120 The Philippine Journal of Science 1921 
Table 2. — Tests of pressure required to inject water into Citrus leaves, 
detached from twigs and kept with their petioles in water in the 
diffused light of the laboratory for one-half day longer. 
Test 
No. 
Date. 
Hour. 
Kind of leaf. 
Pressure. 
Width of ridge of 
entrance of stoma. 
Maxi- 
mum. 
Average. 
a. m. p. m. 
cm. Hg t 
#* 
u 
10 
April 4 
3. 10 
Medium Washington navel orange. 
21.5 
9.7 
7.3 
11 
April 4 _ . .. 
3.35 
Half-grown Szinkom mandarin 
orange. 
31.5 
3.0 
2.6 
12 
April 4 ... .. 
4. 10 
Old, yellow Szinkom mandarin 
orange __ . .. . 
31.0 
6.3 
4.0 
13 
4. 60 
6.0 
10.0 
9.0 
14 
April 4 
5.25 
Young Szinkom mandarin orange. 
31.5 
15 
April 5 
3.46 
Young Pernambuco grapefruit... 
25.6 
• 
16 
11. 45 
9.5 
17 
4. 60 
14.0 
7.2 
4.7 

A comparison of the leaves of different apparent ages of the 
three different varieties showed no clear relationship between 
age and injection pressure. The individual variations between 
tests were so great that, in order to eliminate one possible 
variant, subsequent tests were confined to comparatively young 
leaves. Therefore, no conclusion can be drawn from these obser- 
vations concerning the effect of age of the leaves upon the 
injection pressure. All of the above tests show exceedingly 
variable and generally high injection pressures, probably due to 
the unfavorable conditions to which they were subjected in the 
laboratory. 
Several tests were made on leaves which had been immersed 
in water and placed in the window in bright diffuse light previous 
to testing. The results of these are assembled in Table 3. Some 
of these were tested before placing them in the window, and 
are thus entered in Table 2. These repeated tests on the same 
leaves are particularly valuable for comparison and to show the 
effect of conditions of exposure on the leaf properties. 
The injection pressures shown in Table 3, in which the leaves 
were immersed in water and kept in bright light before testing, 
are generally lower than those in Tables 1 and 2. Thus the 
values in Table 3 range from 2 and 5 millimeters in tests 23 and 
29 for Pernambuco grapefruit, to 15.5 and 18.0 centimeters 
in tests 27 and 28 for Szinkom mandarin. Further, a compari- 
son of the same leaf, tested in each of the two series, is illuminat- 
