﻿274 
  

  

  INTKODUCTION 
  TO 
  BOTANY 
  

  

  (fig. 
  215). 
  Within 
  the 
  sporangium 
  (fig. 
  210) 
  many 
  heavy- 
  

   walled 
  asexual 
  spores 
  are 
  produced. 
  In 
  most 
  of 
  the 
  common 
  

   ferns 
  the 
  sporangia 
  are 
  of 
  the 
  form 
  shown 
  in 
  figure 
  210. 
  There 
  

   is 
  a 
  heavy 
  ring 
  of 
  cells 
  which 
  extends 
  over 
  the 
  wall 
  of 
  the 
  

   sporangium. 
  When 
  ripe 
  these 
  cells 
  become 
  dry, 
  straighten 
  out, 
  

   and 
  tear 
  the 
  sporangium 
  open; 
  the 
  ring 
  of 
  cells 
  then 
  springs 
  

   forcibly 
  back 
  to 
  its 
  former 
  position, 
  and 
  in 
  doing 
  so 
  scatters 
  

   the 
  spores. 
  

  

  259. 
  Sexual 
  reproduction 
  of 
  a 
  fern. 
  The 
  asexual 
  fern 
  spore 
  

   germinates 
  upon 
  moist 
  earth, 
  pots 
  in 
  greenhouses, 
  etc. 
  It 
  soon 
  

  

  grows 
  into 
  a 
  broad, 
  flat, 
  

   heart-shaped 
  plant 
  (fig. 
  -1 
  _ 
  i 
  

   not 
  at 
  all 
  like 
  the 
  fern 
  plant 
  

   that 
  we 
  ordinarily 
  see. 
  This 
  

   plant 
  is 
  one 
  layer 
  of 
  cells 
  in 
  

   thickness 
  at 
  the 
  margin, 
  but 
  

   along 
  the 
  midrib 
  a 
  cushion 
  

   of 
  several 
  layers 
  of 
  cells 
  is 
  

   formed. 
  From 
  the 
  under- 
  

   side 
  and 
  near 
  the 
  base 
  many 
  

   rhizoids 
  gr< 
  >w. 
  The 
  presence 
  

   of 
  chlorophyll 
  and 
  contact 
  

   with 
  moist 
  surfaces 
  aid 
  it 
  in 
  

   manufacturing 
  its 
  own 
  food. 
  

   Antheridia 
  may 
  lie 
  developed 
  almost 
  anywhere 
  upon 
  the 
  

   plant 
  before 
  it 
  lias 
  become 
  fully 
  heart-shaped, 
  and 
  thereafter 
  

   they 
  usually 
  develop 
  on 
  the 
  underside, 
  toward 
  the 
  basal 
  

   region 
  (tig. 
  212). 
  The 
  antheridinm 
  is 
  a 
  globular 
  structure 
  

   with 
  a 
  single 
  layer 
  of 
  wall 
  cells 
  (tig. 
  213) 
  and 
  a 
  central 
  

   region 
  in 
  which 
  usually 
  thirty-two 
  or 
  sixty-four 
  sperms 
  are 
  

   formed. 
  In 
  size 
  ami 
  number 
  of 
  cells 
  this 
  antheridium 
  is 
  much 
  

   simpler 
  than 
  that 
  seen 
  in 
  the 
  br\ 
  "ph\ 
  tes. 
  The 
  sperm 
  is, 
  how- 
  

   ex 
  IT, 
  quite 
  complex 
  and 
  seems 
  well 
  constructed 
  for 
  swimming. 
  

   On 
  the 
  underside 
  of 
  the 
  plant 
  and 
  nearer 
  the 
  tip 
  region 
  

   are 
  the 
  archegonia 
  (figs. 
  21 
  2 
  and 
  214). 
  ( 
  )nly 
  the 
  necks 
  extend 
  

   outward 
  from 
  the 
  surface, 
  and 
  these 
  usually 
  turn 
  

  

  FIG. 
  212. 
  Gametophyte 
  of 
  a 
  fern 
  

  

  A, 
  the 
  gametophyte 
  (magnified 
  about 
  ten 
  

  

  times) 
  ; 
  B, 
  younjj 
  sporophytc 
  ?ir<>\viiiLj 
  

  

  from 
  the 
  gametophyte 
  (mauniliiMl 
  about 
  

  

  tlnvr 
  times) 
  

  

  