558         Root- Structure  of  Spigelia  Marilandica.    { ADecimbe^ isoe?' 
In  cases  where  the  rhizome  of  Ruellia  may  be  less  represented, 
the  roots  alone  are  sufficient  to  prove  that  they  belong  to  this  genus 
and  not  to  Spigelia.  The  most  conspicuous  and  reliable  features  by 
which  the  roots  of  Ruellia  may  be  characterized  consist  in  as  fol- 
lows :  The  hairy  epidermis  and  thin-walled  exodermis  surround  the 
cortical  parenchyma,  which  consists  of  about  twelve  layers  of  cells, 
thick-walled  in  the  two  peripherals,  but  thin-walled  in  the  others. 
In  this  parenchyma,  the  cortical,  we  notice  (in  transverse  sections, 
Fig.  j)  a  number  of  thick-walled  cells  with  the  walls  porous  and  the 
lumen  quite  narrow ;  our  Fig.  2  shows  one  of  these  "  stone-cells  " 
in  longitudinal  section,  surrounded  by  thin-walled  cortical  cells. 
And  in  this  same  parenchyma,  the  cortex,  we  notice,  furthermore, 
the  presence  of  numerous  cystolithes  with  distinct  granulose  surface 
{Fig.  ^,  longitudinal  section) ;  viewed  in  cross-sections  the  cysto- 
lithes (Fig.  5)  are  seen  to  be  hollow,  but  quite  thick-walled.  In 
other  words,  we  have  before  us  exactly  the  same  structure  as  illus- 
trated so  very  well  by  Professor  Greenish  of  his  supposed  Phlox 
Carolina.  Inside  the  cortex  is  a  thin-walled  endodermis,  which 
borders  on  a  thin-walled  pericambium.  The  stele  consists  generally 
of  four  broad  groups  of  leptome  alternating  with  four  rays  of  vessels 
which  extend  to  the  centre  of  the  root,  there  being  no  conjunctive 
tissue  in  the  central  portion. 
While  thus  the  presence  of  these  large  cystolithes  and  sclerotic 
cells  make  the  structure  readily  distinguishable  from  the  roots  of 
Spigelia  and  Phlox,  there  are  two  other  characters  noticeable  in 
Ruellia,  which  are  equally  striking  :  The  presence  of  stereids  in  the 
leptome  (Fig.  5)  and  of  raphidines  also  in  the  leptome.  Of  these  the 
former  are  thick-walled,  prosenchymatic  cells,  while  the  latter,  first 
detected  by  Russow,1  remind  very  much  of  raphides,  and  are  known 
only  from  the  Acanthacece.  The  raphidines  may  be  single  or  many 
together  in  one  cell  of  the  leptome,  but  they  are  often  difficult  to 
find  on  account  of  their  diminutive  size.  The  cystolithes,  on  the 
other  hand,  can  hardly  escape  the  attention,  and  these  are  very  char- 
acteristic of  the  family  Acanthaceoe,  occurring  in  the  vegetative 
organs  of  these :  the  stem,  the  leaf  and  the  root.  But  they  are  also 
known  from  a  few  other  families,  for  instance :  Cucurbitaceoe,  Bora- 
ginece  and  Urticacece,  none  of  which,  however,  may  be  mistaken  for 
lSHzungsber.  Natutforsch.  Gesellsch.  Dorpat.,  Vol.  V,  1881,  p.  308. 
