14  SMITHSONIAN    MISCELLANEOUS    COLLECTIONS  VOL.    96 
each  series  is  higher.  All  values  in  the  second  series  of  experiments 
are  higher.  If  it  is  assumed  that  the  small  temperature  differences 
between  the  various  experiments  exert  but  little  influence,  then  it  must 
be  concluded  that  the  increased  intensity  in  the  second  series  of  ex- 
periments has  increased  the  percentage  length  of  the  coleoptile.  This 
Table  5. — Comparison  of  Percentage  Length  of  Coleoptiles  in  the  Tzvo  Series  of 
Experiments  for  Corresponding  IVaz'e  Lengths 
'ave  length 
A 
First  series 
Percent 
Second  series 
Percent 
4047 
49 
60 
4358 
48 
59 
5461 
SI 
62 
5780 
57 
72 
is  in  agreement  with  the  observation  that  the  coleoptiles  were  longest 
in  the  exposed  series  (greatest  intensity),  shortest  in  the  completely 
darkened  series  (zero  intensity),  and  intermediate  in  length  in  the 
dark  control  exposed  to  very  diffused  light.  The  reverse  is  true 
regarding  the  first  internodal  values,  which  are  the  coleoptile  values 
subtracted  from  loo. 
DISCUSSION 
In  determining  the  sensitivity  curve  (Johnston,  1934)  of  Avena 
coleoptile,  only  the  tips  were  illuminated.  In  the  experiments  here 
reported  entire  seedlings  were  grown  in  restricted  wave-length  regions 
of  the  spectrum.  Went  (1926)  has  pointed  out  two  distinct  light 
responses  in  the  coleoptile,  one  the  tip  response,  the  other  the  base 
response.  Where  the  entire  coleoptile  is  illuminated,  both  responses 
are  concerned.  Van  Overbe'ek  (1936  b)  calls  attention  to  a  third 
type  of  response  described  by  Tollenaar  and  by  Van  Dillewijn,  which 
is  designated  as  the  "  dark  growth  response."  It  is  the  increased  re- 
sponse to  growth  substance  in  darkness  and  has  nothing  to  do  with 
bringing  the  plants  back  into  darkness.  Plants  continuously  exposed 
show  the  response. 
In  the  present  experiments  the  time  period  was  exactly  four  days, 
and  in  that  length  of  time  the  first  internode  as  well  as  the  coleoptile 
attained  considerable  length.  The  light  response  of  both  organs  must 
be  considered  as  well  as  their  interactions.  Also  the  seed  and  roots 
in  the  exposed  series  received  a  certain  amount  of  radiation.  The 
problem  becomes  further  complicated  when  the  action  of  plant  growth 
substances  with  their  production,  inactivation,  and  transport  is 
considered. 
