4  SMITHSONIAN    MISCELLANEOUS    COLLECTIONS  VOL.    96 
In  order  to  overcome  this  difficulty,  a  slightly  different  method  was 
used.  The  mid  point  of  equal  intensities  was  approximately  located 
with  the  photocell  and  the  seedling  placed  slightly  to  the  left  of  this 
point.  A  distinct  phototropic  bending  then  occurred  toward  the  left. 
Another  seedling  was  placed  to  the  right  of  the  mid  point  and  the 
bending  then  occurred  toward  the  right.  The  right  or  left  displace- 
ments were  never  greater  than  0.75  cm.  Consistent  results  were 
obtained  in  a  series  of  such  experiments  in  which  a  fresh  seedling  was 
used  each  time,  no  matter  whether  the  light  was  polarized  parallel 
or  at  right  angles  to  the  axis  of  the  plant.  No  difference  could  be 
detected  in  the  phototropic  response  of  the  seedling  in  regard  to  the 
plane  of  polarization  of  the  light  impinging  on  its  tip  when  the 
Polaroids  were  placed  in  the  two  positions  mentioned. 
Calculations  of  intensities  based  on  the  lengths  of  light  paths  at 
points  of  maximum  displacement  of  seedlings  give  a  difference  of 
slightly  less  than  3  percent  as  the  maximum  range.  This  clearly  shows 
that  if  polarized  light  had  a  different  effect  on  phototropism  in  one 
plane  than  in  the  other,  as  here  used,  such  an  effect  is  less  than  3  per- 
cent. Crozier  and  Mangelsdorf  (1924)  found  no  difference  in  the 
phototropic  efficiency  of  plane  polarized  and  nonpolarized  light  of 
equal  intensity  on  arthropods.  The  difference  in  phototropic  effect  of 
light  depending  on  the  plane  of  polarization  which  Castle  (1934)  found 
for  the  cells  of  Phycomyces  is  shown  by  him  to  be  due  to  differences 
in  the  reflection  losses  at  the  cell  surface.  What  Castle  concludes  for 
the  growth  of  Ph\comyces  is  undoubtedly  true  for  the  coleoptile  of 
Avena,  namely,  that  plane  polarized  light  has  no  specific  effect  on  its 
growth  processes. 
The  disappearance  of  starch  and  signs  of  starvation  of  plants  grown 
in  polarized  light,  as  reported  by  Semmens,  would  indicate  serious 
disturbances  in  the  photosynthetic  mechanism  of  such  plants.  This 
would  undoubtedly  involve  the  CO2  assimilation  process.  Because  of 
the  disagreement  between  the  results  of  Semmens  and  those  of  Dastur 
and  Asana  it  was  thought  worth  while  to  determine  the  CO2  uptake 
of  wheat  plants  in  polarized  and  nonpolarized  light,  especially  since 
this  was  the  experimental  plant  used  by  Hoover,  Johnston,  and 
Brackett  (1933)  and  by  Hoover  (1937)  in  their  COo  absorption 
studies. 
In  a  series  of  experiments  carried  out  by  McAlister  in  which  his 
recently  described  spectrographic  method  (1937,  1937  a)  for  COo 
determination  was  used,  little  or  no  evidence  was  obtained  that  indi- 
cated a  different  rate  of  photosynthesis  of  wheat  plants  in  polarized 
and  nonpolarized  light  from  a  Mazda  lamp. 
