﻿THE 
  STEM 
  AND 
  THE 
  LEAF 
  

  

  67 
  

  

  The 
  hard-wood 
  trees 
  show 
  great 
  differences 
  in 
  the 
  rate 
  at 
  

   which 
  their 
  trunks 
  increase 
  in 
  thickness. 
  Poplars, 
  basswoods, 
  

   willows, 
  and 
  red 
  oaks, 
  growing 
  in 
  good 
  soil 
  and 
  unshaded, 
  may 
  

   for 
  forty 
  or 
  fifty 
  years 
  form 
  annual 
  rings 
  as 
  much 
  as 
  three 
  

   eighths 
  of 
  an 
  inch 
  thick, 
  but 
  old 
  beeches 
  

   and 
  sugar 
  maples 
  in 
  the 
  forest, 
  after 
  they 
  

   have 
  passed 
  the 
  hundred-year 
  limit, 
  often 
  

   grow 
  not 
  more 
  than 
  about 
  one 
  sixteenth 
  

   of 
  an 
  inch 
  per 
  year. 
  When 
  very 
  old, 
  

   though 
  still 
  sound, 
  they 
  may 
  grow 
  only 
  

   about 
  one 
  twenty 
  -fifth 
  of 
  an 
  inch 
  per 
  year. 
  

  

  Would 
  it 
  be 
  good 
  policy 
  to 
  let 
  beeches 
  

   and 
  maples 
  remain 
  long 
  in 
  the 
  forest 
  

   after 
  they 
  are 
  one 
  hundred 
  years 
  old 
  

   before 
  cutting 
  for 
  timber 
  ? 
  Why 
  ? 
  

  

  65. 
  Growing 
  points. 
  The 
  extreme 
  tip 
  

   of 
  the 
  live 
  stem 
  or 
  root 
  of 
  a 
  dicotyle- 
  

   don 
  consists 
  of 
  a 
  more 
  or 
  less 
  conical 
  or 
  

   cushion-shaped 
  mass 
  of 
  tissue 
  composed 
  

   of 
  thin-walled 
  cells 
  like 
  those 
  of 
  the 
  cam- 
  

   bium 
  layer. 
  This 
  portion 
  of 
  the 
  stem 
  or 
  

   root 
  is 
  called 
  the 
  growing 
  point. 
  Every 
  

   live 
  twig 
  and 
  rootlet 
  is 
  tipped 
  with 
  a 
  

   growing 
  point, 
  and 
  it 
  is 
  by 
  the 
  rapid 
  

   sub-division 
  and 
  consequent 
  multiplica- 
  

   tion 
  of 
  these 
  cells 
  that 
  the 
  lengthening 
  

   of 
  the 
  main 
  stem 
  and 
  its 
  divisions, 
  and 
  

   of 
  every 
  root, 
  takes 
  place. 
  

  

  All 
  branches 
  originate 
  from 
  growing 
  

   points, 
  which 
  are 
  usually 
  developed 
  along 
  

   leaf 
  -bearing 
  portions 
  of 
  the 
  stem, 
  each 
  one 
  just 
  above 
  the 
  point 
  

   where 
  a 
  leaf 
  is 
  attached. 
  In 
  their 
  earliest 
  beginnings 
  both 
  

   leaves 
  and 
  rudimentary 
  branches 
  consist 
  wholly 
  of 
  thin-walled 
  

   cellular 
  tissue. 
  Fibrovascular 
  bundles, 
  connected 
  with 
  those 
  

   of 
  the 
  underlying 
  stem, 
  soon 
  appear 
  in 
  the 
  branches 
  and 
  leaves 
  

   as 
  their 
  development 
  goes 
  on. 
  

  

  FIG. 
  46. 
  Formation 
  of 
  

   knots 
  due 
  to 
  branches. 
  

   The 
  figure 
  gives 
  part 
  of 
  

   a 
  lengthwise 
  section 
  of 
  

   a 
  stick 
  of 
  birch 
  wood 
  

  

  a, 
  section 
  of 
  the 
  base 
  of 
  

   a 
  branch 
  which 
  persisted 
  

   until 
  the 
  tree 
  was 
  felled 
  ; 
  

  

  b, 
  section 
  of 
  the 
  base 
  of 
  a 
  

   branch 
  which 
  died 
  some 
  

   years 
  earlier 
  and 
  is 
  now 
  

   covered 
  by 
  several 
  layers 
  

  

  of 
  younger 
  wood 
  

  

  