﻿434 
  On 
  Transpiration 
  from 
  Leaf-Stomata. 
  

  

  attach 
  any 
  definite 
  value 
  to 
  K. 
  It 
  may 
  be 
  of 
  use 
  to 
  sum 
  up 
  

   in 
  other 
  terms 
  the 
  broad 
  essentials, 
  which 
  seem 
  to 
  include 
  

   all 
  the 
  calculation 
  that 
  the 
  facts 
  are 
  definite 
  enough 
  to 
  

   warrant. 
  

  

  The 
  sensitive 
  surfaces 
  of 
  a 
  leaf 
  are 
  protected 
  from 
  injury, 
  

   by 
  being 
  the 
  walls 
  of 
  interior 
  chambers 
  which 
  are 
  connected 
  

   with 
  the 
  outer 
  air 
  by 
  narrow 
  necks 
  called 
  stomata. 
  Their 
  

   efficiency 
  as 
  absorbers 
  of 
  carbon 
  dioxide 
  or 
  as 
  transpirers 
  of 
  

   water 
  vapour 
  depends 
  of 
  course 
  inversely 
  on 
  the 
  narrowness 
  

   of 
  these 
  necks. 
  The 
  reciprocal 
  of 
  this 
  efficiency 
  is 
  the 
  quantity 
  

   that 
  can 
  be 
  discussed 
  directjy 
  ; 
  for 
  it 
  is 
  proportional 
  to 
  the 
  

   total 
  resistance 
  of 
  the 
  channel 
  of 
  diffusion, 
  which 
  is 
  made 
  up 
  

   by 
  addition 
  from 
  that 
  of 
  the 
  chamber, 
  that 
  of 
  the 
  neck 
  itself, 
  

   and 
  that 
  of 
  the 
  tube 
  of 
  diffusion 
  outside 
  extending 
  to 
  where 
  

   the 
  atmosphere 
  becomes 
  normal 
  as 
  regards 
  its 
  carbon 
  dioxide 
  

   or 
  its 
  vapour. 
  The 
  length 
  that 
  may 
  be 
  assumed 
  for 
  this 
  

   latter 
  tube 
  depends 
  on 
  circumstances 
  involving 
  the 
  degree 
  of 
  

   stillness 
  of 
  the 
  air 
  : 
  and 
  it 
  is 
  this 
  that 
  Dr. 
  Jeffreys 
  proposes 
  

   to 
  elucidate 
  by 
  introducing 
  the 
  modulus 
  K. 
  Perhaps 
  with 
  

   moderate 
  motion 
  of 
  the 
  leaf 
  it 
  has 
  practically 
  no 
  length 
  at 
  all, 
  

   the 
  air 
  sweeping 
  over 
  the 
  leaf 
  and 
  being 
  renewed 
  even 
  close 
  

   to 
  its 
  surface. 
  If 
  a 
  large 
  proportion 
  of 
  the 
  wall 
  of 
  a 
  chamber 
  

   is 
  fully 
  efficient, 
  the 
  air 
  in 
  the 
  chamber 
  is, 
  except 
  near 
  the 
  

   neck, 
  practically 
  free 
  from 
  carbon 
  dioxide 
  or 
  saturated 
  with 
  

   vapour, 
  as 
  the 
  case 
  may 
  be 
  : 
  this 
  follows 
  from 
  the 
  usual 
  

   electrostatic 
  analogy. 
  If 
  both 
  these 
  conditions 
  hold 
  good, 
  

   the 
  one 
  just 
  stated 
  on 
  the 
  inside 
  and 
  free 
  sw 
  r 
  eep 
  of 
  air 
  close 
  

   to 
  the 
  leaf 
  on 
  the 
  outside, 
  the 
  total 
  resistance 
  is 
  that 
  of 
  the 
  

   neck 
  alone 
  including 
  the 
  corrections 
  usual 
  in 
  electric 
  problems 
  

   for 
  its 
  ends, 
  there 
  being 
  none 
  for 
  its 
  outer 
  end 
  if 
  the 
  air 
  is 
  

   renewed 
  constantly 
  close 
  over 
  that 
  end. 
  If 
  the 
  stoma 
  were 
  

   a 
  mere 
  aperture 
  in 
  a 
  thin 
  plate, 
  its 
  resistance 
  would 
  then 
  

   reduce 
  to 
  this 
  correction 
  for 
  the 
  inner 
  end 
  alone, 
  as 
  

   Dr. 
  Horace 
  Brown 
  remarks 
  : 
  it 
  would 
  be 
  inversely 
  as 
  the 
  

   linear 
  dimensions, 
  being 
  for 
  a 
  circular 
  aperture 
  measured 
  by 
  

   the 
  reciprocal 
  of 
  its 
  diameter, 
  multiplied 
  by 
  the 
  specific 
  

   resistance 
  for 
  air. 
  This 
  is 
  the 
  reason 
  that 
  a 
  large 
  number 
  of 
  

   very 
  small 
  stomata 
  can 
  permit 
  almost 
  free 
  interchange 
  

   between 
  the 
  interior 
  of 
  the 
  leaf 
  and 
  the 
  outside. 
  

  

  