Fig.  20,  portion  of  transverse  section  of  a  twig  of  two  years'  growth  of  Pinus 
Taeda,  magnified  100  diameters.  A,  epidermis  ;  b,  sclerotic  strengthening  cells 
beneath  the  epidermis  ;  c,  layer  of  thin-walled  parenchyma ;  d,  thick-walled 
cells  of  periderm  ;  e,  lacuna  ;  f,  secretion  reservoir  in  cortex  ;  g,  bast  layer ; 
k,  cambium  ;  k,  tracheids  of  the  xylem. 
