114 *n 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. 1 7) ; 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. Each 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, 



