52     The  Iniernal  Structure  of  the  Stem  and  Leaf.  {ArebJrXvPi9ao7m' 
the  leaf  is  very  hairy,  the  hairs  long  and  very  distinct  along  the 
veins  especially.  A  full  description  of  these  plants  is,  moreover,  to 
be  found  in  Gray's  Manual  and  in  his  Synoptical  Flora  of  North 
America. 
When  thus  the  specimens  are  complete  their  identification  is  very 
easy ;  when  neither  the  flowers  nor  fruits  are  represented  the  plants 
may  be  identified  by  means  of  their  leaves  and  stems,  not  speaking 
of  the  excellent  characters  furnished  by  their  rhizomes  and  roots. 
A  microscopical  examination  may  sometimes  be  necessary,  and 
in  order  to  facilitate  the  work  we  might  offer  a  brief  description  of 
the  anatomical  structure  possessed  by  these  plants  as  an  appendix  to 
our  paper  (1.  c.)  on  their  root- structure.  Let  us  first  examine  the 
stem. 
Ruellia  ciliosa  Pursh. — In  the  Acanthaceoe  the  nodes  of  the  stem 
are  usually  quite  swollen  and  thicker  than  the  internodes,  which  is 
due  to  a  different  development  of  some  of  the  tissues.  The  internodes 
are  obtusely  quadrangular  in  cross-sections.  The  cuticle  is  smooth, 
and  the  outer  cell  walls  of  epidermis  are  moderately  thickened. 
Cystolithes  are  frequent  in  the  epidermis.  A  collenchymatic  tissue 
of  several  separate  strands  is  to  be  observed  outside  the  cortex  ;  this 
parenchyma,  the  cortical,  is  thin-walled  and  consists  of  about  nine 
layers  with  cystolithes  and  raphides,  but  without  sclerotic  cells.  A 
thin-walled  endodermis  surrounds  the  stele,  which  is  obtusely  quad- 
rangular in  transverse  section.  We  find  in  the  stele  an  almost  con- 
tinuous ring  of  stereome  located  between  endodermis  and  the  leptome. 
The  mestome-strands  are  all  collateral ;  the  hadrome  consists  of  rather 
short  rays  of  vessels  separated  by  mostly  four  rows  of  thick-walled 
parenchyma.  The  central  portion  of  the  stele  is  occupied  by  a  thin- 
walled  pith  with  many  raphides  and  crystals,  but  without  cystolithes, 
sclerotic  cells  and  deposits  of  starch. 
In  the  swollen  nodus be  collenchyma  is  developed  much  stronger, 
i.  e.}  it  constitutes  a  closed  ,heath  around  the  cortex ;  the  cells  are 
more  thick-walled  and  there  is  a  larger  number  of  layers.  The 
cortex  represents  a  broader  zone  in  the  node  than  in  the  internode  ; 
furthermore,  the  stereome  inside  endodermis  is  reduced  to  a  few 
scattered  cells.  In  regard  to  the  pith  this  is  much  broader  in  the 
node  than  in  the  internode. 
Phlox  ovata  L. — The  stem  is  cylindric,  glabrous  and  smooth,  and 
covered  by  a  thick,  longitudinally  wrinkled  cuticle.    Epidermis  is 
