126  THE  CARBOHYDRATES

cloudiness  or  a  precipitate  which  is  partially  or  wholly  soluble
on  adding  an  excess.  Arabic  acid  either  does  not  respond  to
this  reagent,  or  merely  gives  a  slight  turbidity,  and  the  same
applies  to  the  gums  obtained  from  certain  species  of  Cactus,
Albizzia,  Acacia  catechu,  Acacia  leucophloea  and  other  plants.
3.  Stannate  of  potash  gives  similar  reactions,  and  in  the
case  of  arabic  acid  produces  a  precipitate  which  is  soluble  in
excess.
4.  A  solution  of  neutral  sulphate  of  aluminium  (10  per
cent)  generally  gives  a  precipitate  which  is,  in  many  cases,
soluble  in  potash.
5.  Basic  lead  acetate  yields  a  precipitate  which  is  entirely
or  partially  soluble  in  excess.
GUM  TRAGACANTH.
This  gum  occurs  in  species  of  Astragalus,  and  consists
of  about  8-10  per  cent  of  soluble  calcium,  magnesium,  and
potassium  salts,  together  with  about  60-70  per  cent  of  in-
soluble salts,  which  only  swell  up  in  water  to  a  jelly.  The
water  soluble  portion  is  said  to  contain  a  substance,  poly-
arabinon-trigalactan-geddic  acid,  which  on  hydrolysis  breaks
up  into  arabinose,  galactose,  and  geddic  acid,  an  isomer  of
arabic  acid.  The  part  soluble  in  water,  when  treated  with
baryta  water,  gives  two  isomeric  tragacanthan-xylan-bassoric
acids,  which  on  hydrolysis  yield  a  pentose  sugar  tragacanthose,
xylose,  and  bassoric  acid  C14H20O13.
WOOD  GUM  AND  CERASIN  OR  CHERRY  GUM.
These  are  other  examples  of  pentosanes,  but  too  little  is
known  of  their  chemistry  to  warrant  any  description.
WOUND  GUM.
A  gum-like  substance,  termed  wound  gum,  is  frequently
found  in  the  tracheae  of  plants,  in  the  immediate  neighbour-
hood of  wounds,  and  stopping  up  the  lumina ;  it  is  secreted
by  the  surrounding  living  cells.  Wound  gum  does  not  swell
in  water,  and  is  insoluble  in  sulphuric  acid  and  in  caustic  soda.
