164 
FORMATION  OF  GUM  TRAGACANTH. 
with  starch,  between  which  the  fibres  of  the  mother  plant  were 
placed.  The  walls  of  these  cells,  formed  by  several  thick  lay- 
ers, consisted  of  bassorine,  and  were  lined  with  a  tender  mem- 
brane of  cellulose.  Proofs  of  the  fungoid  nature  are  looked  for 
in  vain.  They  would  not  have  been  superfluous,  even  if  only  on 
account  of  the  great  curiosity  which  a  fungus  with  cellulose 
membrane  and  starch  granules  would  present. 
An  essentially  different  origin  is  assigned  by  Unger  (Anatom. 
und  Physiolog.  der  Pflanzen,  119)  to  gum  tragacanth.  He 
states  it  forms  in  several  species  of  Astragalus  the  secondary 
layers  of  the  medullary  rays. 
Other  microscopical  investigations  of  tragacanth,  from  which 
something  might  be  learned,  are  unknown  to  me.  Those  of 
Guibourt  [Hist.  Naturelle  des  Drogues  Simples,  4  e*dit.,  torn,  iii., 
420)  are  without  interest. 
In  order  to  obtain  an  explanation  of  the  nature  of  gum  traga- 
canth, I  considered  it,  above  all,  necessary  to  examine  not  only 
the  different  kinds  of  commercial  gum,  but  also  the  stems  of  a 
greater  number  of  Astragali  of  the  section  Tragaeanthce.  Un- 
fortunately my  materials  were  so  far  incomplete,  that  there  were 
no  thick  stems  at  my  disposal,  the  largest  being  herbarium  spec- 
imens, about  the  thickness  of  a  little  finger,  on  the  bark  of 
which  no  exudation  of  gum  tragacanth  was  visible.  The  follow- 
ing will  show,  however,  that  these  materials  were,  notwithstand- 
ing, sufficient  to  reveal  the  manner  in  which  the  gum  is  formed. 
For  the  examination  of  exuded  tragacanth,  thin,  flat  pieces  of 
the  gum  are  best  suited.  A  transverse  section  of  such  a  flat 
piece,  macerated  in  water,  shows  a  considerable  number  of  thick- 
walled  cells  lying  in  an  amorphous  mucilaginous  mass.  The 
walls  of  these  cells  are  colorless  and  gelatinous,  and  consist  of 
thick,  partly  well-marked  layers,  resembling  in  this  respect  the 
stratified  substance  of  a  starch  granule.  In  the  cavity  of  this 
cell  small  starch  granules,  more  or  less  numerous,  are  deposited. 
A  closer  investigation  of  these  cells  requires  the  application  of 
iodine.  This  acts  but  very  slowly,  whilst  an  iodized  solution  of 
chloride  of  zinc  produces  during  the  first  few  hours  no  other  al- 
teration than  that  of  coloring  the  starch  granules  blue  and  the 
walls  of  the  cells  pale  yellow.  It  is  only  after  the  action  of 
the  solution  for  twenty-four  hours  or  more  that  the  walls  of  the 
