﻿280 
  Some 
  Problems 
  of 
  Evaporation. 
  

  

  side 
  o£ 
  diffusion. 
  It 
  does 
  not 
  preclude 
  the 
  possibility 
  that 
  a 
  

   reduction 
  of 
  the 
  stomatal 
  aperture 
  may 
  he 
  associated 
  with 
  a 
  

   redaction 
  of 
  the 
  rate 
  o£ 
  evaporation 
  ; 
  but 
  it 
  does 
  show 
  

   that 
  in 
  most 
  cases 
  the 
  cause 
  o£ 
  such 
  reduction 
  is 
  not 
  the 
  

   mere 
  extra 
  mechanical 
  obstruction 
  to 
  the 
  passage 
  of 
  water 
  

   vapour, 
  but 
  must 
  depend 
  on 
  the 
  internal 
  conditions. 
  The 
  

   importance 
  o£ 
  these 
  is 
  obvious. 
  For 
  instance, 
  in 
  the 
  

   problem 
  considered 
  here 
  the 
  air 
  has 
  been 
  supposed 
  saturated 
  

   when 
  in 
  contact 
  with 
  a 
  stoma 
  and 
  perfectly 
  dry 
  at 
  a 
  great 
  

   distance. 
  Actually 
  the 
  concentration 
  at 
  a 
  great 
  distance 
  

   has 
  the 
  finite 
  value 
  Y^ 
  ; 
  and 
  that 
  at 
  a 
  stomatal 
  aperture 
  is 
  

   probably 
  somewhat 
  less 
  than 
  the 
  saturation 
  concentration. 
  

   Let 
  it 
  be 
  Vi. 
  The 
  latter 
  question 
  is 
  further 
  complicated 
  by 
  

   the 
  facts 
  that 
  the 
  dissolved 
  substances 
  within 
  the 
  cells 
  must 
  

   diminish 
  the 
  pressure 
  of 
  saturated 
  vapour 
  ; 
  that 
  the 
  leaf 
  

   is 
  normally 
  at 
  a 
  somewhat 
  higher 
  temperature 
  than 
  its 
  

   surroundings, 
  so 
  that 
  the 
  pressure 
  of 
  saturated 
  vapour 
  will 
  

   on 
  this 
  account 
  be 
  greater 
  than 
  that 
  at 
  the 
  temperature 
  of 
  

   the 
  surroundings 
  ; 
  and 
  that 
  owing 
  to 
  internal 
  restrictions 
  to 
  

   the 
  supply 
  of 
  water 
  to 
  the 
  stoma 
  the 
  vapour-pressure 
  may 
  

   be 
  reduced. 
  The 
  effect 
  of 
  these 
  changes 
  is 
  that 
  in 
  all 
  the 
  

   formulae 
  we 
  must 
  substitute 
  Vx 
  — 
  Yd 
  for 
  V 
  . 
  

  

  Another 
  complication 
  arises 
  from 
  the 
  fact 
  that 
  the 
  

   stomata 
  are 
  not 
  usually 
  mere 
  pores 
  with 
  saturated 
  air 
  in 
  

   their 
  planes 
  ; 
  in 
  most 
  cases 
  they 
  are 
  pits 
  sunk 
  in 
  the 
  leaf- 
  

   surface. 
  As 
  long 
  as 
  their 
  number 
  is 
  large 
  this 
  is 
  not 
  likely 
  

   to 
  produce 
  any 
  great 
  effect 
  on 
  the 
  rate 
  of 
  evaporation, 
  for 
  in 
  

   exactly 
  the 
  same 
  way 
  as 
  with 
  flat 
  stomata 
  the 
  earlier 
  ones 
  

   met 
  by 
  the 
  air 
  will 
  partially 
  saturate 
  it, 
  and 
  the 
  air 
  when 
  it 
  

   meets 
  the 
  later 
  ones 
  will 
  be 
  nearly 
  at 
  the 
  same 
  saturation 
  

   as 
  that 
  inside 
  them. 
  When 
  the 
  number 
  is 
  small, 
  on 
  the 
  

   other 
  hand, 
  the 
  formula 
  for 
  evaporation 
  from 
  depressed 
  

   stomata 
  must 
  be 
  used. 
  For 
  circular 
  cylindrical 
  stomata 
  this 
  

   gives 
  for 
  the 
  rate 
  of 
  evaporation 
  when 
  the 
  depth 
  I 
  is 
  great 
  

   compared 
  with 
  the 
  radius 
  

  

  7mV* 
  2 
  (Vi-V<*)A 
  

   l 
  + 
  \ira 
  

  

  