﻿NATURAL 
  RUBBER 
  — 
  COOK 
  403 
  

  

  lower 
  figure 
  shows 
  a 
  planted 
  field 
  with 
  the 
  milkweed 
  behaving 
  like 
  

   a 
  grass 
  crop, 
  from 
  which 
  successive 
  cuttings 
  could 
  be 
  made. 
  

  

  The 
  rubber 
  content 
  ranged 
  usually 
  from 
  2 
  to 
  5 
  percent 
  in 
  the 
  wild 
  

   plants, 
  in 
  exceptional 
  cases 
  reaching 
  6 
  percent. 
  The 
  stems 
  have 
  a 
  

   rather 
  heavy 
  coating 
  of 
  wax 
  and 
  a 
  layer 
  of 
  rather 
  strong 
  bast 
  fibers, 
  

   to 
  be 
  considered 
  as 
  byproducts 
  ; 
  also, 
  the 
  floss 
  from 
  the 
  seeds 
  of 
  milk- 
  

   weeds 
  is 
  reported 
  as 
  being 
  of 
  industrial 
  value. 
  A 
  general 
  difficulty 
  is 
  

   that 
  the 
  leaves 
  are 
  difficult 
  to 
  harvest, 
  as 
  in 
  the 
  case 
  of 
  the 
  goldenrods 
  

   already 
  mentioned. 
  Another 
  desert 
  milkweed, 
  Asclepias 
  erosa, 
  with 
  

   very 
  large 
  leaves, 
  was 
  also 
  investigated, 
  as 
  reported 
  in 
  1938. 
  The 
  

   leaves 
  were 
  found 
  to 
  contain 
  about 
  90 
  percent 
  of 
  the 
  rubber, 
  and 
  the 
  

   rubber 
  content 
  of 
  the 
  leaves 
  was 
  definitely 
  higher 
  than 
  that 
  of 
  the 
  

   stems 
  of 
  A. 
  subulata, 
  with 
  a 
  mean 
  of 
  50 
  determinations 
  showing 
  more 
  

   than 
  8 
  percent 
  and 
  a 
  maximum 
  of 
  13 
  percent. 
  Recent 
  observations 
  

   by 
  Dr. 
  Walter 
  T. 
  Swingle 
  indicate 
  that 
  this 
  species 
  may 
  be 
  adapted 
  

   to 
  a 
  wider 
  range 
  of 
  conditions 
  than 
  S. 
  subulata, 
  and 
  may 
  be 
  better 
  

   suited 
  to 
  regular 
  cultivation. 
  

  

  CRYPTOSTEGIA 
  AS 
  A 
  SOIL 
  COVER 
  

  

  An 
  incident 
  that 
  occurred 
  during 
  rubber 
  experiments 
  in 
  Haiti 
  

   showed 
  an 
  unexpected 
  cultural 
  relation 
  between 
  two 
  types 
  of 
  rubber- 
  

   bearing 
  plants, 
  the 
  Para 
  rubber 
  tree 
  and 
  Cryptostegia 
  grandiftora, 
  a 
  

   woody 
  trailing 
  vine 
  of 
  the 
  dogbane 
  family, 
  native 
  in 
  Madagascar. 
  The 
  

   latex 
  of 
  Cryptostegia 
  yields 
  rubber 
  of 
  good 
  quality 
  that 
  was 
  formerly 
  

   exported 
  from 
  Madagascar. 
  It 
  is 
  one 
  of 
  the 
  plants 
  that 
  was 
  tested 
  

   by 
  Edison 
  in 
  Florida 
  and 
  is 
  now 
  being 
  tested 
  on 
  a 
  large 
  scale 
  in 
  Haiti 
  

   and 
  in 
  Mexico 
  as 
  an 
  emergency 
  rubber 
  resource. 
  

  

  A 
  casual 
  planting 
  of 
  Cryptostegia 
  near 
  Port-au-Prince 
  was 
  made 
  

   in 
  a 
  small 
  seed 
  bed 
  previously 
  used 
  for 
  Para 
  rubber, 
  but 
  from 
  which 
  

   all 
  the 
  Para 
  seedlings 
  had 
  been 
  removed 
  except 
  a 
  few 
  stunted, 
  leafless, 
  

   and 
  moribund 
  survivors, 
  overlooked 
  or 
  disregarded 
  when 
  the 
  Crypto- 
  

   stegia 
  seeds 
  were 
  planted. 
  A 
  vigorous 
  growth 
  of 
  Cryptostegia 
  soon 
  

   covered 
  the 
  ground, 
  so 
  that 
  the 
  stunted 
  Para 
  rubber 
  seedlings 
  were 
  

   concealed 
  and 
  forgotten. 
  It 
  was 
  a 
  surprise 
  to 
  find 
  a 
  few 
  months 
  later 
  

   that 
  two 
  plants 
  of 
  the 
  Para 
  rubber, 
  instead 
  of 
  being 
  finally 
  smothered 
  

   and 
  suppressed 
  by 
  the 
  Cryptostegia, 
  were 
  making 
  vigorous 
  growth, 
  

   with 
  normal, 
  full-sized 
  leaves. 
  It 
  was 
  plain 
  that 
  the 
  Cryptostegia, 
  

   instead 
  of 
  competing 
  with 
  the 
  Para 
  rubber, 
  had 
  served 
  to 
  advantage 
  

   as 
  a 
  soil 
  cover 
  or 
  nurse 
  crop 
  for 
  the 
  seedlings 
  of 
  the 
  Para 
  rubber. 
  

  

  As 
  the 
  planting 
  was 
  done 
  in 
  a 
  heavy 
  but 
  very 
  shallow 
  soil, 
  a 
  con- 
  

   dition 
  definitely 
  unfavorable 
  for 
  open 
  planting 
  of 
  young 
  Para 
  rubber 
  

   trees, 
  it 
  would 
  have 
  been 
  impossible 
  for 
  these 
  depauperate 
  seedlings 
  

   to 
  have 
  recovered 
  and 
  grown 
  vigorously 
  if 
  the 
  Cryptostegia 
  had 
  not 
  

   been 
  planted. 
  This 
  favorable 
  reaction 
  seems 
  worthy 
  of 
  further 
  study 
  

  

  