AFetoua^yf?9ao7m*}     The  Internal  Structure  of  the  Stem  and  Leaf.  53 
very  thick-walled,  especially  the  outer  walls,  and  does  not  contain 
crystals  of  any  kind.  The  cortex  consists  of  about  ten  layers,  of 
which  the  peripheral,  the  one  that  borders  on  epidermis,  is  thickened 
like  collenchyma.  Endodermis  is  thin- walled  and  surrounds  a  closed 
sheath  of  thick-walled  stereome  in  one  or  two  layers.  The  mestome- 
bundles  are  collateral,  and  the  vessels  are  arranged  in  long  rows 
with  thick- walled  medullary  rays.  The  pith  is  thin-walled,  solid  and 
contains  no  starch. 
Spigelia  Marilandica  L. — In  this  plant  the  stem  is  four-winged, 
and  the  wings  quite  scabrous  from  short,  pointed  hairs.  The  cuticle 
is  wrinkled  longitudinally,  and  epidermis  very  thick-walled.  A 
collenchyma  of  several  layers  is  to  be  observed  in  the  wings,  but  of 
only  one  layer  between  these,  and  completely  encircling  the  cortex ; 
the  latter  consists  of  about  four  layers  with  chlorophyll  and  starch. 
The  stele  is  here  supported  by  a  few  strata  of  heavily  thickened 
stereome  in  an  almost  uninterrupted  zone.  Very  characteristic  are 
the  bicollateral  mestome  strands,  in  which  the  leptome  is  distributed 
on  the  outer  and  inner  face  of  the  hadromatic  rays.  The  vessels 
are  narrow,  about  ten  in  each  row,  and  the  medullary  rays  consist 
only  of  one  layer  between  each  two  of  the  hadromatic  rays. 
If  we  compare  now  the  stem-structure  of  our  three  plants,  it 
seems  as  if  this  may  be  sufficient  to  the  distinguishing  of  Spigelia 
from  both  Phlox  and  Ruellia.  The  four-winged  stem  and  the  bicol- 
lateral mestome-strands  are  characters  that  are  possessed  by  Spigelia 
in  contrast  to  the  quadrangular  stem  of  Ruellia  with  cystolithes, 
and  the  cylindric  stem  of  Phlox  destitute  of  crystals  and  with  simply 
collateral  mestome-bundles.  That  the  stem-leaves  of  these  same 
plants  offer  equally  distinct  peculiarities  may  be  seen  from  the  fol- 
lowing notes  : 
Ruellia  ciliosa  Pursh. — On  the  leaf-blade  the  cuticle  is  thin  and 
smooth  ;  epidermis  is  slightly  thickened  f*1*  .  ^uter  wall)  and  viewed 
"  en  face  "  {Fig.  10)  the  lateral  cell-walls1  snow  prominent  undula- 
tions, and  on  both  faces.  Stomata  occur  only  on  the  lower  face, 
and  they  have  one  pair  of  subsidiary  cells  which  are  vertical  on 
the  stoma  {Fig.  id).  Large  cystolithes  abound  in  epidermis  on 
both  faces,  and  two  kinds  of  hairs  were  observed :  small,  almost 
sessile  glandular  {Fig.  //),  which  are  scattered  over  both  faces  of 
the  leaf,  and  some  long,  pericellular,  pointed,  which  are  very  fre- 
quent along  the  nerves,  especially  on  the  lower  face.    The  chloren- 
