^pdmbe^S'}        Gelsemium  Semper  vir  ens  >  Ait.  425 
whose  walls  are  thickened  and  deeply  pitted.  Large  vessels  are 
numerous  in  the  outer  portion  of  the  zone. 
On  the  inner  side  of  the  wood  lie  four  large  rounded  patches  of 
internal  phloem  extending  into  the  pith.  These  patches  are  two  to 
three  times  broader  than  the  external  phloem  zone,  and  consist  also 
of  sieve  tubes  and  undifferentiated  phloem  elements.  The  inner 
margins  of  the  phloem  patches  are  bounded  by  a  two-celled  layer, 
which  may  be  termed  a  phloem  sheath  (Fig.  1).  This  is 
sharply  differentiated  alike  from  the  adjoining  pith  cells  and  from 
the  phloem.    A  row  of  somewhat  similar  but  smaller  cells  separates 
Fig.  1 
Fig.  2. 
the  outer  margin  of  the  phloem  patches  from  the  wood,  and  imme- 
diately internal  to  this  row  are  the  patches  of  medullary  cambium. 
The  cambium  cells  have  the  usual  brick-like  shape,  thin  walls  and 
large  nuclei.  The  cells  of  the  sheath  are  rounded  and  in  close  con- 
tact with  each  other.  They  have  thickened  pitted  walls  and  are 
conspicuous  by  their  size  and  the  large  amount  of  chlorophyl  and 
starch  they  contain.  The  pith  cells  are  much  larger,  have  thin  but 
slightly  pitted  walls,  and  a  scanty  supply  of  chlorophyl  and  starch, 
while  the  intercellular  spaces  are  larger  than  those  of  the  phloem 
sheath.    A  few  short  sclerenchymatous  or  "  stone  "  cells  are  some- 
