﻿66 
  INTRODUCTION 
  TO 
  BOTANY 
  

  

  63. 
  Limited 
  thickening 
  of 
  annual 
  stems. 
  In 
  stems 
  of 
  large 
  

   dicotyledons 
  which 
  die 
  to 
  the 
  ground 
  every 
  year, 
  such 
  as 
  the 
  

   sunflowers, 
  ironweeds, 
  hemp, 
  and 
  giant 
  ragweed, 
  growth 
  in 
  

   thickness 
  goes 
  on 
  throughout 
  the 
  summer. 
  The 
  outer 
  cells 
  of 
  

   the 
  cambium 
  continually 
  split 
  up 
  by 
  the 
  formation 
  of 
  tangen- 
  

   tial 
  partitions 
  parallel 
  to 
  the 
  bark, 
  and 
  so 
  form 
  new 
  layers 
  of 
  

   bark. 
  In 
  a 
  similar 
  way 
  and 
  to 
  a 
  still 
  greater 
  extent 
  the 
  inner 
  

   cells 
  of 
  the 
  cambium 
  form 
  new 
  wood, 
  and 
  thus 
  the 
  stem 
  goes 
  

   on 
  increasing 
  in 
  thickness. 
  But 
  in 
  such 
  plants 
  as 
  those 
  just 
  

   mentioned 
  the 
  activity 
  of 
  the 
  cambium 
  is 
  strict 
  ly 
  limited 
  ; 
  after 
  

   it 
  has 
  given 
  rise 
  to 
  a 
  certain 
  amount 
  of 
  new 
  tissue, 
  growth 
  

   stops 
  and 
  the 
  stem 
  dies 
  down 
  to 
  the 
  ground. 
  The 
  death 
  of 
  

   annual 
  stems 
  in 
  the 
  autumn 
  is 
  often 
  thoughtlessly 
  supposed 
  

   to 
  be 
  due 
  to 
  the 
  arrival 
  of 
  winter, 
  but 
  it 
  occurs 
  just 
  as 
  cer- 
  

   tainly, 
  and 
  often 
  after 
  a 
  briefer 
  period 
  of 
  growth, 
  in 
  regions 
  

   where 
  there 
  is 
  no 
  cold 
  weather. 
  

  

  64. 
  Annual 
  thickening. 
  In 
  stems 
  such 
  as 
  those 
  of 
  dicotyle- 
  

   donous 
  trees 
  and 
  the 
  trees 
  of 
  the 
  Pine 
  family 
  and 
  other 
  cone- 
  

   bearers, 
  which 
  live 
  for 
  many 
  years, 
  the 
  cambium 
  forms 
  each 
  

   year 
  a 
  new 
  layer 
  of 
  bark 
  and 
  one 
  of 
  wood. 
  These 
  annual 
  layers 
  

   are 
  usually 
  more 
  noticeable 
  in 
  the 
  wood 
  than 
  in 
  the 
  bark, 
  

   because 
  the 
  wood 
  cylinders 
  thus 
  formed 
  remain 
  closely 
  joined 
  

   together 
  (fig. 
  45). 
  The 
  newer, 
  lighter-colored 
  portions 
  of 
  the 
  

   wood 
  are 
  known 
  as 
  sapwood, 
  and 
  the 
  older 
  portions, 
  often 
  

   darkened 
  by 
  the 
  deposit 
  of 
  coloring 
  matter, 
  are 
  known 
  as 
  

   heartwood. 
  Not 
  infrequently 
  the 
  heartwood 
  decays 
  and 
  leaves 
  

   the 
  tree 
  hollow. 
  

  

  How 
  old 
  is 
  the 
  stick 
  of 
  wood 
  shown 
  in 
  figure 
  45? 
  Did 
  it 
  

   grow 
  at 
  the 
  same 
  rate 
  during 
  all 
  the 
  years 
  of 
  its 
  life? 
  Dis- 
  

   cuss 
  this 
  <|iiestinn. 
  Why 
  is 
  the 
  name 
  annual 
  rings 
  not 
  an 
  

   accurate 
  one 
  ? 
  What 
  are 
  they 
  really 
  ? 
  Is 
  each 
  year's 
  growth 
  

   uniform 
  all 
  round 
  the 
  stem 
  ? 
  Had 
  this 
  stem 
  any 
  branches 
  

   in 
  the 
  portion 
  shown 
  by 
  the 
  section 
  ? 
  How 
  does 
  figure 
  46 
  

   suggest 
  a 
  method 
  of 
  determining 
  the 
  age 
  of 
  the 
  stem 
  at 
  the 
  

   time 
  when 
  a 
  branch 
  began 
  ? 
  What 
  seems 
  a 
  probable 
  cause 
  of 
  

   the 
  death 
  of 
  the 
  branch, 
  figure 
  46, 
  b 
  ? 
  

  

  