Note.  As  we  have  already  learned  in  Chapter  V  the  veins  contain  the
vascular  bundles  of  the  leaf.  Through  them  the  water  and  food  solutions
are  distributed  to  all  parts  of  the  leaf,  and  the  return  current  of  food  ma-
terial elaborated  in  the  leaf  moves  back  through  the  bast  portion  into  the
shoot.  The  veins  also  possess  a  small  amount  of  mechanical  tissue.  This
forms  the  framework  of  the  leaf  and  aids  in  giving  rigidity  to  the  leaf  and


FOLIAGE   LEAVES.  389

in  holding  it  in  the  expanded  position.  The  mechanical  tissue  in  the
framework  alone  couid  not  support  the  leaf.  Turgescence  of  the  meso-
phyli  is  needed  in  addition.
757.  Cat  or  lob«d  leavei. — In  many  leaves,  the  indentatbns
on  the  margin  are  few  and
deep.  Such  leaves  pre-
sent several  lobes  the  pro-
portionate size  of  which
is  dependent  upon  the
depth  of  the  indentation
or  "incision."  Several
of  the  maples,  oaks,
birches,  the  poison  ivy,
thistles,  the  dandelion,
etc.,  have  lobed  leaves.
Where  the  indentation
reaches  to  or  very  near
the  midrib  the  leaf  is
said  to  be  cut.  A  study
of  various  leaves  will
show  all  gradations  from
simple  leaves  with  plane  edges  to  those  which  are  cut  or  divided,  as
in  compound  leaves,  and  the  lobes  are  often  variously  indented.
