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 fibre-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 (Pkuitago), with the petiole 



holds nearly as much water as pos- 



sible in the form of vapor. From fibro-vascular bundles that 



, . continue up into the five 



the intercellular spaces the vapor ma i n 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. 



