412 
North  American  Conifer  ce. 
<  Am.  Jour.  Phaim. 
\      August,  1816. 
MICROSCOPICAL  STRUCTURE. 
A  twig  of  several  years'  growth,  from  which  the  epidermis  and 
exterior  parts  of  the  primary  cortex  had  disappeared,  was  studied 
microscopically.  The  superficial  periderm  layers,  composed  partly 
of  stone  cells  and  partly  of  thin-walled  cells,  contained  much  tannin 
and  coloring  matter.  The  sublying  cortical  layers  contained  many 
tannin  and  crystal  cells,  some  cells  enclosing  brown  coloring  matter, 
and  a  few  stone  cells.  The  bast  layer  was  destitute  of  bast  fibres,  and, 
like  P.  alba,  had  its  numerous  rather  large  tannin  cells  arranged  in 
tangential  rows.  There  were  a  few  secretion  reservoirs  in  the  bark. 
The  wood  was  similar  in  structure  to  that  of  P.  alba,  but  the  secre- 
Fig.  4.1. — Cross-section  of  leaf  of  Picea  nigra,  magnified  75  diameters.  Ep, 
epidermis ;  hy,  hypoderma  ;  m,  mesophyll ;  en ,  endodermis ;  st,  stoma  ;  xy, 
xylem  of  fibro-vascular  bundle  ;  /,  fibres  at  the  outer  margin  of  the  phloem, 
ph  ;  t,  transfusion  tissues. 
tion  reservoirs  were  few  and  small.  Tannin  was  observed  to  be 
present  in  the  medullary  ray  cells  of  the  wood. 
The  leaves  of  this  species  are  from  y2  inch  to  S/%  inch  long,  and 
Jv-  inch  in  diameter,  abruptly  pointed,  usually  curved,  quadrangular- 
prismatic,  possessing  from  three  to  five  more  or  less  interrupted 
rows  of  stomata  on  each  side.  The  epidermis  was  observed  to  be 
composed  of  small,  very  thick-walled  cells,  and  the  hypoderma  of 
one  layer  of  thick-walled  cells.  The  mesophyll  was  composed  of 
wavy-walled  parenchyma,  the  central  bundle  was  small,  distinctly 
double,  and  had  at  the  outer  border  of  the  phloem  a  few  thick- 
