LOSS OF WATER 
37 
seven bundles extends out into the blade as the prominent 
mid-vein of a leaflet. In a common "trick" of child- 
hood, the epidermis of the petiole of the common, broad- 
leaved plantain is broken by sharply bending the petiole, 
or by carefully cutting with a 
knife. The petiole may then be 
carefully pulled apart, so as to dis- 
close the nbro-vascular bundles 
without breaking them (Fig. 29). 
These bundles are the channels 
through which liquids pass between 
the leaf-blade and the branch. 
38. Transpiration. In order to 
understand transpiration, we 
should have in mind a clear pic- 
ture of the conditions within a 
leaf. 1 Because of moisture in the 
cells, the cell- walls are saturated. 
From their moist surfaces water is 
continually evaporating into the 
intercellular spaces (Fig. 27), so 
that the air in those spaces is al- 
. . FIG. 29. Leaf of plantain 
ways nearly saturated; that is, it (Plantago), with the petiole 
holds nearly as much water as pos- 
sible in the form of vapor. From fibro-vascular bundles that 
,, , 11 i ,v continue up into the five 
tne intercellular spaces the vapor m ain veins of the leaf-blade. 
diffuses out through the stomata, 
and passes off into the air. If the outer air is also very 
humid, as frequently near the ground after sunset, the 
1 While loss of water is not confined to leaves, they are the chief organs 
of transpiration, and if we understand the process in them, we shall 
understand it elsewhere. 
