19,1 McLean: Permeability of Citrus Leaves 123 
will be the subject of a later contribution. The data’thus far 
obtained on the leaves on the trees bear out in a general way 
the tentative conclusions stated above. 
SUMMARY 
1, A method is described for determining the pressure required 
to force water through the stomata of leaves. 
2. The pressure required to inject leaves of the same variety 
seems to vary greatly in accordance with the treatment before 
testing, and seems to be lowest when the leaves are exposed to 
bright diffused light and well supplied with moisture. 
3. Apparently Szinkom mandarin orange leaves require on the’ 
average more than twice as much pressure to inject them with 
water as is required for the leaves of Pernambuco grapefruit. 
4. The conclusion stated in 3 seems to be correlated with the 
average width of the stomatal aperture through the epidermis, 
that of Szinkom mandarin orange being a little more than half 
as wide as that of Pernambuco grapefruit. 
