TRANSPIRA TION. 



43 



in a previous chapter, with reference to the physical properties of the living 

 cell, we shall be able to give a partial explanation of the comparative slow- 

 ness with which the water escapes from the leaves. The inner surfaces of 

 the cell walls are lined with the membrane of protoplasm, and within this 

 is the cell-sap. These cells have become turgid by the absorption of the 



Fig. 530. 



Cross-section of leaf of wintergreen. Cu, cuticle; Epid., epidermis; v.d., vascular 

 duct; Int. c. sp., intercellular space; L. ep., lower epidermis; St., stoma. 



water which has passed up to them from the roots. While the protoplas- 

 mic membrane of the cells does not readily permit the water to filter through, 

 yet it is saturated with water, and the elastic cell wall with which it is in 

 contact is also saturated. From the cell wall the water evaporates into the 

 intercellular spaces. But the water is given up slowly through the proto- 

 plasmic membrane, so that the water vapor cannot be given off as rapidly 

 from the cell walls as it could if the protoplasm were dead. The living 

 protoplasmic membrane then which is only slowly permeable to the water of 

 the cell-sap is here a very important factor in checking the too rapid loss of 

 water from the leaves. 



