﻿FOODS 
  IN 
  PLANTS 
  

  

  35 
  

  

  From 
  the 
  upper 
  and 
  the 
  lower 
  surfaces 
  of 
  leaves 
  such 
  as 
  

   those 
  of 
  live-forever, 
  Wandering 
  Jew, 
  Easter 
  lily, 
  corn, 
  and 
  

   spiderwort 
  one 
  may 
  peel 
  a 
  thin, 
  almost 
  colorless 
  layer 
  which 
  

   is 
  known 
  as 
  the 
  epidermis 
  (fig. 
  25). 
  The 
  epidermis 
  is 
  com- 
  

   posed 
  of 
  cells 
  more 
  or 
  less 
  compactly 
  arranged. 
  Different 
  

   plants 
  show 
  much 
  variation 
  in 
  the 
  way 
  in 
  which 
  these 
  

   epidermal 
  cells 
  fit 
  together. 
  

  

  One 
  or 
  both 
  epidermal 
  

   layers 
  may 
  include 
  special 
  

   structures 
  known 
  as 
  stomata 
  

   (singular, 
  stoma) 
  (fig. 
  25). 
  

   Usually, 
  when 
  viewed 
  from 
  

   the 
  surface, 
  the 
  stoma 
  is 
  

   readily 
  seen 
  to 
  consist 
  of 
  

   two 
  crescentic 
  or 
  kidney- 
  

   shaped 
  cells 
  with 
  their 
  

  

  concave 
  sides 
  facing 
  one 
  

  

  FIG. 
  25. 
  A 
  surface 
  view 
  of 
  leaf 
  epider- 
  

   mis 
  from 
  the 
  geranium 
  (Pelargonium) 
  

  

  Among 
  the 
  ordinary 
  epidermal 
  cells 
  (c) 
  are 
  

   four 
  stomata, 
  each 
  with 
  two 
  guard 
  cells 
  (gc) 
  

   and 
  the 
  mouth 
  of 
  an 
  air 
  cavity 
  (p). 
  Con- 
  

   siderably 
  magnified 
  

  

  another, 
  so 
  as 
  to 
  leave 
  an 
  

  

  opening 
  between 
  the 
  two 
  

  

  cells. 
  The 
  opening 
  is 
  really 
  

  

  the 
  mouth 
  of 
  a 
  larger 
  space 
  

  

  extending 
  within 
  the 
  leaf. 
  

  

  It 
  is 
  known 
  as 
  the 
  stomatal 
  

  

  opening, 
  and 
  the 
  two 
  cells 
  

  

  that 
  are 
  about 
  the 
  mouth 
  

  

  are 
  known 
  as 
  the 
  guard 
  cells. 
  Unlike 
  other 
  epidermal 
  cells 
  the 
  

  

  guard 
  cells 
  are 
  green. 
  The 
  stomatal 
  opening 
  serves 
  as 
  a 
  place 
  

  

  of 
  entrance 
  for 
  the 
  carbon 
  dioxide 
  used 
  by 
  the 
  plant, 
  though 
  

  

  carbon 
  dioxide 
  may 
  doubtless 
  enter 
  the 
  plant 
  with 
  the 
  water 
  

  

  from 
  the 
  soil. 
  The 
  guard 
  cells 
  may 
  also 
  press 
  closely 
  together 
  

  

  or 
  may 
  separate 
  until 
  a 
  wide 
  circular 
  opening 
  is 
  formed, 
  and 
  in 
  

  

  thus 
  closing 
  and 
  opening 
  they 
  influence 
  the 
  interchange 
  of 
  air 
  

  

  between 
  the 
  interior 
  and 
  the 
  exterior 
  of 
  the 
  leaf. 
  Obviously 
  

  

  this 
  opening 
  and 
  closing 
  also 
  affects 
  the 
  interchange 
  of 
  such 
  

  

  gases 
  as 
  carbon 
  dioxide 
  and 
  oxygen, 
  as 
  well 
  as 
  the 
  outgo 
  of 
  

  

  moisture 
  from 
  the 
  leaf. 
  

  

  