AmjineSarra'}  North  American  Coniferce.  329 
MICROSCOPICAL  STRUCTURE. 
The  cross-section  of  a  twig  of  two  years'  growth  showed  in 
places  the  persistent  epidermis,  but  there  were  frequent  interruptions 
due  to  exfoliation  caused  by  the  growth  of  the  periderm.  When 
present  it  was  excessively  thick-walled,  lignified  and  cutinized. 
Fig-  33,  cross-section  of  stem,  of  three  years'  growth,  of  Piuus  Montana, 
magnified  100  diameters.  A,  disintegrating  cells  at  the  exterior;  b,  sclerotic 
cells  of  the  periderm;  c,  tannin  cells  ;  d,  tannin  cell  in  bast  layer;  e,  cambium; 
f,  secretion  reservoir  in  secondary  xylem.  Drawing  made  from  specimen 
treated  with  solution  of  anhydrous  ferric  chloride  in  absolute  alcohol. 
Intervening  between  it  and  the  periderm  were  a  few  layers  of  thin- 
walled  parenchyma.  The  outer  layers  of  the  periderm  were  scler- 
otic, and  the  newer  ones  beneath  thin-walled  and  rich  in  tannic 
matters.     The  cortical  parenchyma,  the  bast  layer,  the  cambium 
