FORMATION  OF  GUM  TRAGACANTH.  247 
oring,  perhaps  only  in  consequence  of  their  larger  mechanical 
extension. 
The  observations  here  detailed  will  leave  no  doubt  that  gum 
tragacanth  is  neither  a  secreted  sap,  dried  by  exposure  to  the 
air,  nor  an  independent  cryptogamic  organism  ;  but  that  its  for- 
mation is  owing  to  a  more  or  less  complete  transformation  of 
the  cells  of  the  medullary  rays  into  a  gelatinous  mass,  which, 
when  brought  into  contact  with  water,  increases  in  volume  to 
several  hundred  times  the  original  size  of  the  cells. 
Whether  the  production  and  exudation  of  the  gum  takes  place 
in  one  and  the  same  place  of  the  stem  only  once,  or  whether  it  is 
repeated  several  years,  can  of  course  only  be  found  out  in  the 
native  country  of  the  tragacanth  plant ;  but  perhaps  the  conjec- 
ture, that  the  appearance  of  the  gum  continues  through  a  long 
time,  is  not  too  bold.  The  transformation  of  the  pith  can  of 
course  only  take  place  once  in  any  given  part  of  the  stem,  and 
this  source  will  become  exhausted  after  the  exudation  of  the 
gum  formed  has  sooner  or  later  been  effected.  It  may  be  differ- 
ent, with  respect  to  the  medullary  rays,  as  not  all  medullary 
rays  of  a  given  part  of  the  stem  undergo  their  transformation 
at  the  same  time.  At  least  in  the  younger  stems  examined  by 
me,  only  a  part  of  the  medullary  rays  had  undergone  this  trans- 
formation, while  the  remainder  still  exhibited  the  ordinary  struc- 
ture of  thin- walled  cells.  We  may  also  venture  to  assume  thai; 
the  great  solidity  of  the  periderm  covering  the  stem,  is  the  reason 
why  the  forcing  of  the  gum  through  the  bark  takes  place  every 
year  out  of  a  limited  portion  of  the  medullary  rays,  and  that 
consequently,  perhaps,  many  years  may  elapse  before  all  the 
medullary  rays  of  a  stem  have  discharged  themselves. 
Looking  around  in  the  vegetable  kingdom  for  analogous  trans- 
formations of  cells  into  mucilage,  we  find  them  to  be  by  no  means 
of  rare  occurrence.  Alexander  Braun  (  Verjungung  der  Pflanze, 
p.  203)  remarks  that,  in  a  manner  quite  analogous,  a  softening 
of  the  membrane  of  the  cells,  their  swelling  up  into  a  gelatinous 
form  and  diffluence  [zerflie&seri],  are  phenomena  of  common  occur- 
rence in  the  natural  order  Palmellacece  and  Clirooooccacece,  and 
that  analogous  changes  in  the  membranes  of  the  cells  are  met 
with  in  Hydrodictyon  and  Botrydium,  and  that  the  jelly-like 
softening  of  the  membranes  of  the  mother  cell  of  pollen  granules 
