THE PERMEABILITY OF CITRUS LEAVES TO WATER 
By F. T. McLean 
Professor of Botany, College of Agriculture, Los Baiios 
ONE TEXT FIGURE 
The resistance of leaves to injection hag been little investi- 
gated. In most of the physiological studies thus far reported, 
leaves have been treated mainly as instruments for gaseous 
interchange with the air and for photochemical reactions. A 
few ecological studies have been made of the relation of the 
form of leaves to the removal of water, and the opinion has 
been expressed that certain structures are beneficial in prevent- 
ing the clogging of the stomata with water and the flooding 
of the leaf tissues. So far as the writer is aware, there have 
been no studies of the pressures required to cause flooding. The 
resistance of leaves to the penetration of water may be important 
not only to prevent waterlogging of the tissues but also to pre- 
vent the entrance of certain leaf-disease organisms. Thus 
Pseudomonas citri Hasse, the cause of citrus canker, may be 
dependent mainly for its spread upon the permeability of the 
citrus leaves to water, as is suggested by McLean.’ He found 
that the differences in the structure of the stomata of a disease- 
resistant mandarin orange variety and a susceptible grapefruit 
were of such a character that water could enter the stomata of 
the former more easily than those of the latter. The structure 
and behavior of the canker bacteria are such that infection most — 
Probably takes place by means of continuous water columns, 
either through the stomata or through wounds. Whether in- 
fection will take place in this manner through the stomata or 
not depends upon the amount of pressure necessary to force 
water through the stomata. There appears to be need for more 
data on this particular point. 
There has been little study of the infiltration of the leaves 
of terrestrial plants with water, except in the case of a few 
epiphytic plants with special absorbing structures on the leaves. 
The infiltration of leaves with other liquids having a lower 
*McLean, Forman T., A study of the structure of the stomata of two 
Species of Citrus in relation to citrus canker, Bull. Torr. Bot. Club 48 (1921) 
101-106. 
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