2  SMITHSONIAN    MISCELLANEOUS    COLLECTIONS  VOL.    96 
mirror,  and  to  CO2  assimilation  studies  of  wheat  plants  in  which  light 
intensities  are  increased  by  mirrors.  The  present  paper  discusses  the 
phototropic  response  of  the  oat  coleoptile  and  the  CO2  assimilation  of 
young  wheat  plants  in  polarized  light. 
The  general  method  of  illuminating  the  plant  by  two  opposing 
beams  of  light  was  used  in  the  phototropic  experiments.  This  balancing 
'   \  ^ol/>ss  filter 
I  \ 
I  \ 
I  \ 
I  \ 
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Iff  ON  80^ 'y  \ 
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Fig.  I. — Diagram  showing  position  of  Avcna  coleoptile  between 
two  opposing  beams  of  light  which  originate  from  a  single  source 
and  are  polarized  at  right  angles  to  each, other. 
action  of  light  on  phototropic  response  has  been  discussed  by  Johnston 
(1934)  and  by  Castle  (1931).  In  the  experiments  here  discussed  a 
single  light  source  was  used.  Two  beams  were  reflected  by  right-angle 
prisms  in  such  a  manner  that  the  first  0.5  to  i.o  mm  of  the  tip  of  a 
coleoptile  was  illuminated  from  opposite  sides.  The  general  arrange- 
ment of  apparatus  is  illustrated  in  figure  i. 
