470 
Structure  of  Heuchera  Americana. 
/Am.  Jour.  Pharm. 
\     October,  1894. 
the  starch- bearing  cortical  parenchyma  at  by  at  g,  the  inner  layer  of 
the  bark  wholly  composed  of  soft  tissues  and  containing  little  if  any 
starch.  But  it  is  the  woody  cylinder  that  has  the  most  charac- 
teristic structure.  In  many  roots  it  appears  crossed  through  its 
centre  by  a  straight  band  of  lignified  tissue,  consisting  almost  wholly 
of  scalariform  tracheids  and  ducts,  mainly  the  former.  This  band  is 
indicated  at  c  in  the  Figure.  Nearly  all  of  the  lignified  tissue  of 
the  root  is  concentrated  in  this  band,  but  a  few  small  areas  may  be 
found  on  either  side  of  it  adjacent  to  the  cambium  zone.  One  of 
these  is  indicated  at  e  in  the  Figure. 
This  appearance  of  the  mature  root  is  due  to  the  fact  that  the 
Fig.  5. 
original  radial  bundle  in  the  root  is  commonly  di-arch,  and  during  the 
course  of  the  secondary  changes  it  undergoes,  it  preserves  its  two- 
rayed  character,  the  secondary  formations,  except  narrow  areas  just 
back  of  each  original  phloem  mass,  producing  but  little  lignified 
tissue. 
The  original  bundles  of  the  root,  however,  are  not  always  di- 
arch  ;  sometimes  they  are  tri-arch,  and  sometimes  tetrarch.  Hence, 
an  old  or  mature  root  may  sometimes  show  three  or  four  narrow 
xylem  rays,  instead  of  the  straight  band,  shown  in  Fig.  J. 
Fig.  4.  shows  the  cross-section  of  a  younger  root,  which,  how- 
ever, has  undergone  considerable  secondary  changes,  and  which 
4 
