NO.    6  GROWTH    OF   AVENA    COLEOPTILE JOHNSTON  7 
intensity  acts  as  a  factor  redistributing  these  sul)stances.  This  is 
evidenced  by  ditYerences  in  lengths  of  first  internodes  and  coleoptiles 
of  exposed  and  darkened  series  of  plants.  If  the  total  amount  of 
growth  substance  were  changed  at  all,  there  is  the  suggestion  that  a 
smaller  amount  is  present  in  plants  exposed  to  light  than  in  plants 
kept  in  darkness. 
The  ratio  of  the  length  of  the  tirst  internode  to  the  total  length  of 
seedling  was  determined  for  each  plant  and  averaged  for  each  experi- 
ment and  again  averaged  for  the  corresponding  wave-length  group. 
These  data  are  presented  in  table  2.  There  is  surprisingly  small  varia- 
tion between  the  duplicate  and  triplicate  experiments.  There  was 
such  small  variation  between  the  different  seedlings  of  a  given  group 
that  the  first  internode  was  found  to  be  a  fairly  definite  percentage 
of  the  total  length  of  the  seedling  regardless  of  whether  the  seedling 
was  long  or  short.  In  the  exposed  series,  the  average  ratio  values 
vary  from  a  maximum  of  0.56  for  wave-length  regions  4798  A, 
5005  A,  and  7600  A  to  a  minimum  of  0.36  at  wave-length  regions 
6395  A  and  6595  A.  In  the  dark  series  the  average  ratio  values  vary 
from  0.89  in  the  first  two  wave-length  experiments  to  0.65  in  the  third 
from  the  last  experiment. 
The  natural  question  to  be  asked  is  why  should  the  ratio  of  the 
dark  controls  vary  to  that  extent  especially  since  the  temperatures 
(see  table  i)  of  these  different  experiments  were  about  the  same. 
If  it  were  a  matter  of  total  growth  alone,  then  it  is  to  be  expected 
that  the  seedlings  of  the  first  two  experiments  would  be  longer  be- 
cause of  a  higher  average  temperature.  As  previously  mentioned, 
the  dark-control  plants  of  the  first  nine  experiments  were  grown  in 
a  separate  growth  chamber  where  there  was  no  possibility  of  scattered 
light  reaching  the  young  plants.  The  ratio  values  of  the  (jther  dark 
control  experiments  are,  in  general,  lower,  esi:)ecially  for  wave-length 
regions  5403  A  to  7005  A,  inclusive.  Here  a  partition  of  black  paper 
held  between  two  glass  plates  separated  the  exposed  from  the  dark- 
ened seedlings. 
To  correct  the  ratio  values  for  differences  in  temperature.  atuKJs- 
pheric  conditions,  and  viability,  each  exposed  value  was  dix'ided  by 
the  corresponding  dark  value.  These  are  recorded  in  the  sixth  column 
and  the  average  for  each  wave-length  group  placed  in  column  7. 
However,  since  the  dark  control  values  (except  the  first  nine)  appear 
not  to  be  true  values  because  of  scattered  light,  all  the  average  ratio 
values  (exposed/dark  control)  have  been  made  relative  to  the  aver- 
age of  the  first  nine  dark  control  ratios  (0.89).  These  have  been 
placed  in  the  last  column  of  table  2. 
