l6  SMITHSONIAN    MISCELLANEOUS    COLLECTIONS  VOL.    96 
])lasinic  streaming.  The  cells  of  the  coleoptile  will,  therefore,  retain 
more  growth  regulators  than  are  transported  to  the  mesocotyl  cells 
at  high  temperatures  and  when  illuminated.  Hence,  under  such  con- 
ditions the  coleoptile  grows  faster  than  the  first  internode  or  meso- 
cotyl. The  length  of  path  that  the  growth  substance  must  travel  to 
reach  the  cells  of  the  mesocotyl  is  thus  increased  and  less  growth 
in  this  latter  tissue  will  result.  Their  data  (1934)  and  those  of 
Hamada  (1931)  show,  as  one  would  expect  from  this  theory,  the 
mesocotyl  cells  of  illuminated  plants  to  be  smaller  than  those  of  dark- 
grown  plants.  A  second  explanation  is  that  mentioned  by  Van  Over- 
beek  (1935),  who  states  that  a  temporary  increase  in  temperature 
increases  the  use  of  auxin,  thereby  decreasing  the  amount  available 
to  the  mesocotyl.  According  to  DuBuy  and  Nuernbergk  (1935),  it 
is  very  probable  that  a  decreased  protoplasmic  streaming  and  an 
increased  use  of  auxin  together  cause  an  increased  growth  of  the 
coleoptile  and  a  decreased  growth  of  the  mesocotyl.  The  reaction 
capacity  of  cells  to  growth  substances  may  also  enter  into  the  prob- 
lem since  such  reaction  capacities  appear  to  be  different  for  light  and 
for  dark  exposed  plants,  as  shown  by  Du  Buy  (1933),  and  for  plants 
grown  under  different  humidity  conditions,  as  shown  by  Gorter  and 
Funke  (1937). 
The  results  of  several  experimenters  show  that  the  growth  of  th^ 
first  internode  may  be  depressed  by  light,  high  temperatures,  and 
decapitation  of  the  coleoptile  tip.  Van  Overbeek  (1936  a)  depressed 
mesocotyl  growth  of  maize  seedlings  by  exposure  to  a  temperature 
of  48°  C.  for  half  an  hour.  The  amount  of  growth  substance  taken 
from  the  tips  of  similarly  treated  seedlings  was  less  than  that  from 
the  controls.  That  there  is  a  linkage  between  the  amount  of  growth 
substance  obtainable  from  the  tip  of  the  coleoptile  and  the  growth  of 
the  first  internode  seems  evident.  It  is  reasonable  to  suppose  than  any 
condition  that  would  inactivate  the  growth  substance  in  the  tip  of 
the  coleoptile  or  that  would  interfere  with  its  transport  to  the  first 
internode  would  depress  the  growth  of  the  first  internode.  From  the 
data  here  presented,  it  would  appear  that  radiant  energy,  even  as  low 
in  intensity  as  1.2  ergs/cmVsec,  has  a  very  definite  influence.  Whether 
this  influence  is  exerted  through  a  direct  effect  on  the  growth  sub- 
stance itself  or  on  its  mode  of  transportation  through  protoplasmic 
streaming  or  other  means  cannot  be  stated.  There  also  appears  to  be 
a  wave-length  effect.  But  whether  this  is  a  direct  effect  on  the  first 
internode  or  a  secondary  one  through  its  action  on  the  coleoptile,  which 
in  turn  would  control  the  growth  substance  reaching  the  first  inter- 
node, cannot  be  concluded  from  these  data.    Under  one  set  of  con- 
