THE MECHANICS OF FOLIAR TRANSPIRATION 157 



of a leaf are filled with water, which also saturates the protoplasm and the 

 cell walls, this water being supplied to the leaf cells through the water con- 

 ducting tissues of the vascular bundles. Hence evaporation of water will 

 occur from these wet cell walls into the internal atmosphere of the inter- 



paZ/'sode 

 ce//s \ 



spongy 

 cells 



epidermis 

 sc/erenchijma 



■sc/erenchc/ma 



afvmafe 

 juard cell 



Fig. 34. Cross section of a portion of a leaf of tulip tree {Liriodendron tulipifera). 



cellular spaces just as it will occur from any wet surface into the surrounding 

 air. The intercellular spaces constitute a connected system, ramifying 

 throughout the leaf. Under certain unusual conditions the intercellular spaces 



rea/n duct 



■phloem 



■xylem 



endodermia 

 'guard ce// 

 ■stoma fe 

 - cti/orenctiyma 



epidermis 

 't)i/poderma/ sc/erenchyma 



Fig. 35. Cross section of a leaf of -vvhite pine {Pinus strobus). 



can become injected with liquid water but under all normal conditions they 

 are occupied by air. 



If the stomates are closed the only effect of evaporation from the meso- 

 phyll cell walls will be the saturation of the entire volume of the intercellular 



