420 
Structure  of  Podophyllum. 
/Am.  Jour.  Pharm. 
X  September,  1894. 
structure  quite  different  from  that  of  the  rhizome.  The  bundles, 
instead  of  being  arranged  in  a  circle,  are  scattered  without  apparent 
order  through  the  stem  as  they  are  in  the  stems  of  monocotyls. 
This  deviation  from  the  dicotyl  type  is  rather  remarkable. 
The  cork  cells  of  the  rhizome  are  formed  by  the  tangential  divis- 
sion  of  the  exterior  layer  of  collenchyma  cells,  that  is,  of  the  one 
immediately  underneath  the  epidermis.  Only  a  few  tiers  of  cork 
cells  are  formed,  usually  three  or  four,  before  the  epidermis  ruptures, 
and  thereafter  the  thickness  of  cork  does  not  increase,  the  scaling  off 
at  the  surface  keeping  pace  with  the  growth  from  within.  Fig.  2 
shows  a  small  portion  of  a  cross-section  of  a  rather  young  rhizome, 
Fig.  3. 
including  the  epidermis  and  sublying  collenchyma,  magnified  175 
diameters.  The  section  from  which  the  drawing  was  made  was 
cleared  by  means  of  hydrate  of  chloral  solution,  which  also  swells 
the  cell  walls,  so  that  in  the  drawing  they  are  shown  of  greater 
thickness  than  the  normal,  a  is  the  rather  thick  cuticle ;  £,  an  epi- 
dermal cell ;  c,  a  tangential  partition  which  has  recently  been  formed 
across  the  outer  layer  of  collenchyma  cells,  the  outer  of  the  two  tiers 
of  celis  thus  formed  being  the  young  cork,  and  the  inner  the  phello- 
gen  or  cork  cambium;  d,  collenchyma  cell  farther  interior.  The 
collenchyma  in  its  inner  layers  passes  gradually  into  parenchyma. 
