428  Gelsemium  Sempervirens,  Ait.       { ^pfembe^i™ 
The  petiolar  bundles  are  at  first  distinctly  bicollateral.  Numer- 
ous patches  of  external  phloem  border  upon  the  outer  or  lower  face 
of  the  wood,  and  on  its  inner  or  upper  face  are  two  clearly  defined 
patches  of  internal  phloem.  Almost  immediately  after  the  petiole 
has  separated  from  the  stem,  the  main  petiolar  bundle  gives  off  two 
small  lateral  branches.  These  bundles  consist  chiefly  of  external 
phloem  with  a  little  xylem.  They  continue  upward  through  the 
petiole  and  along  the  sides  of  the  leaf,  where  their  branches  anasto- 
mose with  branches  from  the  main  leaf  bundle.  A  remarkable 
change  soon  takes  place  in  the  main  petiolar  bundle  of  a  kind  which, 
so  far  as  I  am  aware,  has  not  previously  been  described.  Just  above 
the  point  where  the  lateral  petiolar  bundles  branched  off,  the  two 
internal  phloem  patches,  one  after  the  other,  pass  downward  and  out- 
ward through  the  wood  to  join  the  external  phloem.  In  a  transverse 
section  of  a  petiole,  the  phloem  strands  may  be  seen  in  longitudinal 
section  passing  between  the  xylem  cells.  They  bend  outward  alon^ 
a  radius  of  the  bundle,  and  in  a  definite  position,  at  about  one-half 
of  the  distance  from  the  periphery  to  the  mid-line  of  the  bundle. 
After  the  passage  of  these  strands,  there  is  no  further  trace  of  in- 
ternal phloem  in  the  petiole  or  leaf. 
HISTOLOGY  OF  THE  ROOT. 
The  structure  of  a  very  young  root,  in  transverse  section,  is 
illustrated  in  Fig.  J.  The  loose-celled  starch-bearing  cortex, 
about  seven  to  eight  cells  deep,  is  separated  by  a  thin- walled  endo- 
dermis  from  the  axial  vascular  cylinder.  The  bundle  is  typically 
diarch.  The  two  groups  of  the  protoxylem  consist  each  of  about 
six  spiral  tracheae,  and  between  them  at  the  sides  of  the  bundle  lie 
two  small  patches  of  phloem,  separated  from  the  protoxylem  by  the 
procambium,  a  layer  of  large  prominently  nucleated  cells.  Outside 
the  xylem  and  phloem  elements,  and  just  within  the  endodermis,  is 
the  pericambial  zone.  Later,  by  secondary  growth,  the  xylem  is 
united  into  a  central  cylinder,  surrounded  externally  by  a  ring  of 
phloem,  but  internal  phloem  is  entirely  absent  in  the  root. 
On  older  roots  irregular  warts  or  swellings  are  frequently  found, 
which,  when  sectioned,  reveal  a  vigorous  fungoid  growth.  The 
fungoid  hypha?  ramify  through  the  cells  of  the  inner  and  especially 
the  middle  cortex,  and  in  some  places  large  cavities  occur,  resulting 
from  the  breaking  down  of  the  cortex  cells.    These  are  filled  with 
