NO.    6  GROWTH    OF    AVENA    COLEOPTILE JOHNSTON  1/ 
ditions  (darkness)  the  first  internode  is  long  and  the  coleoptile  short. 
Under  other  conditions  (iUumination)  the  first  internode  is  short 
and  the  coleoptile  long.  It  would  appear  that  in  the  first  case  the 
growth  substances  were  not  utilized  for  growth  in  the  coleoptile,  but 
transported  to  the  first  internode  where  the  greater  amount  of  growth 
was  found.  In  the  second  case  considerable  more  growth  took  place 
in  the  coleoptile.  Here  it  would  appear  that  some  growth  substance 
was  prevented  from  acting  on  the  cells  of  the  first  internode.  In 
these  experiments  light  was  evidently  the  controlling  factor  in  the 
distribution  of  the  growth  substance. 
Reference  to  figure  4  shows  that  wave  lengths  4050  A  to  5500  A 
slightly  depressed  the  growth  of  the  coleoptile.  In  this  region  growth 
of  the  first  internode  was  somewhat  increased.  In  the  wave-length  re- 
gion 5500  A  to  7200  A  the  reverse  efifect  is  noted.  Here  the  coleop- 
tile growth  is  increased  and  that  of  the  first  internode  decreased.  Since 
there  is  very  little  difterence  in  total  growth  of  the  entire  seedling 
between  light  and  dark  conditions,  one  is  led  to  the  conclusion  that 
light  of  this  low  intensity  functions  mainly  as  a  distributing  rather 
than  an  inactivating  factor  on  growth  substance.  However,  since 
there  was  actually  a  slight  difi^erence  in  total  growth,  there  may  also 
be  a  slight  inactivating  influence  of  light,  for  the  experimental  data 
show  slightly  greater  average  growth  for  the  dark  control  plants  than 
for  the  illuminated  ones. 
SUMMARY 
Two  series  of  experiments  were  conducted  in  which  the  growth  of 
the  coleoptile  and  first  internode  (mesocotyl)  of  Avefia  sativa,  Mark- 
ton  variety,  was  studied  in  different  wave-length  regions  of  the 
visible  spectrum. 
Radiation  of  low  intensity  greatly  depressed  the  growth  of  the 
first  internode  as  compared  to  that  in  darkness.  On  the  other  hand, 
growth  of  the  coleoptile  was  depressed  in  darkness.  Total  growth  of 
the  entire  seedling  (coleoptile  plus  first  internode)  for  the  4-day 
periods  of  all  these  experiments  was  slightly  greater  in  darkness 
than  in  light.  Although  the  total  length  of  seedling  was  not  greatly 
dependent  on  intensity  of  illumination,  the  ratios  of  first  internode 
and  coleoptile  to  total  length  were  extremely  critical  indices  to  in- 
tensity of  illumination.  It  is  extremely  difficult  to  get  conditions  dark 
enough  to  avoid  light  effects  on  these  ratios. 
All  wave  lengths  give  much  the  same  tendency  to  decrease  the 
ratio  of  first  internode  to  total  length  and  this  effect  increases  with 
light  intensity.  Yet  there  is  a  wave-length  effect  yielding  a  minimum 
ratio  at  about  6500  A. 
