﻿Vegetable 
  Assimilation 
  and 
  Respiration. 
  

  

  263 
  

  

  0*103 
  with 
  a 
  chlorophyll-content 
  of 
  only 
  1"9, 
  the 
  indications 
  are 
  against 
  

   chlorophyll 
  being 
  limiting 
  in 
  the 
  spring 
  leaves. 
  Secondly, 
  these 
  indications 
  

   are 
  borne 
  out 
  by 
  the 
  fact 
  that 
  young 
  leaves 
  18 
  days 
  older 
  than 
  the 
  first 
  

   sample 
  exhibit 
  an 
  increase 
  of 
  assimilation 
  of 
  about 
  100 
  per 
  cent, 
  with 
  an 
  

   increase 
  in 
  chlorophyll-content 
  of 
  only 
  30 
  per 
  cent. 
  Here 
  the 
  assimilation 
  

   number 
  has 
  increased 
  with 
  increase 
  of 
  chlorophyll-content 
  ; 
  a 
  comparison 
  of 
  

   mature 
  golden 
  and 
  mature 
  green 
  leaves 
  shows 
  the 
  opposite 
  relation 
  between 
  

   assimilation 
  number 
  and 
  chlorophyll-content. 
  It 
  is 
  impossible 
  to 
  say 
  in 
  this 
  

   case 
  whether 
  the 
  limiting 
  factor 
  is 
  involved 
  in 
  the 
  photochemical 
  or 
  the 
  

   chemical 
  part 
  of 
  the 
  mechanism. 
  If 
  in 
  both 
  cases 
  of 
  spring 
  leaves 
  the 
  

   external 
  limiting 
  factor 
  be 
  temperature, 
  as 
  it 
  is 
  in 
  the 
  mature 
  leaves, 
  then 
  

   the 
  internal 
  limiting 
  factor 
  would 
  be 
  concerned 
  with 
  the 
  chemical 
  part 
  ; 
  but 
  

   if, 
  on 
  the 
  other 
  hand, 
  light 
  be 
  the 
  external 
  limiting 
  factor, 
  as 
  it 
  is 
  in 
  the 
  

   leaves 
  of 
  the 
  golden 
  variety, 
  then 
  the 
  internal 
  factor 
  would 
  be 
  concerned 
  

   with 
  the 
  photochemical 
  part. 
  

  

  It 
  has 
  thus 
  been 
  shown 
  that 
  there 
  are 
  three 
  lines 
  of 
  evidence 
  indicating 
  

   that 
  chlorophyll 
  is 
  not 
  the 
  limiting 
  factor 
  in 
  the 
  young 
  stage 
  of 
  leaf 
  develop- 
  

   ment. 
  In 
  Irving's 
  experiments 
  the 
  chlorophyll 
  continued 
  to 
  increase, 
  to 
  a 
  

   certain 
  point, 
  without 
  any 
  sign 
  of 
  assimilation 
  ; 
  in 
  the 
  case 
  of 
  Willstatter's 
  

   young 
  leaves, 
  as 
  in 
  the 
  case 
  of 
  those 
  of 
  Irving, 
  when 
  they 
  once 
  reached 
  the 
  

   stage 
  of 
  beginning 
  to 
  assimilate, 
  the 
  assimilation 
  increased 
  out 
  of 
  proportion 
  

   to 
  the 
  increase 
  in 
  chlorophyll 
  ; 
  and 
  thirdly, 
  in 
  the 
  writer's 
  experiments, 
  the 
  

   assimilation 
  increased 
  with 
  no 
  increase 
  in 
  chlorophyll. 
  

  

  Summary. 
  

  

  (1) 
  The 
  development 
  of 
  a 
  leaf 
  begins 
  with 
  a 
  phase 
  in 
  which 
  it 
  has 
  no 
  

   photosynthetic 
  power. 
  The 
  development 
  of 
  this 
  power 
  is 
  shown 
  to 
  lag- 
  

   behind 
  greening, 
  so 
  that 
  a 
  young 
  green 
  leaf 
  may 
  exhibit 
  slight 
  or 
  zero 
  

   photosynthesis. 
  This 
  means 
  that 
  photosynthetic 
  activity 
  demands 
  the 
  

   development 
  of 
  some 
  internal 
  factor 
  other 
  than 
  chlorophyll. 
  The 
  photo- 
  

   synthetic 
  potentiality 
  of 
  this 
  factor 
  rapidly 
  increases 
  with 
  age, 
  day 
  by 
  day, 
  

   whether 
  the 
  leaf 
  is 
  in 
  the 
  light 
  or 
  in 
  the 
  darkness, 
  and 
  even 
  though 
  there 
  is 
  

   no 
  concurrent 
  increase 
  in 
  the 
  amount 
  of 
  chlorophyll. 
  

  

  (2) 
  It 
  has 
  been 
  possible 
  to 
  demonstrate 
  this 
  by 
  keeping 
  the 
  leaf 
  when 
  

   lighted 
  for 
  photosynthesis 
  in 
  an 
  atmosphere 
  of 
  hydrogen 
  with 
  only 
  a 
  trace 
  of 
  

   oxygen, 
  in 
  which 
  condition 
  there 
  is 
  no 
  increase 
  in 
  the 
  initial 
  amount 
  of 
  

   chlorophyll. 
  

  

  (3) 
  The 
  photosynthesis 
  is 
  measured 
  in 
  an 
  atmosphere 
  of 
  hydrogen, 
  to 
  

   which 
  carbon 
  dioxide 
  has 
  been 
  added. 
  The 
  partial 
  pressure 
  of 
  the 
  oxygen 
  

  

  