54     The  Internal  Structure  of  the  Stem  and  Leaf.     { AF%JrOuarVPf9a07m' 
chyma  is  differentiated  into  a  palisade-tissue  [P  in  Fig.  8)  and  a 
pneumatic,  more  open  tissue  underneath  this  (Px  in  the  same  figure). 
The  chlorenchyma  is  interrupted  by  collenchyma  and  by  a  thin-wal- 
led water-storage  tissue,  which  follows  the  stronger  veins  and  which 
is  especially  well  developed  on  the  dorsal  face  of  the  blade.  The 
mestome-strands  are  collateral,  and  the  mediane,  which  is  the  thick- 
est,  is  composed  of  several  strands  united  so  as  to  form  a  broad  arch 
with  the  concave  face  turned  upwards.  While  the  lateral  nerves 
possess  distinct  and  completely  closed  parenchyma-sheaths,  the 
midrib  shows  merely  an  open  sheath  covering  the  leptome  side  of 
the  mestome  strands,  but  not  the  hadrome. 
Phlox  ovata  L. — The  thick  cuticle  is  smooth  on  the  lower,  but 
wrinkled  on  the  upper  surface  of  the  leaf,  showing  striations  in  all 
directions  around  the  stomata ;  where  it  covers  the  hairs  {Fig.  ij)  it 
shows  very  distinct  spiral  ridges.  The  stomata,  which  are  confined 
to  the  dorsal  face  of  the  blade,  have  no  subsidiary  cells,  but  are  sur- 
rounded by  mostly  three  ordinary  epidermis  cells.  The  outer  cell- 
wall  of  epidermis  is  very  thick,  especially  on  the  lower  face.  Small 
wart-like  projections  of  epidermis  (Fig.  if)  abound  along  the  mar- 
gins of  the  leaf,  but  besides  these  there  are  also  some  short,  peri- 
cellular hairs  (Fig.  ij)  scattered  over  both  faces  and  along  the  mar- 
gins, but  they  are  very  minute ;  glandular  hairs  (Fig.  15)  consisting 
of  a  unicellular,  globose  head  borne  on  a  short  stalk  of  three  to 
four  cells  in  a  single  row,  occur  above  the  broad  midvein,  but  they 
do  not  seem  to  be  very  -numerous. 
In  regard  to  the  chlorenchyma  we  find,  also,  here  a  distinct 
bifacial  structure,  there  being  two  layers  of  typical  palisades  and 
about  five  strata  of  very  irregular  cells  with  wide  intercellular  spaces 
representing  a  pneumatic  tissue.  The  leaf  has  no  stereome,  but  a 
few  strata  of  collenchyma  accompany  the  midrib,  which  is,  more- 
over, surrounded  by  several  broad  layers  of  thin-walled  water  stor- 
age tissue.  The  midvein  shows  the  same  outline  as  in  Ruellia,  but 
has  a  closed  parenchymatic  sheath  all  around ;  the  lateral  veins  are 
very  thin  and  are  directly  embedded  in  the  chlorenchyma. 
Spigelia  Marilandica  L. — The  leaf  is  much  thicker  than  in  the 
two  other  plants,  and  the  veins  much  more  prominent  on  the  dorsal 
face.  The  cuticle  shows  striations,  but  these  are  not  so  distinct  as 
in  Phlox  ovata.    Epidermis  is  moderately  thickwalled  and  bears 
