'"^"^  o^t^fif^.^'""'"}    Formation  of  Gums  and  Gum  Resins.  505 
and  even  in  the  dry  state  are  very  readily  soluble  in  cold  water.  The 
barium  salt  is  obtained  by  neutralizing  an  alcoholic  solution  with 
baryta-water.  A  solution  of  the  magnesium  salt  gives  with  lead, 
silver,  and  zinc  salts  white  precipitates  which  form  resinous  masses 
when  warmed. 
Shellac  freed  from  wax  yields  20  per  cent,  of  azelaic  acid  when 
boiled  with  potash  and  potassium  permanganate,  products  smelling 
like  butyric  acid  being  also  formed.  Shellac  is  completely  converted 
into  azelaic  acid  and  fatty  acids  by  potassium  permanganate,  when 
the  residual  resin  is  again  boiled  with  the  permanganate,  and  the 
process  repeated  until  the  whole  is  oxidized. 
MODE  OF  FORMATION  OF  GUMS  AND  GUM-RESINS. 
Although  these  two  classes  of  substances  differ,  as  a  general  rule,  in 
their  mode  of  formation — the  former  being  the  result  of  chemical 
changes  in  the  cell- wall,  the  latter  a  secretion  in  the  interior  of  cells — 
this  is  not  altogether  without  exception.  In  some  species  of  Orchis 
true  gum  or  mucilage  is  secreted  in  the  interior  of  cells.  Intercellular 
passages  or  canals  are  known  as  "  schizogenous,^^  when  they  result  from 
the  simple  separation  or  parting  of  cells,  "  lysigenous  when  they  are 
formed  by  the  absorption  or  disappearance  of  cells  or  cell-walls. 
Essential  oils  and  gum-resins  are  generally  formed  in  a  layer  of 
so-called  "  epithelial cells  lining  the  cavity  or  receptacle,  which  may 
be  either  of  schizogenous  or  lysigenous  origin,  and  into  which  they 
are  diffused  through  the  very  thin  cell- walls  of  the  epithelial  cells. 
There  are,  on  the  other  hand,  instances  in  which  the  cell-wall  takes 
part  in  the  formation  of  essential  oils  and  resins,  as  in  the  lysigenous 
oil-passages  of  the  Aurantiacese. 
In  a  recent  paper  in  the  Beriehte  of  the  German  Botanical  Society, 
Herr  A.  Tschirch  describes  the  mode  of  formation  of  a  number  of 
essential  oils  and  gum-resins.  Copaiva  balsam,  derived  from  Copaifera 
Langsdorffii  and  offioinalis,  is  not  formed,  as  is  sometimes  stated,  in 
schizogenous,  but  in  lysigenous  canals.  The  absorption  of  the  cell- 
walls  and  the  formation  of  the  resin  commences  in  the  parenchyma  of  the 
wood,  advancing  from  there  to  the  medullary  rays,  the  libriform  and 
the  vessels.  The  resin-passages  in  species  of  Bipterocarpus,  which 
yield  gurjun  balsam,  and  those  in  Eperna  falcata,  which  yield  Bal- 
samum  antharthriticum,  are  formed  in  the  same  way.    In  Styrax 
