^"^^ikme  way  droppeil  from  the  combinations  in  course  of  formfr  '
^^i-on,  und,  eeizeJ  by  the  ever-present  oxj'gcn,  is  i-cturnod  to
^-\ie  atmosphere  iis  carbonic  acid  giiB.  This  function,  whicb  U
^liat  of  true  re»jnratio/i,  ia  now  known  to  be  as  universal  in

^'%^1lC  vegetable  as  in  the  animal  kingdom.  Tho  chief  nutrition
iof  plants  consists  in  the  ussiinilation  of  cirbon  and  the  funnap
"^ion  of  ccllulinc  or  wood  by  its  combination  wilh  oxygoftl
^nd  hydrogen,  or  perhaps  directly  with  water,  as  the  fonnulA^
'Xrould  suggest.  But  this  form  of  nutrition  is,  after  all,  onljh
Ibe  sapply  of  strengthening  material.     Another  form  of  niK

I'ftrition  is  likewise  going  on  at  the  roots,  in  which  thii  quantity
«f  protoplasm  is  increased  by  the  contact  o£  tlie  ultimate  cclla
^>f  tlie  roots  with  the  matters  contained  in  tho  soil,  which
'Xunst  posiiess  a  certain  proportion  of  nitrogen  in  some  form,
■«r  the  plant  will  perish.  Tliia  miiy  be  regarded  iiB  the  true
"process  of  nutrition.  The  other  process  is  to  tho  tree  what
~tlie  absorption  of  carbonate  of  lima  is  to  the  coral,  and  tho
"trunk  of  the  tree,  as  well  as  all  its  true  woody  filier,  may
xeprcsent  the  polypidom  upon  which  the  animals  live,  and
which  they  secrete  from  their  gelatinous  bodies.
