Am^T,S^xm'}  Botany  and  Chemistry  of  Essential  Oils.  399 
formed  in  the  secondary  bast  of  certain  UmbellifercB  and  Com- 
posites. 
In  the  Coniferce,  when  the  cortical  parenchyma  disappears  owing 
to  secondary  thickening,  the  resin  vessels  are  replaced  by  sacs  of 
lysigenous  origin. 
According  to  the  researches  of  Mesnard,  the  oil  is  produced  from 
tannoid  substances  (formed  from  the  chlorophyll)  in  the  absence  of 
light  and  oxygen.  Thus,  in  the  sepal  the  oil  is  found  in  the  cells 
on  the  upper  surface,  protected  from  light  and  oxygen  in  the  bud, 
and  the  tannin  and  pigments  in  the  cells  of  the  lower  surface  which 
have  been  exposed  to  light  and  oxygen. 
He  finds  that  the  oils  are  thus  formed  in  the  jasmine,  rose  and 
violet,  but  in  the  orange  there  are  several  oils,  and  in  the  Tuberacece 
the  oil  is  found  on  the  lower  surface,  and  this  abnormal  result  is 
probably  due  to  the  presence  of  a  fixed  oil  and  abundance  of 
chlorophyll,  for  these  oils  are  always  found  in  the  cells  of  palisade 
parenchyma,  and  thus  result  from  chlorophyll. 
In  some  cases  the  oil  is  produced  by  enzymes  from  other  complex 
bodies,  e.  g.}  Valerian,  oil  of  bitter  almonds,  and  possibly  salicin. 
Some  doubt  exists  as  to  whether  the  oil  exists  as  such  in  the 
vessels  of  the  Conifera,  or  whether  the  oil  is  produced  by  the 
decomposition  of  this  body. 
We  have  thus  left  the  question  of  the  physiological  function  of 
these  oils  to  the  last,  and  the  answer,  it  must  be  confessed,  is  more 
of  a  conjecture  than  statement  of  facts. 
It  is  now  well  known  that  the  food  taken  into  an  organism  may 
be  divided  into  two  parts — that  assimilated  and  used  in  the  meta- 
bolic processes  of  the  plant,  and  that  which  is  not  so  used,  and  is 
thus  rejected  by  the  organism.  In  animals  the  portion  not  assimi- 
lated is  excreted,  and  passes  away  from  the  organism,  but  in  plants 
this  is  not  always  the  case.  In  a  few  plants,  as  ^.  Incrustata,  special 
apparatus  exists  for  the  excretion  of  material  not  required  by  the 
plant,  but  in  most  cases  the  plant  disposes  of  the  matter  by  secre- 
tion in  certain  sacs  or  vessels  or  renders  them  harmless  by  combi- 
nation with  other  bodies,  as  calcium  oxalate. 
There  is  little  doubt  but  that  essential  oils,  resins,  alkaloids,  etc., 
fall  under  this  category,  and  must  be  viewed  as  bye-products  in  the 
metabolic  processes  of  the  plant.  They  are  not  exactly  degrada- 
tion products,  as  gums,  etc.,  but  are  substances  produced  by  the 
plant  which  are  of  no  use  to  it. 
