140 ELEMENTS OF PLANT ANATOMY. 
The lenticels of the superficial periderm arise generally in 
the ground tissue under a stoma or a group of the same. They 
are usually lens-shaped or oval with a fissure running length- 
wise through the middle. They are generally raised above the 
surface of the periderm, more rarely sunk below or on a level 
with it. Many are extremely small, appearing only as small 
points. The greater number are larger, reaching sometimes 
more than a centimeter in length. Examples, lenticels of Betula. 
FIG. 62. 
Lenticel from Sambucus nigra. /f filling cells. ec phellogen. v rejuvenating 
layer. ph phelloderm. 0b bast bundle. —(Acced. to Stahl.) 
In some instances lenticels originate before the formation of 
periderm, by certain of the ground cells below a stoma be- 
coming meristematic and forming a phellogen layer which 
eventually joins that of the periderm. (Figs. 60 and 61). This 
phellogen develops parenchymatic tissue in both directions, that 
toward the center becoming ordinary chlorophyll-holding cells, 
while that without forms what is known as the filling tissue. 
This consists at first of ordinary isodiametric cells, which after 
a time separate at the corners and finally become quite free. 
By their rapidly increasing numbers (Fig. 62) they crowd up 
into the air space, and finally rupture the epidermis, thus leaving 
