.8 
2 
April 3 
4.30 
Young Kishiu mandarin 
orange 
11.5 
4.0 
3.0 
3 
April 3 
4.50 
Young Szinkom mandarin 
orange 
23.0 
3.8 
3.0 
4 
April 4 - 
9.00 
Old Washington navel orange- 
15.0 
9.0 
7.3 
5 
April 4 
9.00 
Medium Washington navel 
orange 
22.5 
7.7 
7.3 
6 
April 4 _ 
9. 00 
Young Washington navel 
orange _ 
12.0 
7.7 
6.3 
7 
April 4 . __ . 
11.30 
Half-grown Washington navel 
orange 
10.3 
6.0 
4.8 
8 
April 5 . 
10. 45 
Medium Pernambuco grape- 
fruit 
12.0 
9.7 
6.0 
9 
April 5 _ ... 
12.00 
Young Pernambuco grape- 
fruit - 
12.5 
8.0 
6.3 
