116 THE OAK 
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 in- 
creases 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 colour gradually 
passing into a silver-grey as the periderm develops, and 
in a few years (when the shoot is from five to twenty 
years old, or thereabouts) 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—.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 
