are developed again, and these absorb water and bulge, bursting the closing layer and reopening the lenticel for the season. As the branch ages and its surface increases new lenticels are developed between the earlier ones, and, of course, with no reference to stomata.
The exterior of the very young stem or branch is smooth or slightly pubescent, the green color gradually passing into a silver-gray as the periderm develops, and in a few years (when the shoot is from five to twenty years old, or thereabout) the gradually thickening bark is shining and turning browner, flecked with lenticels and lichens. Later still the bark is rugged, brown, and fissured, and usually covered with small lichens and fungi. Bark begins to exfoliate at about the thirtieth year.
The epidermis cracks and peels off when the twigs are a year old, and shreds of the dead membrane may be detected on the outside of the young cork, which begins to form very early during the first year. It is, in fact, owing to the impervious nature of this cork that the epidermis dies, and to the stretching of the cortex as the stem grows in thickness that the dead membrane cracks and peels off (see Figs. 17 and 18).
The first indication of the development of the cork is the conversion of the sub-epidermal layer of cortex-cells into a meristem — i. e., the cells become capable of active growth and division.Each cell of the layer referred to may be termed an initial cell of the cork-cambium (or phellogen), and the