﻿THE 
  STEM 
  AND 
  THE 
  LEAF 
  

  

  65 
  

  

  commonly 
  found 
  in 
  the 
  bark 
  of 
  dicotyledons 
  and 
  are 
  often 
  the 
  

   main 
  factors 
  in 
  strengthening 
  young 
  stems. 
  Collenchyma 
  cells 
  

   (J) 
  are 
  like 
  the 
  thin-walled 
  cells 
  of 
  the 
  pith, 
  but 
  are 
  reenforced 
  

   at 
  the 
  angles, 
  just 
  as 
  some 
  packing 
  boxes 
  have 
  strips 
  of 
  board 
  

   nailed 
  fast 
  on 
  the 
  inside 
  of 
  the 
  box 
  at 
  the 
  junctions 
  of 
  the 
  sides. 
  

   Bast 
  fibers 
  (7?) 
  are 
  extremely 
  slender 
  tubes 
  with 
  closed 
  and 
  

   pointed 
  ends, 
  much 
  like 
  a 
  piece 
  of 
  thermometer 
  tubing 
  drawn 
  

   to 
  a 
  point 
  in 
  a 
  gas 
  flame 
  and 
  

   thus 
  closed. 
  Collenchyma 
  gives 
  

   moderate 
  stiffness 
  to 
  the 
  parts 
  in 
  

   which 
  it 
  occurs 
  and 
  is 
  highly 
  

   elastic, 
  so 
  that 
  it 
  does 
  not 
  hinder 
  

   the 
  growth 
  of 
  the 
  stem 
  which 
  it 
  

   incloses. 
  Bast 
  fibers 
  are 
  flexible 
  

  

  but 
  very 
  tough, 
  and 
  therefore 
  

   enable 
  the 
  parts 
  of 
  the 
  root, 
  

   stem, 
  or 
  leaf 
  in 
  which 
  they 
  occur 
  

   to 
  resist 
  being 
  pulled 
  apart. 
  In 
  

   many 
  stems, 
  particularly 
  those 
  

   which 
  are 
  more 
  than 
  a 
  year 
  old 
  

   (fig. 
  45), 
  a 
  great 
  part 
  of 
  the 
  total 
  

   strength 
  is 
  due 
  to 
  the 
  presence 
  of 
  

   several 
  kinds 
  of 
  fibers, 
  of 
  which 
  

   the 
  wood 
  is 
  largely 
  made 
  up. 
  

  

  Which 
  stem 
  is 
  more 
  like 
  a 
  wire 
  

  

  cable 
  in 
  its 
  structure, 
  that 
  of 
  Dutchman's-pipe 
  (fig. 
  42, 
  A) 
  

   or 
  that 
  of 
  the 
  sunflower 
  (fig. 
  42, 
  5) 
  ? 
  

  

  62. 
  Stiffness 
  of 
  stems. 
  It 
  is 
  a 
  familiar 
  fact 
  that 
  a 
  metal 
  

   tube 
  is 
  stiffer 
  than 
  a 
  solid 
  rod 
  of 
  the 
  same 
  kind 
  of 
  metal 
  and 
  

   the 
  same 
  weight 
  per 
  foot 
  of 
  length. 
  So 
  in 
  many 
  plants, 
  just 
  as 
  

   in 
  the 
  long 
  bones 
  of 
  animals, 
  the 
  stems 
  are 
  at 
  once 
  stiff 
  and 
  

   light, 
  because 
  the 
  material 
  is 
  arranged 
  in 
  the 
  form 
  of 
  a 
  tube, 
  as 
  

   in 
  the 
  bamboo, 
  the 
  straw 
  of 
  the 
  small 
  grains, 
  and 
  such 
  flower 
  

   stalks 
  as 
  that 
  of 
  the 
  dandelion. 
  In 
  other 
  cases, 
  as 
  in 
  the 
  corn- 
  

   stalk 
  and 
  in 
  the 
  stems 
  of 
  elder, 
  the 
  harder 
  parts 
  of 
  the 
  stem 
  

   constitute 
  a 
  tube 
  inside 
  of 
  which 
  is 
  much 
  soft, 
  light 
  pith. 
  

  

  FIG. 
  45. 
  One 
  quarter 
  of 
  a 
  cross 
  

   section 
  of 
  a 
  stick 
  of 
  oak 
  wood 
  

  

  H, 
  medullary 
  rays, 
  running 
  from 
  

   hark 
  to 
  pith; 
  r, 
  annual 
  rings 
  ; 
  b, 
  

   boundaries 
  between 
  rings, 
  porous 
  

   from 
  presence 
  of 
  many 
  ducts; 
  i, 
  inte- 
  

   rior 
  fibrous 
  layers 
  of 
  dead 
  bark 
  ; 
  pi, 
  

   hard 
  plates 
  of 
  dead 
  bark 
  splitting 
  

   away 
  from 
  each 
  other 
  but 
  attached 
  

   to 
  bark 
  beneath. 
  Reduced 
  

  

  