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 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, necked with leuticels 

 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 



