﻿INTRODUCTION 
  TO 
  BOTANY 
  

  

  constantly 
  thicker, 
  until 
  finally, 
  in 
  older 
  branches 
  and 
  stems, 
  

   the 
  light, 
  penetrates 
  through 
  it 
  poorly, 
  if 
  at 
  all, 
  and 
  chloro- 
  

   phyll 
  ceases 
  to 
  be 
  developed. 
  The 
  ridging 
  of 
  bark 
  is 
  due 
  to 
  

   the 
  fact 
  that 
  bark 
  on 
  young 
  branches 
  and 
  stems 
  is 
  so 
  spread 
  

   by 
  growth 
  within 
  that 
  longitudinal 
  cracks 
  are 
  formed. 
  As 
  more 
  

   wood 
  is 
  developed 
  within, 
  the 
  spreading 
  and 
  thickening 
  are 
  

   increased, 
  and 
  ridges 
  and 
  crevices 
  become 
  more 
  pronounced 
  

   ( 
  li<_r. 
  222}, 
  as 
  is 
  true 
  in 
  most 
  perennial 
  stems. 
  

  

  272. 
  Rate 
  of 
  thickening 
  of 
  the 
  stem. 
  Two 
  of 
  the 
  most 
  im- 
  

   portant 
  of 
  our 
  gymnosperm 
  trees 
  are 
  the 
  white 
  pine 
  and 
  the 
  

  

  long-leaf 
  pine. 
  .V 
  white-pine 
  tree 
  

   overtopping 
  most 
  of 
  its 
  fellows 
  in 
  

   the 
  forest 
  is, 
  on 
  the 
  average, 
  at 
  ten 
  

   years 
  0.9 
  inch 
  in 
  diameter, 
  at 
  one 
  

   hundred 
  years 
  17.2 
  inches, 
  and 
  at 
  

   two 
  hundred 
  years 
  31 
  inches. 
  The 
  

   average 
  thickness 
  of 
  the 
  annual 
  

   rings 
  during 
  the 
  life 
  of 
  the 
  tree 
  

   throughout 
  its 
  second 
  century 
  is 
  

   therefore 
  about 
  -J^ 
  inch. 
  In 
  the 
  

   Southern 
  pine 
  the 
  growth 
  is 
  slower. 
  

   The 
  increase 
  in 
  thickness 
  of 
  a 
  tree 
  

  

  FIG. 
  223. 
  Seed 
  conn 
  of 
  Sc-.tch 
  two 
  hundred 
  twenty 
  years 
  old 
  and 
  

  

  17 
  : 
  l 
  inches 
  in 
  diameter 
  was 
  only 
  1 
  

   inch 
  during 
  the 
  last 
  forty 
  years, 
  or 
  

   -^ 
  inch 
  per 
  year. 
  

   The 
  tallest 
  and 
  least 
  shaded 
  white-pine 
  trees 
  at 
  fifty 
  years 
  

   develop 
  new 
  wood 
  at 
  the 
  rate 
  of 
  about 
  -\ 
  cubic, 
  foot 
  per 
  year; 
  

   at 
  seventy-five 
  years 
  at 
  the 
  rate 
  of 
  about 
  1 
  cubic 
  foot 
  per 
  

   year, 
  and 
  at 
  one 
  hundred 
  years 
  at 
  the 
  rate 
  of 
  about 
  1* 
  cubic 
  

   feet 
  per 
  year. 
  1 
  

  

  273. 
  Significance 
  of 
  the 
  stem. 
  The 
  gymnosperm 
  stem 
  is 
  

   more 
  complex 
  than 
  that 
  of 
  any 
  other 
  plant 
  in 
  the 
  series 
  of 
  

   groups 
  that 
  we 
  have 
  been 
  studying. 
  There 
  was 
  vascular 
  

  

  1 
  For 
  further 
  discussion 
  of 
  the 
  rate 
  of 
  growth 
  of 
  pine 
  trees 
  see 
  "The 
  

   White 
  Pine," 
  Bulletin 
  22^ 
  U.S. 
  Dept. 
  Agr., 
  Division 
  of 
  Forestry. 
  

  

  pine 
  (Pinus 
  .S///JVN//-/.S) 
  which 
  

  

  has 
  opened 
  and 
  dropped 
  its 
  

  

  seeds 
  

  

  