1 34        The  Rhubarbs  and  a  Probable  A  du  Iter  ant.    { Awm2^  uSE*1' 
size  from  the  Rheum  starch.  A  No.  60  powder  of  this  substance 
also  exhibits  a  much  larger  proportion  of  the  parenchyma  debris, 
the  cells  of  which  are  much  larger  than  those  of  the  Rheums. 
The  distinction  between  the  two  forms  of  rhubarb  that  could  not 
be  made  out  by  the  use  of  the  microscope  appeared  very  distinctly 
under  the  influence  of  chemical  reagents.  Normally,  the  two  pow- 
ders differ  materially  in  their  appearance.  For  instance,  the 
Chinese  gives  a  fine,  soft  powder  of  a  distinctly  pure  yellow  color, 
and  with  the  odor  that  is  so  characteristically  that  of  rhubarb  ;  while 
the  rhaponticum  affords  a  granular,  pinkish-yellow  powder,  the  odor 
of  which  is  not  so  strongly  rhubarb-like,  but  is  more  woody  in  char- 
FiG.  6. — Rheum  Rhaponticum.    x  450  diameters,    a,  starch  ;  b,  fragments  of 
pitted  vessel ;  c,  calcium  oxalate  ;  d,  chrysophanic  acid. 
acter.  Under  the  action  of  alkalis  these  differences  in  color  become 
more  manifest.  Ammonium  hydrate  proved  the  most  suitable  reagent 
for  this  purpose.  Under  its  influence  Chinese  rhubarb  turns  a  dark, 
brick- red  color,  while  the  rhaponticum  exhibits  a  distinctly  salmon- 
red  shade.  Canaigre  with  the  same  reagent  gives  a  brownish  color. 
To  apply  the  test,  place  upon  a  glass  surface  a  small  amount  of  pow- 
der, and  moisten  it  with  a  drop  or  two  of  ordinary  ammonia  water. 
At  once  the  color  reaction  becomes  apparent.  A  mixture  of  the 
two  rhubarbs  cannot  thus  be  detected,  as  the  pink  of  the  rhaponti- 
cum is  masked  by  the  stronger  yellow  of  the  Chinese  variety.  The 
