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water that otherwise would cling to the outer surface 
of the leaf blade is kept at a distance from the little 
mouths, and these are not interrupted in the per- 
formance of a duty so necessary to the health of the 
plant. 
This same habit of coating its lower leaf surfaces 
with hair, you notice in the speckled or swamp alder, a 
shrub which grows also in wet places, and therefore 
runs the same risk of having its leaf mouths clogged 
with water. 
So when you see only the upper surface of a leaf 
-covered with hair, you can guess that the object of the 
plant is to prevent too much perspiration; but when 
you see only its lower side clothed in this same way, 
you can guess that the plant fears too little perspiration. 
Sometimes you find a plant with leaves which have a 
coating of what looks almost like dust on one or both of 
their surfaces. This dust we call “bloom.” We see it 
in apples and grapes, as well as on leaves. It is made 
up of a waxy material which is put forth by the plant 
just as it puts forth hair. This bloom the plant uses 
also as a help to free perspiration. By thus clothing its 
leaves it shields the little mouths from water clogging; 
. and so you can be sure that the little mouths have not 
been filled with water, and thus prevented from doing 
their work. 
The cabbage leaf has mouths on both of its surfaces, 
and so both sides are covered with this protecting bloom. 
If you dip a cabbage leaf in water and then shake it, the 
drops roll off and leave it quite dry. 
