CHAPTER X. 



HOW WATER MOVES THROUGH THE PLANT— 

 Concluded. 



III. Part which the Leaf plays in Transpiration. 

 Demonstration 1 9. 



98. Structure of a leaf. — We are now led to inquire why it is that a 

 living leaf loses water less rapidly than dead ones, and why less water 

 escapes from a given leaf surface than from an equal surface of water. To 

 understand this it will be necessary to examine the minute structure of a 

 leaf. For this purpose we will select the leaf of an ivy, though many other 

 leaves will answer equally well. From a por- 

 tion of the leaf we should make very thin 

 cross-sections with a razor or other sharp in- 

 strument. These sections should be perpen- 

 dicular to the surface of the leaf, and should 

 be then mounted in water for microscopic 

 examination. * 



Let the pupils examine the preparations and 

 make sketches of the structure of the leaf, 

 naming the different kinds of cells, and de- 

 scribing the function of the different groups 

 of cells. (See paragraphs 99-101.) 



99. Epidermis of the leaf. — In this 



Fig. 47. . , , 



Section through ivy leaf show- section we see that the green part of 



ing communication between sto- , , . . , , 



mate and the large intercellular the leat is bordered on what are its 



spaces of the leaf ; stoma closed. 



upper and lower surfaces by a row 

 of cells which possess no green color. The walls of the cells 

 of each row have nearly parallel sides, and the cross walls are 

 perpendicular. These cells form a single layer over both sur- 



* Demonstrations may be made with prepared sections of leaves. 



56 



