19, 1 _ McLean: Permeability of Citrus Leaves 121 
TABLE 3.—Tests of pressure required to inject water into Citrus leaves, 
detached from the plant, immersed in water, and placed in bright light 
in the laboratory window. 
Width of ridge of 
entrance of stoma. 
LS ty Date. Hour. Kind of leaf. Pressure, 
oe Average. 
p.m, : em, Hg. Bs BL 
13 | April $2.2222.- 2.30 | Medium Pernambuco grapefruit___ 9.5 
19 | April5____.-_. 8.40 |... BB cu cctecn liicacenae |, Sum TE, 62 
20 | April 5_...-__. 4,00 | Young Pernambuco grapefruit __-- 6.5 9.7 7.0 
21 | April6_..____. 2.00 | Medium Pernambuco grapefruit ___ 3.0 10.0 7.9 
i Aprig. 2.40 | Young Szinkom mandarin orange-. 5.0 6.0 4.8 
23 |} April6_-_..... 3.45 | Young Pernambuco grapefruit __-- Lg eile aie Ge fille be 
24} April 6__._.__. 4.07 58 OO cance ara cepeuucls cums 5.5 9.3 19 
25 | April6__.____- 4,42 | Young Szinkom mandarin orange.. 12.5 6.7 3.5 
26 | April7__...... G10555 co a naan ie ac Mic tea ERAN es NSE 1.6 1.3 5.6 
2 | April... a a i Mee OG ses ean cae BOW Fides ckalatewnctuu 
201 April Tocco 535 Ci ere do 18.0 5.7 4.3 
a Lae 4.30 | Young Pernambuco grapefruit -__- 0.5 9.3 8.7 
ing. Thus a medium Pernambuco grapefruit leaf in the first 
series (No. 8, Table 1) gave an injection pressure of 12.0 cen- 
timeters, while the same leaf, under the more favorable treat- 
ment in the second series (Nos. 18, 19, and 21, Table 3), gave 
values of 9.5, 2.0, and 3.0 centimeters successively. The lowest 
pressures obtained in this series, as shown in Table 3 for Per- 
nambuco, are of such magnitude as might easily occur in leaves 
on the trees, due to changes of temperature when the leaves 
are wet, or even to rapid expansions and contractions of the air 
chambers when the leaves are bent in being blown about by the 
wind. The combination of abundant moisture and light provided 
in these tests appears to have caused the stomata to open and 
make the entrance of water easy. 
The tests in Tables 1, 2, and 3 do not nearly approach the nor- 
mal conditions to which the leaves on the trees are subjected 
during bright sunny weather. Therefore, a third lot of leaves — 
were left attached to twigs, which were put in water and placed 
so as to be fully exposed to the sun. These were tested at inter- 
vals during the day, and the results are tabulated in Table 4. 
The young leaves on twigs and exposed to full insolation show 
great variations in the pressures required to inject them with 
water. Using similar Szinkom leaves for all tests, the values 
varied from 5.5 to more than 38.0 centimeters. Since these 
