300 STRUCTURE OF LEAVES 



nutrients are withdrawn into the stem, and a layer of corky- 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. 57, 87, 

 91 show a number of leaf-scars. Fig. 474 shows the leaf- 

 scar in the form of a ring surrounding 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 that cover 

 the leaf-scar surface. Com- 

 pare 216. 



472. Stomates of geranium leaf. 473. Grouped stomates on a begonia leaf. 



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 

 in others? What is the purpose of epidermis? What are stomates? Draw 

 on the board a section through a stomate showing epidermis and 

 mesophyll. Give some idea of number of stomates in various plants. 

 Name several types of epidermal hairs. What utility could be suggested 

 for the dense coat of hairs on leaves of shepherdia? (Fig. 469.) 



Note. — To study leaf tissues: A number of leaves can be com- 



