432 
Gelsemmm  Sempervirens,  Ait. 
I  Am.  Jour.  Pharm. 
t  September,  18  9 
pear  groups  of  very  small,  thin-walled  cells,  whose  division  walls 
lie  in  all  planes.  Soon  thereafter  similar  groups  of  small  cells  are 
differentiated  on  the  border  of  the  pith  area.  These  represent  the 
internal  phloem  patches.  The  course  of  the  internal  phloem  has 
been  traced  in  older  stems  into  the  petioles,  so  it  may  be  regarded 
as  an  integral  part  of  the  leaf  trace  bundle.  It  owes  its  origin  to 
the  same  primary  meristem  that  gives  rise  to  the  external  phloem, 
and  the  protoxylem.  Certain  primary  meristematic  cells  o'n  the 
inner  face  of  the  protoxylem  represent  the  medullary  cambium.  To 
the  later  activity  of  these  cells  the  secondary  growth  of  the  medul- 
lary phloem  is  due.  A  radial  arrangement  of  the  later-formed 
medullary  phloem  cells  is  to  be  observed,  and  is  an  indication  of 
their  cambial  origin.  The  medullary  phloem  appears  in  the  hypo- 
cotyl  some  time  after  the  differentiation  of  protoxylem  and  external 
phloem.  Its  origin,  however,  is  from  embryonic  cells  that  are  a  part 
of  the  original  primary  meristem  of  the  bundle.  The  appearance 
of  these  embryonic  cells,  on  the  inner  side  of  two  bundles  in  the 
hypocotyl,  at  a  definite  point  below  the  cotyledonary  node,  and  of 
similar  cells  in  the  two  opposite  bundles  in  the  epicotyl,  just  below 
the  first  leaf  node,  may  be  explained  as  follows,  on  phylogenetic 
grounds.  Internal  phloem  is  a  secondary  character  acquired  during 
the  evolution  of  the  plant.  Since  the  hypocotyl  and  cotyledons  are 
embryonic  structures  representing  the  primitive  stages  of  growth  of 
the  plant,  characters  that  have  been  acquired  by,  and  are  adapted 
to,  the  adult  stem,  may  reasonably  be  found  absent  throughout  the 
whole,  or  a  part,  of  the  hypocotyl.  In  this  plant  the  lower  portion 
of  the  hypocotyl  exhibits  the  ancestral  condition  in  the  absence  of 
internal  phloem.  The  upper  portion  of  the  hypocotyl  and  of  the 
epicotyl  are  transition  stages,  for  two  bundles  have  acquired  internal 
phloem,  while  two  bundles  are  as  yet  devoid  of  it.  The  region  of 
the  first  leaf  node  shows  the  acquired  condition  of  the  presence  of 
internal  phloem  in  all  four  bundles. 
The  physiological  significance  of  this  acquisition,  and  the  causes 
that  led  to  it,  are  not  clear.  It  is  a  noteworthy  fact  that  internal 
phloem  appears  only  in  parts  of  this  plant  where  pith  is  present. 
Although  present  in  the  stem,  internal  phloem  is  absent  throughout 
the  greater  length  of  the  petiole.  It  is  present  in  the  upper  portion 
of  the  hypocotyl,  but  is  absent  in  the  lower  part  where  the  pith  area 
is  becoming  constricted  by  inward  growth  of  the  xylem.    Both  in- 
