ANATOMY OF TISSUES. 139 
the periderm is developed in the ordinary manner. The cork 
wings are gradually worn away. Stems of Liquidamber illus- 
trate this. 
The outer walls of the primary epidermal cells taken collec- 
tively form a membrane around the plant organ which is more 
Fie. 60. 
Beginning of the growth of a lenticel on a birch branch. astoma, b air space. c begin- 
ning of divisions which give rise to the filling tissue. d epidermal cells. c cuticle 
raised up. — (De Bary.) 
or less impervious to air and water. The corky layers of the 
secondary epidermis or periderm form a covering with similar 
Fig. 61. 
Finished lenticel. astoma. 6 intercellular space in the filling tissue. e lenticel- 
phellogen. d beginning of periderm. — (De Bary.) 
function. This latter covering is suppled with breathing 
places or pores corresponding to the stomata of the primary 
epidermis ; to these organs De Candolle gave the name lenticel. 
