﻿Vegetable 
  Assimilation 
  and 
  Respiration. 
  

  

  255 
  

  

  by 
  assimilation. 
  It 
  must, 
  therefore, 
  be 
  concluded 
  that 
  the 
  extent 
  of 
  the 
  

   increase 
  in 
  " 
  apparent 
  " 
  assimilation 
  that 
  could 
  be 
  attributed 
  to 
  decrease 
  in 
  

   respiration 
  is 
  most 
  probably 
  less 
  than 
  0"5 
  units. 
  In 
  all 
  cases, 
  the 
  increase 
  of 
  

   assimilation 
  was 
  of 
  a 
  considerably 
  greater 
  magnitude. 
  

  

  Evidence 
  that 
  the 
  leaves 
  respired 
  normally 
  in 
  the 
  photosynthetic 
  apparatus 
  

   in 
  the 
  light 
  is 
  afforded 
  by 
  the 
  fact 
  that 
  they 
  appeared 
  quite 
  normal 
  at 
  the 
  

   end 
  of 
  the 
  experiments, 
  while 
  some 
  younger 
  leaves 
  which 
  were 
  used, 
  but 
  

   which 
  failed 
  to 
  put 
  out 
  sufficient 
  oxygen, 
  died 
  as 
  a 
  result 
  of 
  the 
  anaerobic 
  

   conditions. 
  These 
  injured 
  leaves 
  developed 
  the 
  characteristic 
  odour 
  of 
  the 
  

   pod 
  of 
  the 
  French 
  bean, 
  an 
  odour 
  absent 
  from 
  the 
  normal 
  leaf. 
  

  

  From 
  the 
  above 
  consideration, 
  it 
  would 
  appear 
  that 
  the 
  inevitable 
  

   conclusion 
  to 
  be 
  drawn 
  is 
  that 
  this 
  increase 
  in 
  assimilation 
  with 
  progress 
  of 
  

   time 
  cannot 
  be 
  due 
  to 
  the 
  factors 
  just 
  examined, 
  but 
  must 
  be 
  attributed 
  to 
  

   increase 
  in 
  some 
  " 
  internal 
  " 
  factor 
  other 
  than 
  chlorophyll. 
  This 
  aspect 
  of 
  

   the 
  matter 
  will 
  be 
  further 
  analysed 
  in 
  the 
  next 
  section. 
  

  

  Section 
  II. 
  — 
  The 
  Bearing 
  of 
  these 
  Results 
  on 
  the 
  Conception 
  of 
  the 
  Photo- 
  

   synthetic 
  Mechanism. 
  

  

  If 
  the 
  only 
  data 
  available 
  were 
  those 
  given 
  in 
  Table 
  A, 
  it 
  might 
  be 
  

   assumed 
  that 
  some 
  " 
  protoplasmic 
  " 
  factor 
  was 
  developing 
  from 
  day 
  to 
  day 
  

   and 
  throughout 
  acting 
  as 
  a 
  limiting 
  factor. 
  

  

  Before 
  considering 
  the 
  question 
  further, 
  it 
  will 
  be 
  well 
  to 
  be 
  clear 
  as 
  to 
  

   what 
  is 
  meant 
  by 
  a 
  " 
  limiting 
  factor." 
  A 
  factor 
  is 
  said 
  to 
  be 
  limiting 
  a 
  

   reaction 
  when 
  an 
  increase 
  in 
  that 
  factor 
  results 
  in 
  an 
  increase 
  in 
  the 
  rate 
  of 
  

   the 
  reaction. 
  To 
  take 
  the 
  case 
  of 
  chlorophyll 
  and 
  assimilation, 
  chlorophyll 
  is 
  

   said 
  to 
  be 
  limiting 
  assimilation 
  when 
  an 
  increase 
  in 
  chlorophyll 
  leads 
  to 
  an 
  

   increase 
  in 
  the 
  rate 
  of 
  assimilation. 
  It 
  is 
  conceivable, 
  however, 
  and 
  even 
  

   probable 
  that 
  an 
  amount 
  of 
  chlorophyll-content 
  which 
  would 
  be 
  limiting 
  

   under 
  conditions 
  of 
  weak 
  illumination 
  would 
  no 
  longer 
  be 
  limiting 
  under 
  

   conditions 
  of 
  stronger 
  illumination. 
  

  

  In 
  considering 
  a 
  process 
  such 
  as 
  photosynthesis, 
  it 
  is 
  perhaps 
  better 
  to 
  

   think 
  of 
  the 
  reaction 
  as 
  consisting 
  of 
  stages 
  such 
  as 
  diffusion, 
  photochemical, 
  

   and 
  " 
  dark 
  " 
  or 
  chemical 
  stages. 
  Each 
  of 
  these 
  stages 
  conceivably 
  depends 
  

   upon 
  two 
  or 
  more 
  factors. 
  For 
  example, 
  it 
  is 
  possible 
  to 
  think 
  of 
  the 
  photo- 
  

   chemical 
  stage 
  as 
  dependent 
  upon 
  light 
  intensity 
  and 
  chlorophyll-content. 
  

   This 
  stage 
  then 
  is 
  limiting 
  when 
  an 
  increase 
  in 
  either 
  of 
  these 
  factors 
  leads 
  

   to 
  an 
  increase 
  in 
  the 
  rate 
  of 
  assimilation. 
  Similarly, 
  the 
  chemical 
  stage 
  may 
  

   depend 
  upon 
  protoplasmic 
  development 
  and 
  upon 
  temperature, 
  and 
  conse- 
  

   quently 
  a 
  degree 
  of 
  protoplasmic 
  development 
  which 
  is 
  limiting 
  at 
  a 
  lower 
  

  

  vol. 
  xci. 
  — 
  b. 
  x 
  

  

  