272 STRUCTUKE OF LEAVES 



of corky cells is completed over the surface of the 

 stem where the leaf is attached. The leaf soon falls. It 

 often falls even before killed by frost. Deciduous leaves 

 begin to show the surface line of articulation in the early 

 growing season. This articulation may be observed at 

 any time during the summer. The area of the twig once 

 covered by the petioles is called the leaf-scar after the leaf 

 has fallen. Figs. 53, 83, 86 show a numl)er of leaf-scars. 



415. Stomates of geranium leaf. 416. Grouped stomates on a begonia leaf. 



Fig. 417 shows the leaf -scar in the form of a ring sur- 

 rounding the bud, for in the plane tree the bud is covered 

 by the hollowed end of the petiole; sumac is a similar 

 case. Examine with a hand-lens leaf-scars of several 

 woody plants. Note the number of bundle -scars in each 

 leaf-scar. Sections may be cut through a leaf-scar and 

 examined with the microscope. Note the character of cells 

 which cover the leaf- scar surface. Compare 204. 



Review. — Name three tissues found in leaves. On the board 

 draw a sketch showing the structure of a leaf as seen in cross-section. 

 What cells of leaves bear protoplasm and chlorophyll? Why do 

 some leaves have palisade cells near both surfaces? Describe 

 epidermal cells. Why are their walls much more thickened in 

 some plants than others? What is the purpose of epidermis? What 

 are stomates? Draw on the board a section through a stomate 

 showing epidermis and mesophyll. What is the work of guard cells? 

 Give some idea of number of stomates in various plants. Name five 



