114 THER OAK 
of which are traversed by very numerous pit-canals ; 
cells containing crystals also accompany these groups. 
In consequence of the above arrangements the 
secondary cortex presents a more or less stratified 
appearance on the transverse section, the strata con- 
sisting chiefly of alternate tangential layers of hard 
bast and soft bast (fig. 17); the elements of the latter also 
showing a decided tendency to be arranged in layers. 
After the first year the young stem or branches 
covered with thin periderm are seen to be dotted with 
lenticels or cortical pores. Structures similar in every 
respect and subserving the same function—viz. the 
exchange of gases with the environment—are formed on 
the roots as soon as the periderm is developed. 
The lenticel is a local interruption of the periderm, 
where the cells are loosened so as to allow air to pass 
between the loosened cells into the intercellular spaces 
between the cortical cells. Hach lenticel may be de- 
scribed as a biconvex projecting swelling of the periderm, 
the swelling being caused by the increased radial 
diameter of the loosened cells. ‘This is the condition 
during the spring and summer, but in the winter the 
cork-cambium is continuous across beneath the lenticel, 
and forms periderm in an uninterrupted sheet, to be 
ruptured again at the lenticel during the formation and 
swelling of the looser cells (complementary or packing 
cells) in the following spring. These loose packing- 
cells are at first quite similar to young cork-cells, and 
are developed as such, but they loosen and round off, 
