﻿THE 
  ALG.E 
  225 
  

  

  from 
  one 
  cell 
  to 
  the 
  other. 
  Then 
  the 
  protoplasm 
  from 
  one 
  

   cell 
  passes 
  through 
  this 
  tube 
  to 
  the 
  other 
  cell, 
  and 
  the 
  two 
  

   masses 
  of 
  protoplasm 
  unite. 
  The 
  nuclei 
  unite 
  and 
  one 
  cell 
  is 
  

   formed 
  from 
  what 
  were 
  two. 
  About 
  this 
  new 
  cell 
  a 
  heavy 
  

   wall 
  is 
  formed, 
  and 
  this 
  specially 
  formed 
  structure 
  is 
  a 
  spore 
  ; 
  

   that 
  is, 
  a 
  specially 
  formed 
  cell 
  which 
  can 
  reproduce 
  the 
  kind 
  

   of 
  plant 
  that 
  formed 
  it. 
  It 
  must 
  be 
  noted 
  also 
  that 
  this 
  spore, 
  

   being 
  formed 
  by 
  cell 
  union 
  and 
  not 
  by 
  cell 
  division, 
  is 
  sexual 
  ; 
  

   also, 
  sexual 
  cells 
  that 
  are 
  formed 
  by 
  union 
  of 
  similar 
  cells 
  are 
  

   called 
  zygospores, 
  that 
  is, 
  yoked 
  spores. 
  It 
  is 
  customary 
  to 
  

   speak 
  of 
  the 
  cells 
  that 
  unite 
  to 
  form 
  spores 
  as 
  gametes, 
  and 
  in 
  

   the 
  case 
  of 
  the 
  plant 
  Spirogyra 
  the 
  gametes 
  are 
  similar, 
  so 
  

   there 
  need 
  be 
  no 
  special 
  name 
  here 
  for 
  the 
  two 
  gametes, 
  to 
  dis- 
  

   tinguish 
  them 
  one 
  from 
  another. 
  In 
  asexual 
  reproduction, 
  as 
  

   will 
  be 
  seen 
  in 
  other 
  plants, 
  reproduction 
  is 
  carried 
  on 
  by 
  means 
  

   of 
  spores 
  that 
  are 
  formed 
  by 
  cell 
  division, 
  not 
  by 
  cell 
  union. 
  

  

  In 
  two 
  Spirogyra 
  plants 
  there 
  may 
  be 
  many 
  cells 
  uniting, 
  

   or 
  conjugating, 
  at 
  the 
  same 
  time. 
  These 
  pairs 
  are 
  usually 
  in 
  

   about 
  the 
  same 
  stage 
  of 
  spore 
  formation. 
  Occasionally 
  cells 
  

   from 
  one 
  plant 
  will 
  unite 
  with 
  those 
  from 
  more 
  than 
  one 
  other 
  

   plant. 
  Also, 
  occasionally 
  one 
  cell 
  may 
  unite 
  with 
  the 
  adjoin- 
  

   ing 
  one 
  in 
  the 
  same 
  plant. 
  When 
  ripe, 
  the 
  zygospores 
  are 
  set 
  

   free 
  by 
  the 
  decay 
  of 
  the 
  old 
  walls 
  and 
  may 
  fall 
  to 
  the 
  bottom 
  

   of 
  the 
  pond 
  or 
  stream. 
  After 
  a 
  period 
  of 
  rest, 
  sometimes 
  after 
  

   a 
  drought 
  or 
  in 
  the 
  spring, 
  these 
  spores 
  germinate 
  and 
  pro- 
  

   duce 
  new 
  Spirogyra 
  plants. 
  It 
  is 
  obviously 
  an 
  advantage 
  to 
  

   the 
  plant 
  to 
  have 
  a 
  heavy-walled 
  spore 
  to 
  carry 
  it 
  through 
  

   unfavorable 
  periods. 
  

  

  211. 
  A 
  branching 
  alga: 
  Cladophora. 
  This 
  is 
  a 
  green 
  alga 
  

   of 
  very 
  wide 
  distribution. 
  It 
  usually 
  grows 
  attached 
  to 
  objects 
  

   along 
  shoals 
  in 
  streams, 
  over 
  dams, 
  and 
  about 
  waterfalls. 
  

   Sometimes 
  it 
  appears 
  in 
  heavy 
  floating 
  mats 
  along 
  margins 
  of 
  

   ponds, 
  lakes, 
  and 
  even 
  oceans. 
  It 
  is 
  one 
  of 
  the 
  few 
  green 
  

   algse 
  ever 
  found 
  in 
  salt 
  water. 
  Cladophora 
  is 
  extensively 
  

   branched 
  (fig. 
  176), 
  and 
  since 
  its 
  branched 
  filaments 
  are 
  

   abundantly 
  supplied 
  with 
  chlorophyll, 
  it. 
  is 
  clear 
  that 
  this 
  

  

  