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 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 becomes 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 characteristic markings.

Fibro-vascular Bundles.—We have studied the fibro-vascular bundles of stems (Chap. X). These stem bundles 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the Leaf.—In most common deciduous plants, when the season's work for the leaf is ended, the nutritious matter may be withdrawn, and ''a layer of corky cells is completed over the surface of the stem where the leaf is attached.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 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. 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 surrounding 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 microscope. Note the character of cells that cover the leaf-scar surface.