There  is  some  difference  in  the  breathing  of  the  leaves
and  the  blossoms  of  a  plant.  The  blossoms  breathe  faster
than  the  leaves,  especially  when  they  are  just  opening;
and  they  are  therefore  slightly  warmer — in  some  cases,
indeed,  so  warm  that  the  heat  may  actually  be  felt!  And
the  same  is  true  of  sprouting  seeds  when  there  are  many
together.  The  sprouting  barley,  for  instance,  from  which
malt  is  made,  gives  off  quite  a  high  degree  of  heat.
As  the  plant,  like  the  animal,  must  breathe  in  order  to
live,  it  must  needs  have  a  constant  supply  of  air  for  breath-
ing, as  well  as  feeding,  purposes.  And  this  air  is  neces-
sary, more  or  less,  not  only  for  leaves  and  blossoms,  but
for  all  parts,  including  stems  and  roots.  It  breathes
through  all,  though  chiefly  by  its  leaves  and  blossoms  and
the  roots  are  always  breathing  out  carbon  dioxide  into  the
soil.  If  the  soil  be  too  close,  or  baked  on  the  top,  the
roots  cannot  get  enough  air  to  breathe  properly,  and
the  plant  is  sickly.  And  the  same  thing  happens  when
soil  is  piled  up  round  the  stem,  for  the  air  is  then  kept
away  both  from  it  and  from  the  roots,  and  many  a  tree
has  been  killed  in  consequence.
QUESTIONS  FOR  REVIEW

1.  Name  some  of  the  most  common  nitrogenous  substances.
Why  are  they  so  called?

2.  W^hy  is  it  rash  to  conclude  that  they  are  equally
nutritious?

3.  What  is  the  most  nutritious  part  of  the  plant,  and  when?
Illustrate.
148  The  Great  World's  Farm

4.  For  what  does  the  plant  need  carbon?

5.  How  is  carbon  supplied  to  the  air?

6.  What  gases  make  up  the  air?

7.  Illustrate  the  fact  that  gases  will  mix,  whatever  their
weight.
