108.  Irritability  of  Plants. — Irritability  in  either  plants  or
animals  is  the  power  of  responding  to  a  stimulus.  For  ex-
ample, if  a  frog  be  touched  suddenly  (mechanical  stimulus),
the  leg  muscles  contract  and  the  animal  jumps.  The  frog  also
responds  to  heat  stimulus  (goes  into  shade  when  the  sun's
heat  is  too  great) ;  to  light  stimulus  (sees  enemies  and  jumps)  ;
to  sound  stimulus  (hears  sounds  and  jumps) ;  electrical
stimulus  (jumps  if  touched  by  a  slightly  charged  electric
wire).  These  are  phases  or  kinds  of  irritability  common  in
animals  which  have  a  brain,  spinal  cord,  and  nerves.
AN  INTRODUCTION  TO  PLANT  BIOLOGY         117

We  shall  later  study  some  microscopic  animals  which
respond  to  mechanical,  heat,  light,  and  electrical  stimuli;
but  they  have  no  visible  nerves  or  nerve  organs.  These
lower  animals  have  irritability  or  nervous  functions  without
special  organs  to  perform  the  functions.  This  is  essentially
the  case  in  plants.  In  recent  years  there  have  been  many
magazine  articles  discussing  "  the  nerves  of  plants."  The
truth  is  that  no  one  has  seen  any  nerves  or  brains  or  similar
nervous  organs  in  plants,  although  they  do  exhibit  the
various  forms  of  irritability  and  respond  to  the  diflFerent
kinds  of  stimuli  which  affect  animals,  as  the  following  ex-
amples will  show :  —

Mechanical  or  touch  stimuli  (such  as  a  jar  by  passing
animals)  causes  the  sudden  folding  of  the  leaflets  and  drooping
of  the  branches  of  the  Mimosa  or  sensitive  plant  (see  Fig.  1).
The  leaf  of  the  Venus  fly-trap  (Fig.  2)  closes  quickly  and
grasps  insects  which  happen  to  touch  it.
