90 PLANT BIOLOGY 



dermis, but as the twig ages the epidermis perishes and 

 the bark becomes the external layer. Gases continue to 

 pass in and out through the lenticels, until the branch be- 

 comes heavily covered with thick, corky bark. With the 

 growth of the twig, the lenticel scars enlarge lengthwise 

 or crosswise or assume other shapes, often becoming char- 

 acteristic markings. 



Fibro-vascular Bundles. We have studied the fibro- 

 vascular bundles of stems (Chap. X). These stem bun- 

 dles continue into the leaves, ramifying into the veins, 

 carrying the soil water inwards and bringing, by diffusion, 

 the elaborated food out through the sieve-cells. Cut 

 across a petiole and notice the hard spots or areas in it; 

 strip these parts lengthwise of the petiole : what are they ? 



Fall of the Leaf. In most common deciduous plants, 

 when the season's work for the leaf is ended, the nutritious 

 matter may be withdrawn, and a layer of corky cells is com- 

 pleted over the surface of the stem where the leaf is attached. 

 The leaf soon falls. It often falls even before it is killed 

 by frost. Deciduous leaves begin to show the surface line 

 of articulation in the early growing season. This articula- 

 tion 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. In Chap. XV are shown 

 a number of leaf-scars. In the plane tree (sycamore or 

 buttonwood), the leaf-scar is in the form of a ring surround- 

 ing the bud, for the bud is covered by the hollowed end of 

 the petiole ; the leaf of sumac is similar. 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 micro- 

 scope. Note the character of cells that cover the leaf- 

 scar surface. 



