NO.    6  GROWTH    OF   AVENA    COLEOPTILE — JOHNSTON  II 
The  similarity  of  this  curve  to  that  found  by  FHnt  and  McAHster 
(i935)  1937)  representing  the  germination  of  hght-sensitive  lettuce 
seed  was  so  marked  that  their  curve  has  been  plotted  as  the  dash-dot 
line  in  this  same  figure.  The  general  similarity  of  these  two  curves 
is  suggestive  of  a  common  physiological  process.  The  retarding  action 
of  radiation  on  growth  in  the  region  4000  A  to  5000  A  is  likewise 
seen  by  inverting  the  phototropic  sensitivity  curve  found  by  Johnston 
(1934)  and  plotting  it  as  the  dash  curve  in  figure  4.  In  the  photo- 
tropic  studies,  however,  no  acceleration  in  growth  was  noted  in  the 
region  6000  A  to  7000  A  as  would  have  been  indicated  by  negative 
bending.  This  lack  of  response  may  be  due  to  an  insufficient  light 
gradient  in  this  region. 
Second  series  {with  mercury  lamp). — In  the  experiments  so  far 
described,  the  wave-length  bands  averaged  in  width  about  300  ang- 
stroms. By  using  a  capillary  high-pressure  mercury  lamp  (H-3  type) 
as  the  light  source,  a  greater  amount  of  energy  could  be  directed  on 
the  plants  and  most  of  the  energy  concentrated  in  the  mercury  line 
selected  for  the  exposure.  In  the  second  series  of  experiments  four 
mercury  lines  were  used.  The  intensity  at  the  beginning  of  each  ex- 
periment was  set  at  13  ergs/cniYsec.  This  intensity  sometimes 
dropped  as  low  as  7  ergs/cmYsec.  at  the  end  of  the  experiment  be- 
cause of  the  creeping  of  the  lamp  in  its  socket.  However,  it  was 
still  considerably  higher  than  that  in  the  first  series  (1.2  ergs/cm'/ 
sec). 
Two  dark  controls  were  run  with  each  set  of  exposed  plants.  One 
control  was  in  the  same  moisture  chamber  with  the  exposed  plants 
but  separated  from  it  by  a  black  partition,  as  was  the  case  in  the  first 
series  of  experiments.  The  other  dark  control  was  placed  in  another 
moisture  chamber  wrapped  with  black  paper  and  covered  with  a 
l)lack  cloth.  Here  there  was  no  opportunity  for  exposure  to  scattered 
light.  Each  wave-length  experiment  was  run  three  or  four  times  with 
an  average  of  six  seedlings  for  each  run  of  the  three  conditions.  Data 
showing  the  average  length  of  first  internode,  coleoptile,  and  total 
length  of  seedlings  after  four  days  of  growth  are  presented  in  table  3. 
The  maximum  and  minimum  temperatures  are  also  included  in  the 
table.  The  average  data  for  each  wave  length  are  shown  in  boldface 
type. 
In  the  exposed  series,  the  first  intcrnodes  were  always  shorter  than 
the  coleoptiles.  In  the  dark-control  series  in  the  same  chamber  with 
the  exposed,  the  first  intcrnodes  were  always  longer  than  the  coleop- 
tiles, while  in  the  other  dark  control,  where  no  light  of  any  kind  could 
