78  North  American  Conifer ce.  {^ixZryM^' 
P.  Strobus,  was  observed.  The  bast,  cambium  and  xylem  layers  were 
observed  to  be  similar  in  structure  to  those  already  described  in  the 
other  species.  The  principal  structural  difference  between  this 
stem  and  the  stems  of  the  other  species  was  the  larger  average 
Fig.  16,  small  portion  of  cortex  of  P.  palustris,  magnified  300  diameters,  a, 
tannin  cell,  containing  tannin  in  great  quantity;  b,  stone  cell;  c,  c' ,  ordinary 
parenchyma  cells,  containing  a  little  tannin. 
size  of  the  bark  cells  and  the  presence  of  scattered  stone  cells  in  the 
middle  layer.  The  cells  were  also  obviously  much  richer  in  oleo- 
resin.  The  alkannin  test  showed  its  presence  in  nearly  all  the  tissues. 
