﻿THE 
  PLANT 
  AS 
  A 
  WORKING 
  MACHINE 
  

  

  11 
  

  

  9. 
  Stems 
  as 
  supporting 
  structures. 
  In 
  an 
  examination 
  of 
  

   the 
  stem 
  of 
  almost 
  any 
  woody 
  plant 
  there 
  appears 
  an 
  outer 
  dead 
  

   bark, 
  an 
  inner 
  green 
  bark, 
  a 
  cylinder 
  of 
  wood, 
  and 
  sometimes 
  a 
  

  

  small 
  area 
  of 
  pith 
  

   in 
  the 
  center 
  of 
  

   the 
  woody 
  cyliri- 
  

   der. 
  Although 
  

   some 
  of 
  our 
  com- 
  

   monest 
  agricul- 
  

   tural 
  plants, 
  as 
  

   corn 
  and 
  wheat, 
  

   do 
  not 
  have 
  a 
  cen- 
  

   tral 
  woody 
  cylin- 
  

   der, 
  they 
  have 
  an 
  

   outer 
  region 
  of 
  

   very 
  hard, 
  strong 
  

   tissue, 
  and 
  either 
  

   a 
  hollow 
  central 
  

   region, 
  as 
  in 
  the 
  

   wheat, 
  or 
  a 
  large 
  

   pithy 
  center 
  through 
  which 
  run 
  a 
  number 
  of 
  fibrous 
  bundles, 
  as 
  

   in 
  corn 
  (fig. 
  7). 
  In 
  wheat 
  the 
  fibrous 
  bundles 
  lie 
  close 
  to 
  the 
  hard 
  

   outer 
  portion 
  of 
  

   the 
  stem 
  ; 
  conse- 
  

   quently 
  they 
  are 
  

   not 
  seen 
  nearly 
  

   so 
  easily 
  as 
  are 
  

   the 
  fibrous 
  bun- 
  

   dles 
  in 
  corn. 
  A 
  

   stem 
  could 
  not 
  

   stand 
  alone 
  in 
  an 
  

   upright 
  position 
  

   if 
  it 
  did 
  not 
  pos- 
  

   sess 
  some 
  such 
  rigid 
  tissues 
  as 
  those 
  generally 
  noted 
  in 
  woody 
  

   plants 
  or 
  in 
  the 
  outer 
  portions 
  of 
  corn 
  and 
  wheat 
  stems. 
  Other 
  

   factors 
  that 
  help 
  in 
  maintaining 
  an 
  upright 
  position 
  of 
  the 
  

  

  FIG. 
  6. 
  Root 
  hairs 
  

  

  A, 
  a 
  cross 
  section 
  of 
  a 
  root, 
  showing 
  the 
  central 
  cylinder 
  

  

  (e?/0, 
  the 
  cortex 
  (co), 
  ami 
  the 
  epidermis 
  (PI>), 
  with 
  root 
  

  

  hairs 
  that 
  have 
  grown 
  from 
  epidermal 
  cells. 
  B, 
  a 
  root 
  

  

  hair 
  enlarged 
  to 
  show 
  the 
  soil 
  particles 
  attached 
  to 
  it 
  

  

  FIG. 
  7. 
  A 
  cornstalk 
  broken 
  so 
  as 
  to 
  show 
  the 
  num- 
  

   ber 
  and 
  distribution 
  of 
  the 
  vascular 
  bundles 
  

  

  