He  considers  the  walls  of  cells  are  permeable  to  oil ;  pro-
vided it  be  an  emulsion  sufficiently  fine,  and  especially  if  a  free
fatty  acid  be  present,  the  permeability  being  directly  propor-
tional to  the  amount  of  such  acid  present.  It  is  thought  that
the  acid  forms  a  soap  in  the  walls,  and  thus  facilitates  the
passage.
It  is  not  improbable  that  both  methods  are  adopted  by  the
plant,  viz.  the  translocation  of  the  products  of  the  dissociation
of  the  fat,  and  the  translocation  of  oil  qua  oil.
*  Leathes:  "  The  Fats,"  London,  1910.
FUNCTION  OF  WAXES  43

WAXES.
The  chief  function  of  waxes  in  plants  is  to  form  a  protec-
tive covering  against  undue  evaporation  of  water.  They  are
found  most  commonly  in  or  on  the  cuticle  of  leaves  and  fruits
where  they  give  rise  to  the  glaucous  effect.
As  already  stated,  the  waxes  resemble  the  fats  in  their
chemical  constitution  in  so  far  as  they  are  esters,  but  they
differ  in  the  nature  of  their  alcohol  constituent  which  is  not
glycerol  but  is  usually  a  monohydric  alcohol  such  as  cetyl
alcohol  C16H33OH,  carnaubyl  alcohol  C24H49OH,  pisangceryl
alcohol  C24H49OH,  ceryl  alcohol  C2RH53OH,  myricyl  alcohol
CgoHgiOH,  cholesterol  or  phytosterol  C27H45OH.
In  addition  to  the  acids  already  mentioned  as  occurring  in
fats,  the  following  are  also  met  with  in  waxes  in  the  form  of
esters :  ficocerylic  acid  CigHggOg,  carnaubic  acid  C24H48O2,  and
pisangcerylic  acid  C24H48O2,  as  well  as  acids  belonging  to
series  of  the  general  formula  CnH2n-202  and  CnHgnOg.
The  term  wax  used  in  the  chemical  sense  has  reference
only  to  the  chemical  composition  of  these  substances,  regardless
of  their  physical  state  of  aggregation,  and  consequently  both
liquid  and  solid  waxes  are  known.
