NO.    3         PLANT  RESPONSES  IN   POLARIZED  LIGIIT^ — JOHNSTON  3 
The  light  source  was  a  i  ,ooo-lumens,  i  .6-amperes  street  series  lamp 
with  a  heat-resisting,  heat-absorbing  extra  light  shade  Corning  filter 
(2.75  mm).  After  being  reflected  by  the  prisms  two  beams  of  light 
entered  an  iron  box  (34.5  X  18.5  X  30  cm  high)  through  two  oppositely 
located  side  windows.  In  each  window  was  fitted  a  Polaroid  disk 
(4  cm  diameter).  Each  beam  of  light  then  passed  through  a  hori- 
zontal 0.5-mm  slit  in  a  brass  shield  and  fell  on  the  tip  of  a  coleoptile 
placed  midway  between  these  shields.  These  shields  were  4  cm  apart. 
The  small  Erlenmeyer  flask  (50  ml)  in  which  the  oat  seedling  was 
growing  rested  on  a  small  shelf  which  could  be  raised  or  lowered  by 
means  of  a  worm  gear.  With  this  arrangement  the  tip  of  a  seedling 
could  be  accurately  placed  in  the  path  of  the  narrow  opposing  beams 
of  light.  The  total  length  of  each  light  beam  from  the  lamp  to  the 
mid  point  between  the  brass  shields  was  107  cm. 
The  oats,  Avena  sativa  Markton,  were  germinated  at  approximately 
25°  C.  between  glass  plates  covered  with  moist  filter  paper.  The  plates 
were  so  placed  in  a  moisture  chamber  that  the  seedlings  grew  verti- 
cally. A  careful  selection  of  the  seedlings  was  made  for  straightness 
when  they  had  attained  a  length  of  i  to  2  cm.  One  was  then  trans- 
ferred to  the  small  Erlenmeyer  flask  fitted  with  a  cork  stopper.  It 
was  supported  by  means  of  a  little  cotton  in  a  small  hole  of  the  stopper. 
The  flask  was  filled  with  distilled  water  so  that  the  roots  were  entirely 
immersed.  The  seedlings  were  always  handled  in  darkness  or  photo- 
graphically red  light.  One  Polaroid  disk  was  so  placed  that  the 
plane  polarized  light  was  parallel  to  the  axis  of  the  seedling.  The 
other  Polaroid  was  placed  to  give  a  beam  of  light  polarized  at  right 
angles  to  this.  The  setting  at  right  angles  could  easily  be  accomplished 
by  observing  the  lamp  filament  through  the  two  prisms  and  two 
Polaroids  and  rotating  the  one  Polaroid  until  the  transmitted  light 
reached  its  minimum  visibility.  At  this  point  the  filament  appeared  a 
dark  purple  red  in  color. 
By  means  of  a  specially  constructed  photocell  a  point  was  located 
between  the  slits  of  the  two  shields  where  the  two  beams  were  equally 
intense.  At  this  point  the  seedlings  either  showed  no  phototropic 
bending  or  a  very  slight  one  after  2  to  3  hours.  No  change  so  far  as 
the  reactions  of  the  coleoptile  were  concerned  were  noted  when  the 
Polaroids  were  rotated  through  an  angle  of  90°  in  order  to  reverse 
the  polarity  of  the  light  striking  the  two  sides  of  the  tip.  The  difficulty 
in  this  procedure  was  to  locate  accurately  the  mid  point  of  equal 
intensities.  The  seedling  was  more  sensitive  to  small  dififerences  of 
light  intensity  than  the  photocell. 
