426  Gelsemium  Sempervirens,  Ait.  {^eptimbe^issT" 
times  present.  Very  early  in  the  life  history  of  the  stem  death  of 
the  pith  cells  occurs.  The  cell  contents  dry  up,  the  pith  as  a  whole 
shrinks  away  from  the  sides  and  becomes  detached  from  the  phloem 
sheath,  but  persists  as  an  inert  somewhat  lignified  mass,  until  its 
place  is  usurped  by  the  enlarging  phloem  patches. 
HISTOLOGY  OF  THE  STEM  FROM  THE  SECOND    TO  THE  TENTH  YEAR. 
In  a  transverse  section  of  a  stem  at  the  end  of  the  second  year's 
growth,  the  most  prominent  change  is  the  increased  size  of  the 
internal  phloem  patches.  Each  has  pushed  farther  out  into  the 
pith,  and  as  the  growth  has  been  greater  in  the  middle  than  at  the 
sides,  the  inner  margin  has  a  curved  outline,  with  the  convexity 
toward  the  pith.  The  formation  of  new  cells  from  the  medullary 
cambium  takes  place  centrifugally,  the  newly  formed  cells  lying  ex- 
ternal to  the  old.  On  the  inner  side  of  each  patch,  adjoining  the 
phloem  sheath,  a  dark  crescentic  mass  of  partially  obliterated  tissue 
is  now  evident.  This  is  composed  of  the  older  sieve  tubes  that 
have  collapsed  and  been  pushed  together  by  the  pressure  from  the 
new  elements  laid  down  by  the  active  medullary  cambium. 
The  external  phloem  has  increased  but  little  in  breadth,  in  com- 
parison with  the  internal  patches,  but  the  total  number  of  cells  and 
the  actual  area  of  the  zone  is  greater  than  before.  Here  and  there 
along  the  border  are  darker  areas,  composed  of  four  or  five  com- 
pressed cells,  showing  that  the  same  crowding  and  obliteration  goes 
on,  although  to  a  less  extent  than  in  the  internal  patches. 
In  older  stems  the  increased  size  of  the  internal  phloem  patches 
becomes  more  and  more  prominent.  The  masses  of  crushed  tissue 
or  "  Hornbast "  (Fig.  2)  are  more  numerous  and  broader, 
the  later  formed  ones  lying  in  concentric  layers  external  to  the 
older  masses.  Some  large  phloem  parenchyma  cells  are  often  pres- 
ent between  the  crushed  masses,  for  they  are  better  able  to  resist 
the  crushing  process,  owing  to  their  greater  turgidity.  The  patches 
may  thus  present  a  stratified  appearance  from  the  alternation  of  the 
bands  of  crushed  tissue  and  the  scattered  parenchyma  cells.  Each 
of  the  four  patches  usually  divides  into  two  parts,  so  that  in  the 
oldest  stems  eight  cone-shaped  masses  of  internal  phloem  are  pres- 
ent. The  neighboring  patches  grow  together  laterally,  while  they 
continue  to  encroach  upon  the  pith.  In  the  oldest  stem  examined 
{Fig*  2),  of  about  twelve  years'  growth,  the  internal  phloem  patches 
