﻿278 
  

  

  INTRODUCTION 
  TO 
  BOTANY 
  

  

  D 
  

  

  ". 
  The 
  ciiniinon 
  scouring 
  rush. 
  <n- 
  

   horsetail 
  Kti<ixi'tnin 
  nvfi.se) 
  

  

  . 
  I. 
  a 
  plant 
  in 
  curly 
  spring 
  condition 
  : 
  /... 
  rhi/<>nie: 
  

   /i..v. 
  spore-bearing 
  lirancli: 
  c, 
  collection 
  of 
  spo- 
  

   rophylls 
  (sli-ohilus, 
  orcoiic) 
  ; 
  /.//, 
  foliage 
  branch, 
  

   \\ 
  liidi 
  later 
  cxjianils 
  as 
  in 
  /,' 
  : 
  ( 
  ', 
  one 
  sporopliyll 
  

   from 
  the 
  cone, 
  showing 
  the 
  stalk 
  (xt) 
  and 
  several 
  

   sporangia 
  (\/o. 
  l> 
  and 
  /,', 
  spore 
  with 
  clatcrs. 
  

   A 
  and 
  /;, 
  one 
  half 
  natural 
  si/.c 
  ; 
  I', 
  nia^nitied 
  

   about 
  20 
  times; 
  /> 
  and 
  H, 
  greatly 
  enlarged 
  

  

  patches. 
  In 
  still 
  others, 
  as 
  

   the 
  maidenhair 
  (A<J/<niftini) 
  

   and 
  the 
  bracken 
  fern 
  (YVrr/.s- 
  

   iitj/i///ti<t) 
  and 
  other 
  species 
  of 
  

   Pteris, 
  the 
  sporangia 
  are 
  cov- 
  

   ered 
  by 
  the 
  folded 
  leaf 
  margins 
  

   - 
  the 
  so-called 
  false 
  indusium. 
  

   In 
  some, 
  as 
  the 
  sensitive 
  fern, 
  

   or 
  oak 
  fern 
  (fig. 
  21G), 
  the 
  leaf 
  

   is 
  different 
  ialed 
  into 
  a 
  spore- 
  

   bearing 
  branch 
  and 
  a 
  chloro- 
  

   phyll 
  branch. 
  In 
  such 
  cases 
  

   the 
  former 
  is 
  called 
  the 
  sporo- 
  

   /*////// 
  (spore 
  leaf), 
  and 
  the 
  

   latter 
  the 
  fol/<t</<' 
  leaf, 
  the 
  

   chlorophyll 
  bearer. 
  Setting 
  

   apart 
  special 
  structures 
  for 
  

   special 
  pieces 
  of 
  work 
  (divi- 
  

   sion 
  of 
  labor), 
  as 
  here 
  shown, 
  

   ordinarily 
  increases 
  the 
  quan- 
  

   tity 
  and 
  quality 
  of 
  work 
  done. 
  

   262. 
  Horsetails, 
  or 
  scouring 
  

   rushes. 
  The 
  class 
  of 
  pterido- 
  

   phytes 
  to 
  which 
  these 
  plants 
  

   belong 
  once 
  constituted 
  a 
  

   prominent 
  part 
  of 
  the 
  

   earth's 
  flora 
  as 
  tree-like 
  

   plants. 
  They 
  are 
  no\v 
  

   represented 
  by 
  the 
  sin- 
  

   gle 
  genus 
  J'Ji/it/xi'finit. 
  

   Fossil 
  remains 
  tell 
  in- 
  

   teresting 
  stories 
  of 
  the 
  

   ancestors 
  of 
  these 
  plants 
  

   which 
  lived 
  ages 
  ago 
  

   when 
  coal 
  was 
  being 
  

   formed 
  in 
  abundance. 
  

  

  