﻿On 
  Transpiration 
  from 
  Leaf-Stomata. 
  433 
  

  

  K 
  = 
  1000 
  cm. 
  2 
  /sec, 
  U 
  = 
  400 
  cm./sec, 
  this 
  becomes 
  0'05 
  

   roughly. 
  Thus 
  the 
  isolated-stomata 
  law 
  gives 
  much 
  the 
  

   smaller 
  limit 
  and 
  is 
  therefore 
  correct. 
  For 
  indoor 
  expe- 
  

   riments, 
  however 
  (and 
  most 
  transpiration 
  experiments 
  have 
  

   been 
  done 
  indoors), 
  K 
  and 
  U 
  are 
  both 
  much 
  smaller 
  than 
  is 
  

   above 
  assumed, 
  and 
  the 
  limits 
  will 
  occur 
  in 
  the 
  other 
  order, 
  

   so 
  that 
  the 
  wet-leaf 
  law 
  will 
  apply 
  and 
  evaporation 
  from 
  

   individual 
  stomata 
  will 
  accordingly 
  be 
  restricted. 
  

  

  Consider 
  then 
  a 
  leaf 
  in 
  air 
  moving 
  sufficiently 
  slowly 
  for 
  

   the 
  wet-leaf 
  law 
  to 
  hold. 
  What 
  happens 
  when 
  the 
  stomata 
  

   contract 
  ? 
  At 
  first 
  they 
  are 
  capable 
  of 
  sending 
  into 
  the 
  air 
  

   more 
  water 
  vapour 
  than 
  the 
  turbulent 
  air 
  can 
  carry 
  away; 
  

   and 
  as 
  long 
  as 
  this 
  remains 
  true 
  it 
  seems 
  to 
  me 
  that 
  vapour 
  

   will 
  stay 
  in 
  the 
  layer 
  of 
  shearing, 
  and 
  practically 
  saturate 
  it; 
  

   then 
  the 
  rate 
  of 
  evaporation 
  remains 
  nearly 
  constant, 
  for 
  it 
  

   depends 
  almost 
  entirely 
  on 
  the 
  outer 
  region, 
  which 
  is 
  in 
  the 
  

   same 
  state 
  throughout. 
  As 
  soon 
  as 
  the 
  contraction 
  reduces 
  

   the 
  possible 
  supply 
  of 
  vapour 
  to 
  below 
  the 
  maximum 
  amount 
  

   that 
  the 
  turbulence 
  can 
  remove, 
  the 
  rate 
  of 
  evaporation 
  will 
  

   diminish, 
  finally 
  reaching 
  the 
  limit 
  zero 
  when 
  the 
  stomata 
  are 
  

   quite 
  closed. 
  Thus 
  most 
  of 
  the 
  reduction 
  to 
  zero 
  will 
  take 
  

   place 
  in 
  this 
  last 
  stage. 
  I 
  do 
  not 
  see 
  in 
  what 
  respect 
  this 
  

   result 
  is 
  paradoxical, 
  and 
  it 
  does 
  appear 
  to 
  be 
  well 
  confirmed 
  

   by 
  experiment. 
  

  

  The 
  amount 
  of 
  transpiration 
  possible 
  on 
  the 
  wet-leaf 
  

   hypothesis 
  seems 
  to 
  be 
  quite 
  adequate. 
  Thus, 
  consider 
  a 
  leaf 
  

   of 
  5 
  cm. 
  radius, 
  in 
  an 
  atmosphere 
  containing 
  0*04 
  per 
  cent, 
  

   of 
  carbon 
  dioxide 
  by 
  volume. 
  Then 
  even 
  in 
  still 
  air 
  the 
  

   volume 
  of 
  C0 
  2 
  absorbed 
  per 
  hour 
  could 
  be 
  13 
  c.c., 
  or 
  0*17 
  c.c. 
  

   per 
  sq. 
  cm. 
  Thoday 
  finds 
  experimentally 
  that 
  Helianthus 
  

   annuus 
  in 
  the 
  open 
  air 
  can 
  at 
  the 
  utmost 
  absorb 
  0*14 
  c.c. 
  of 
  

   carbon 
  dioxide 
  per 
  sq. 
  cm. 
  per 
  hour. 
  On 
  the 
  question 
  of 
  

   evaporation 
  I 
  have 
  seen 
  no 
  quantitative 
  statement 
  amenable 
  

   to 
  numerical 
  calculation 
  except 
  Renner's 
  definite 
  assertion 
  

   that 
  the 
  evaporation 
  from 
  a 
  leaf 
  is 
  the 
  same 
  as 
  that 
  from 
  a 
  

   water 
  surface 
  of 
  the 
  same 
  size. 
  

  

  Note 
  by 
  Sir 
  Joseph 
  Larmor. 
  

  

  Dr. 
  Jeffreys 
  agrees 
  that 
  the 
  standard 
  discussions 
  on 
  this 
  

   subject 
  are 
  not 
  in 
  error, 
  though 
  there 
  is 
  a 
  question 
  of 
  how 
  

   closely 
  they 
  conform 
  to 
  the 
  conditions 
  of 
  any 
  particular 
  

   experiment. 
  The 
  idea 
  of 
  an 
  eddy 
  conductivity 
  may 
  prove 
  

   useful 
  with 
  due 
  limitation 
  ; 
  but 
  I 
  find 
  it 
  difficult 
  to 
  

  

  