This  downwar^^d  fmf.wa.rrl  stream  of  living  \  /
matter,  carrying  along  with  it  live  protoplasm
and  other  foods  or  manufactured  materials,  must
be    carefully    distinguished    from    the    upward
stream  of  crude  sap  which  rises  from  the  roots  '
to  the  leaves  and  branches.     The  one  contains
only  such  raw  materials  of  life  as  are  supplied
by  the  soil — namely,  nitrogenous  matter,  water,
and   food-saltst;    the  other  contains  the  things
eaten  from  the  air  by  the  plant  in  its  leaves,  I
and  afterwards  worked  up   by  it   into   sugars,/  £•
starches,  protoplasm,  and  chlorophyll.
Stems  are  usually  covered  outside  for  purposes
of  protection  by  a  more  or  less  thick  integument,
which  in  trees  and  shrubs  assumes  the  corky
form  we  know  as  bark.  Bark  consists  of  d,eacl
and  empty  cells,  thickened  with  a  lighter
thickening  matter  than  wood,  and  presenting
as  a  rule  a  rather  spongy  appearance.  But
beneath  the  bark  comes  a ^i^nci^_layer_Qf  living


192  THE   STOftY    OF   Tftfc   PLANTS.
material,  interposed  between  the  corky  dead  I
cells  of  the  integument  and  the  woody  dead  •
cells  of  the  interior.  This  living  layer  extends
over  stem,  twigs,  and  branches :  it  forms  the
binding  and  connecting  portion  of  the  entire
plant  community  ;  it  links  together  in  one  united
/whole  the  living  material  of  the  leaves,  and  shoots
v  \with  the  living  material  of  the  roots  arid  rootlets.
It  is  thus^the  stem,  above  all,  that  gives  to  the
complex  plant  colony  of  foliage  and  flowers
whatever  organic  unity  and  individuality  it  ever
possesses.
