﻿254 
  Mr. 
  G. 
  E. 
  Briggs. 
  Experimental 
  Researches 
  on 
  

  

  factors 
  to 
  which 
  the 
  increase 
  of 
  assimilation, 
  with 
  progress 
  of 
  time 
  after 
  

   greening, 
  might 
  have 
  been 
  attributed 
  in 
  the 
  first 
  instance. 
  These 
  are 
  

   (1) 
  increase 
  in 
  chlorophyll-content 
  during 
  that 
  time 
  ; 
  (2) 
  increase 
  in 
  leaf 
  

   area 
  ; 
  (3) 
  fall 
  in 
  respiration, 
  since 
  the 
  assimilation 
  measured 
  is 
  the- 
  

   " 
  apparent 
  " 
  not 
  the 
  " 
  real 
  " 
  assimilation 
  ; 
  or, 
  possibly 
  (4) 
  a 
  combination 
  of 
  

   these 
  factors. 
  

  

  It 
  seems 
  most 
  improbable 
  that 
  the 
  increase 
  is 
  due 
  to 
  increased 
  chlorophyll- 
  

   content, 
  since 
  the 
  same 
  increase 
  in 
  assimilation 
  was 
  observed 
  when 
  the 
  leaves 
  

   were 
  kept 
  in 
  the 
  dark. 
  To 
  gain 
  definite 
  information 
  on 
  this 
  point 
  one 
  sample 
  

   of 
  leaves 
  was 
  taken 
  directly 
  after 
  greening, 
  and 
  another 
  sample 
  of 
  the 
  same 
  

   lot 
  after 
  exposure 
  to 
  light 
  in 
  the 
  apparatus 
  ; 
  both 
  samples 
  were 
  dried 
  and 
  

   extracted 
  with 
  85 
  per 
  cent, 
  acetone, 
  in 
  such 
  quantities 
  that 
  the 
  ratio 
  of 
  dried 
  

   leaf 
  material 
  to 
  extract 
  was 
  the 
  same 
  in 
  both 
  cases. 
  These 
  extracts 
  were 
  

   indistinguishable 
  in 
  tone 
  of 
  greenness, 
  thus 
  showing 
  that 
  the 
  conditions 
  of 
  

   the 
  experiment 
  were 
  really 
  such 
  as 
  to 
  inhibit 
  further 
  development 
  of 
  

   chlorophyll. 
  

  

  The 
  change 
  in 
  size 
  of 
  the 
  leaves 
  during 
  the 
  experiments 
  was 
  found 
  to 
  be- 
  

   negligible. 
  

  

  Knowing 
  that 
  the 
  respiration 
  of 
  young 
  leaves 
  is 
  relatively 
  large, 
  it 
  was 
  

   decided 
  to 
  determine 
  its 
  magnitude 
  for 
  material 
  similar 
  to 
  that 
  used 
  in 
  the 
  

   assimilation 
  experiments, 
  and 
  to 
  measure 
  the 
  falling 
  respiration 
  during 
  a. 
  

   three-day 
  period. 
  

  

  To 
  estimate 
  the 
  respiration, 
  about 
  twenty 
  young 
  etiolated 
  leaves 
  of 
  six 
  or 
  

   seven 
  days 
  from 
  sowing 
  were 
  placed 
  in 
  an 
  air-tight 
  flask 
  containing 
  ordinary 
  

   air 
  ; 
  the 
  flask 
  was 
  placed 
  in 
  the 
  dark, 
  and 
  at 
  intervals 
  samples 
  of 
  gas 
  were 
  

   withdrawn 
  and 
  analysed 
  for 
  carbon 
  dioxide, 
  and 
  at 
  the 
  same 
  time 
  the 
  gas 
  in 
  

   the 
  flask 
  was 
  replaced 
  by 
  ordinary 
  air. 
  The 
  results 
  are 
  recorded 
  in 
  Table 
  XII 
  

   in 
  the 
  Appendix, 
  the 
  respiration 
  being 
  expressed 
  as 
  cubic 
  centimetres 
  of 
  

   carbon 
  dioxide 
  per 
  gramme 
  dry 
  weight 
  of 
  leaf 
  per 
  hour. 
  

  

  The 
  first 
  value, 
  1/4, 
  is 
  most 
  probably 
  too 
  high, 
  as 
  it 
  represents 
  the 
  rate 
  of 
  

   respiration 
  during 
  the 
  five 
  hours 
  subsequent 
  to 
  cutting 
  : 
  respiration 
  is 
  usually 
  

   greatly 
  increased 
  by 
  wounding 
  and 
  handling. 
  If 
  it 
  be 
  assumed 
  that 
  the 
  

   respiration 
  of 
  these 
  leaves 
  decreased 
  from 
  1*3 
  to 
  - 
  8 
  units 
  during 
  the 
  three 
  

   days, 
  then 
  the 
  magnitude 
  of 
  the 
  fall 
  in 
  respiration 
  is 
  not 
  likely 
  to 
  be 
  under- 
  

   estimated. 
  Consequently 
  the 
  fall 
  in 
  respiration 
  cannot 
  be 
  held 
  to 
  account 
  for 
  

   a 
  rise 
  in 
  "apparent" 
  assimilation 
  of 
  more 
  than 
  05 
  units, 
  unless 
  the 
  

   coefficients 
  of 
  gaseous 
  exchange 
  depart 
  very 
  widely 
  from 
  unity. 
  Moreover, 
  

   this 
  fall 
  in 
  respiration 
  is 
  due 
  chiefly 
  to 
  starvation, 
  that 
  is, 
  disappearance 
  of 
  

   respirable 
  material. 
  In 
  the 
  case 
  of 
  the 
  leaves 
  used 
  in 
  the 
  experiments 
  on 
  

   photosynthesis, 
  the 
  loss 
  of 
  material 
  by 
  respiration 
  was 
  more 
  than 
  made 
  good 
  

  

  