LOSS OF WATER 



37 



seven bundles extends out into the blade as the prominent 

 mid-veifi 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 fibro-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.^ 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- 

 ways nearly saturated; that is, it 

 holds nearly as much water as pos- 

 sible in the form of vapor. From 

 the intercellular spaces the vapor 

 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 



Fig. 29. — Leaf of plantain 

 {Plantago), with the petiole 

 stretched lengthwise from a 

 transverse cut, showing the 

 fibro-vascular bundles that 

 continue up into the five 

 main veins of the leaf-blade. 



^ 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. 



