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  XXXVIII. 
  On 
  Transpiration 
  through 
  Leaf-Stomata. 
  

   By 
  Sir 
  Joseph 
  Larmok, 
  F.R.S.* 
  

  

  THE 
  acute 
  and 
  valuable 
  paper 
  by 
  Dr. 
  Harold 
  Jeffreys 
  

   (Phil. 
  Mag. 
  for 
  March, 
  pp. 
  270-280) 
  on 
  evaporation 
  

   and 
  diffusion 
  shows, 
  by 
  inadvertence, 
  less 
  than 
  due 
  apprecia- 
  

   tion 
  of 
  the 
  investigations 
  of 
  H. 
  T. 
  Brown 
  and 
  F. 
  Escombe 
  on 
  

   transpiration 
  through 
  the 
  stomata 
  of 
  leaves. 
  The 
  question 
  is 
  

   so 
  important 
  in 
  plant-economy 
  that 
  misunderstandings 
  should 
  

   not 
  be 
  allowed 
  to 
  persist. 
  The 
  statement 
  held 
  to 
  be 
  erroneous 
  

   on 
  p. 
  277 
  (for 
  which 
  originally 
  I 
  had 
  some 
  degree 
  of 
  respon- 
  

   sibility) 
  still 
  appears 
  to 
  me 
  to 
  be 
  quite 
  correct, 
  in 
  its 
  proper 
  

   context. 
  The 
  diffusional 
  suction 
  to 
  or 
  from 
  each 
  stomatal 
  

   opening 
  is 
  local 
  ; 
  thus, 
  when 
  the 
  openings 
  are 
  as 
  much 
  as 
  

   ten 
  diameters 
  apart 
  the 
  interference 
  between 
  the 
  adjacent 
  

   shells 
  of 
  diffusion 
  is 
  surely 
  very 
  slight, 
  as 
  stated. 
  But 
  

   Dr. 
  Jeffreys 
  finds 
  that 
  this 
  would 
  make 
  the 
  transpiration 
  of 
  

   vapour 
  from 
  the 
  stomata 
  very 
  many 
  times 
  greater 
  than 
  the 
  

   evaporation 
  from 
  the 
  entire 
  leaf 
  when 
  wet. 
  Whence, 
  then, 
  

   the 
  discrepancy 
  ? 
  His 
  calculation 
  compares 
  a 
  system 
  of 
  

   actual 
  stomata, 
  each 
  of 
  diameter 
  10~ 
  3 
  cm., 
  and 
  at 
  about 
  

   5 
  .10~ 
  3 
  cm. 
  apart, 
  with 
  a 
  single 
  giant 
  stoma 
  the 
  size 
  of 
  the 
  

   whole 
  wet 
  leaf 
  and 
  so 
  of 
  diameter 
  6 
  cm. 
  But 
  the 
  calcula- 
  

   tion 
  for 
  the 
  latter 
  case 
  implies 
  that 
  the 
  air 
  is 
  absolutely 
  still 
  

   throughout 
  the 
  shells 
  of 
  diffusion, 
  which 
  then 
  extend 
  to 
  

   several 
  diameters 
  from 
  the 
  leaf, 
  so 
  that 
  most 
  of 
  this 
  mass 
  of 
  

   air 
  remains 
  highly 
  saturated 
  with 
  vapour. 
  Under 
  natural 
  

   conditions, 
  where 
  the 
  atmosphere 
  and 
  the 
  leaf 
  are 
  not 
  quite 
  

   still, 
  the 
  evaporation 
  will 
  be 
  at 
  the 
  very 
  least 
  hundreds 
  of 
  

   times 
  greater. 
  

  

  The 
  natural 
  comparison 
  is 
  that 
  discussed 
  later 
  by 
  

   Dr. 
  Jeffre}'s 
  (p. 
  279), 
  where 
  he 
  passes 
  on 
  to 
  consider 
  the 
  

   effect 
  of 
  wind 
  or 
  movement 
  of 
  the 
  air. 
  But 
  here 
  again, 
  by 
  

   reasoning 
  similar 
  to 
  that 
  described 
  above, 
  he 
  seems 
  to 
  arrive 
  

   at 
  misunderstanding 
  of 
  Sir 
  F. 
  Darwin, 
  and 
  holds 
  that 
  when 
  

   the 
  stomata 
  become 
  constricted 
  " 
  until 
  the 
  stomatal 
  aperture 
  

   is 
  reduced 
  to 
  a 
  certain 
  very 
  small 
  value 
  the 
  possible 
  rate 
  of 
  

   transpiration 
  is 
  practically 
  independent 
  of 
  the 
  aperture 
  and 
  

   nearly 
  all 
  of 
  the 
  reduction 
  to 
  zero 
  when 
  the 
  stoma 
  closes 
  

   takes 
  place 
  on 
  the 
  last 
  2 
  per 
  cent, 
  of 
  the 
  reduction 
  of 
  aper- 
  

   ture." 
  The 
  reasoning 
  on 
  which 
  this 
  statement 
  is 
  based 
  should 
  

   be 
  capable 
  of 
  some 
  less 
  paradoxical 
  form 
  of 
  conclusion. 
  If 
  I 
  

   am 
  not 
  mistaken, 
  the 
  underlying 
  idea 
  may 
  be 
  developed 
  in 
  

   altered 
  form 
  as 
  follows. 
  We 
  may 
  imagine 
  partitions 
  erected 
  

   perpendicular 
  to 
  the 
  surface 
  of 
  the 
  leaf, 
  so 
  that 
  each 
  stomatal 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

  

  