NO.    3         I'LANT  RESPONSES  IN    POLARIZED  LIGHT JOHNSTON  5 
Young  wheat  seedlings  were  grown  under  controlled  conditions  in 
the  growth  tube  and  exposed  first  to  nonpolarized  and  then  to  polar- 
ized light.  The  light  source  was  a  i,ooo-watt  Mazda  projection  lamp 
used  with  a  suitable  water  filter.  Light  polarized  parallel  to  the  plant 
axis  was  obtained  by  inserting  a  Polaroid  disk  between  the  plant  and 
the  lamp. 
The  Polaroid  greatly  reduced  the  light  intensity  and  in  order  to 
reduce  the  intensity  of  the  nonpolarized  light  to  that  of  the  polarized, 
a  20o-mesh  screen  was  inserted  between  the  plant  and  the  lamp.  A 
small  G.  E.  photocell  foot-candle  meter  was  used  to  a])proximate  the 
intensities.   Data  from  two  such  experiments  are  presented  in  table  i. 
Table  i. — Carbon  Dioxide  Assimilation  of  Young  Wheat  Seedlings  in 
Nonpolarized  and  Polarized  Light 
Light 
Intensity 
COo  assim 
ilation 
' 
Thermocouple 
reading  with 
s              filter  => 
Foot- 
candle 
(min^ 
10  min.) 
Cor- 
rected 
250 
482 
588 
260 
574 
586 
^72, 
8.2 
565 
689 
282 
9.8 
670 
684 
Character 
Date 
1937 
May  4 Nonpolarized 
Polarized 
May  5 Nonpolarized 
Polarized 
"  With  a  Corning  Aklo  heat-resisting,  heat-absorbing  medivim  (2.46  mm)  filter  together 
with  a  lo-cm  water  filter,  the  wave-length  distribution  was  restricted  to  about  3800-7800  A, 
which  includes  the  major  portion  of  the  spectrum  active  in  photosynthesis.  These  thermo- 
couple readings  were  used  in  obtaining  the  corrected  CO2  assimilation. 
In  such  experiments  it  is  important  that  light  and  not  the  CO,^  of 
the  air  surrounding  the  plants  be  the  limiting  factor.  In  order  to 
make  sure  this  was  the  case,  additional  readings  of  COo  uptake  were 
taken  on  May  5  at  higher  intensities  in  nonpolarized  light.  The  CO2 
uptake  at  700-800  foot-candles  was  found  to  be  1060  mm^  per  10- 
minute  interval.  It  therefore  appears  certain  that  at  the  lower  intensi- 
ties used  in  these  experiments  light  and  not  COo  was  the  limiting 
factor. 
Although  the  experiments  on  this  phase  of  the  work  were  not  many 
and  for  lo-minute  intervals,  yet  because  of  the  extreme  accuracy  and 
quickness  of  this  optical  method  of  determining  the  COo  absorption 
by  plants,  it  may  safely  be  concluded  that  polarized  light  has  no  efifect 
upon  the  uptake  of  COo.  There  is  the  possibility,  although  it  does 
not  seem  probable,  that  if  the  plants  were  grown  for  long  periods 
in  polarized  light  some  secondary  eflfects  on  the  COo  assimilation 
might  appear.    The  measurements  indicate  a  difiference  of  a])])roxi- 
