114 TUE OAK. 



are traversed by very numerous pit-canals ; cells contain- 

 ing crystals also accompany these groups. 



In consequence of the above arrangements the second- 

 ary cortex presents a more or less stratified appearance 

 on the transverse section, the strata consisting chiefly of 

 alternate tangential layers of hard bast and soft bast 

 (Fig. 17) ; the elements of the latter also showing a de- 

 cided tendency to be arranged in layers. 



After the first year the young stem or branches cov- 

 ered with thin periderm are seen to be dotted with len- 

 ticels or cortical pores. Structures similar in every re- 

 spect and subserving the same function viz., the ex- 

 change 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 peri- 

 derm, 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 leuticel, 

 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 de- 

 veloped as such, but they loosen and round off, and their 



