﻿36 
  

  

  INTRODUCTION 
  TO 
  BOTANY 
  

  

  34. 
  Internal 
  structure 
  of 
  the 
  leaf. 
  An 
  idea 
  of 
  the 
  inner 
  

   structure 
  is 
  best 
  obtained 
  by 
  examining 
  a 
  cross 
  section 
  of 
  a 
  

   simple 
  leaf. 
  In 
  general, 
  three 
  kinds 
  of 
  cells 
  appear 
  within 
  the 
  

   epidermis, 
  and 
  these 
  are 
  variously 
  arranged 
  in 
  different 
  kinds 
  

   of 
  plants. 
  In 
  simple 
  leaves, 
  such 
  as 
  that 
  shown 
  in 
  figure 
  26, 
  

   almost 
  all 
  the 
  cells 
  contain 
  the 
  green 
  coloring 
  matter, 
  chloro- 
  

   ;>////// 
  ( 
  meaning 
  " 
  leaf 
  green 
  "). 
  These 
  chlorophyll-bearing 
  cells 
  

   are 
  long 
  and 
  are 
  arranged 
  side 
  by 
  side 
  (palisade 
  tissue) 
  or 
  are 
  

  

  P 
  

  

  FIG. 
  26. 
  Cross 
  section 
  of 
  a 
  geranium 
  leaf 
  

  

  a, 
  air 
  space; 
  a.c, 
  air 
  chamber; 
  e, 
  upper 
  epidermis; 
  e', 
  lower 
  epidermis; 
  77, 
  pali- 
  

   sade 
  cells; 
  s, 
  stoma; 
  xji, 
  spongy 
  parenchyma 
  (usually 
  spongy 
  parenchyma 
  has 
  

   fewer 
  chloroplasts 
  than 
  the 
  palisade 
  tissue) 
  ; 
  v, 
  vein. 
  Magnified 
  150 
  times. 
  After 
  

  

  drawing 
  by 
  Mrs. 
  F. 
  E. 
  Clements 
  

  

  more 
  irregular 
  in 
  form 
  and 
  loosely 
  arranged 
  (spongy 
  tissue). 
  

   Air 
  spaces 
  are 
  abundant 
  between 
  these 
  cells, 
  but 
  they 
  are 
  

   larger 
  and 
  more 
  abundant 
  in 
  the 
  spongy 
  tissue 
  than 
  in 
  the 
  

   palisade 
  tissue. 
  When 
  a 
  single 
  layer 
  of 
  palisade 
  tissue 
  and 
  

   one 
  of 
  spongy 
  tissue 
  are 
  present, 
  the 
  palisade 
  tissue 
  lies 
  next 
  

   to 
  the 
  upper 
  surface 
  of 
  the 
  leaf 
  and 
  the 
  spongy 
  tissue 
  next 
  to 
  

   the 
  lower 
  surface; 
  but 
  frequently 
  there 
  is 
  a 
  lower 
  layer 
  of 
  

   palisade 
  tissue, 
  not 
  so 
  well 
  developed 
  as 
  the 
  upper 
  one. 
  In 
  

   cross 
  sections 
  of 
  leaves 
  the 
  veins 
  appear 
  as 
  masses 
  of 
  small, 
  

   thick-walled 
  cells 
  closely 
  crowded 
  together 
  and 
  usually 
  lying 
  

   about 
  midway 
  between 
  the 
  upper 
  and 
  lower 
  surfaces 
  of 
  the 
  leaf. 
  

  

  