FORMATION  OF  GUM  TRAGACANTH. 
245 
evident  stratification  of  their  membrane,  the  jelly-like  softness 
of  the  latter  when  moistened,  render  them  very  similar  to  the  well 
known  cells  of  the  cotyledons  of  Schotia. 
In  this  transition  state  into  gum  tragacanth,  occur,  according 
to  circumstances,  partly  those  cells  which  border  the  still  unal- 
tered layers,  partly  also  those  which  form  the  centre  of  the  pith 
and  the  medullary  rays,  as  I  observed  in  the  young  stems  of 
Astragalus  eyllenius,  Boiss.,  which  plant,  according  to  Orphan- 
ides,  is  one  of  those  from  which  tragacanth  is  collected  in 
Greece. 
When  the  transformation  has  progressed  a  degree  further, 
the  various  cells  swell  in  water  to  a  globular  form,  and  become 
more  or  less  perfectly  separated  from  each  other,  but  still  re- 
taining their  complete  integrity ;  while,  on  the  application  of 
iodine,  no  mucilage  is  observed  issuing  from  them  into  the  water. 
In  the  latter  respect,  I  found,  however,  in  some  pieces  (Astraga- 
lus aureus,  Willd.,  A.  pseudo-tragacantha,  M.  B.,  A.  compactus, 
Willd.,  A.  pycnocephalus,  Fisch.)  a  striking  exception.  There 
appeared  in  the  water  surrounding  the  cells  a  mucilaginous  sub- 
stance, which,  it  would  seem,  was  soluble  in  water,  and  on  appli- 
cation of  iodized  solution  of  chloride  of  zinc,  instantly  assumed 
a  beautiful  indigo  blue  color.  The  amorphous  contents  of  the 
cells  as  well  as  those  of  the  unchanged  medullary  rays  and  of  a 
part  of  the  cells  of  the  bark,  were  colored  in  like  manner.  The 
tint,  however,  was  not  lasting,  for  within  a  few  hours  the  blue 
had  entirely  disappeared,  giving  place  to  a  yellowish  tinge.  This 
phenomenon  reminds  us  again  of  the  cells  of  the  cotyledons  of 
Schotia,  from  which  water  extracts  a  mucilage,  which  is  rendered 
blue  by  iodine,  and  is  coagulated  by  alcohol.  In  other  cases, 
as  in  Astragalus  eyllenius,  water  extracts  from  the  transformed 
cells  a  mucilage,  which  iodine  renders  yellowish ;  but  this,  like 
the  mucilage  that  is  rendered  blue  by  iodine,  is  an  exception. 
When  the  transformation  of  the  cells  into  gum  tragacanth 
has  advanced  further,  it  is  impossible  to  distinguish  whether  the 
membranes  (which  swell  up  very  much  in  water)  are  composed 
of  many  thin  layers  or  not.  This  transformation  into  a  mass  of 
homogeneous  appearance,  proceeds  in  the  cell  membranes  from 
without  or  within ;  for  I  observed  (in  Astragalus  marinus}  cells 
in  which  the  change  had  extended  only  to  the  outer  half  of  the 
