244 
FORMATION  OF  GUM  TRAGACANTH. 
appear  to  the  naked  eye  as  a  hard,  transparent,  gum-like  mass, 
which  becomes  gelatinous  in  water.  There  is  also  often  to  be 
found  upon  the  cut  surface  of  the  stem  a  dry  projection  of  gum, 
which  has  issued  from  the  interior  of  the  pith. 
Aided  by  the  microscope,  we  perceive  at  first  sight  that  the 
gum-like  mass  which  fills  the  pith  cavity  [MarJckohle],  and  the 
medullary  rays,  or  which  has  issued  from  the  pith  in  the  place 
where  it  has  been  cut,  does  not  consist  of  dried  mucilage  [nicht 
aus  einem  eingetrockneten  Grummischleim']y  but  actually  of  the 
cells  of  the  pith  and  medullary  rays,  which  have  undergone  a 
more  or  less  complete  transformation  into  gum  tragacanth. 
Generally  this  transformation  has  not  occurred  to  all  the  cells 
of  the  pith  and  medullary  rays  ;  but  the  outer  layers  of  medul- 
lary ray,  which  are  nearest  to  the  cells  of  the  wood,  consist  reg- 
ularly (and,  in  like  manner  the  outer  part  of  the  pith,  situated 
nearest  to  the  woody  fibres,  consist,  not  unfrequently)  of  the 
usual,  thin-walled,  parenchyme-cells,  the  membrane  of  which 
becomes  violet  on  the  application  of  an  iodized  solution  of  chlo- 
ride of  zinc  : — -These  unchanged  cells,  however,  form  generally 
only  a  very  thin  stratum,  consisting  of  a  few  cells,  whilst  all  the 
remaining  cells  constituting  the  central  mass  of  the  pith  and 
medullary  rays  are  transformed. 
That  the  peculiar  condition  of  these  cells  is  the  effect  of  a 
transformation  of  ordinary  parenchyme-cells,  and  not  a  condi- 
tion peculiar  to  the  cells  of  these  parts  from  the  first,  is  proved 
by  the  pith  and  medullary  rays  of  the  tops  of  branches,  which 
present  nothing  unusual. 
The  transformed  cells  differ  in  respect  to  their  physical  condi- 
tion from  common  cellular  tissue  in  presenting  a  substance,  which, 
when  dry,  is  very  hard,  transparent,  and  gummy,  and  which, 
when  wet,  is  swollen  and  slippery.  Placed  under  a  microscope, 
these  cells  (if  their  transformation  has  not  advanced  too  far) 
exhibit  the  angular  form  and  the  close  approximation  of  paren- 
chyme-cells, but  their  walls  are  very  thick,  and  evidently  com- 
posed of  numerous  very  thin  strata  ;  their  primary  membrane 
may  be  exactly  distinguished  from  the  secondary  incrassating 
strata  \_Verdichungssohicten,']  and  is  not  incrassated,  as  is  dis- 
tinctly seen  in  a  transverse  section  of  the  pits  in  which  the  pri- 
mary membrane  lies  free.    The  whole  form  of  these  cells,  the 
