﻿THE 
  ALG.E 
  223 
  

  

  207. 
  Reproduction 
  of 
  Pleurococcus. 
  New 
  Pleurococcus 
  plants 
  

   are 
  formed 
  by 
  the 
  division 
  of 
  the 
  old 
  plants. 
  Thus, 
  the 
  grow- 
  

   ing, 
  or 
  vegetative, 
  plant 
  body 
  (one 
  cell) 
  divides, 
  and 
  each 
  

   division 
  is 
  a 
  new 
  plant. 
  Under 
  favorable 
  growing 
  conditions 
  

   reproduction 
  goes 
  on 
  rapidly. 
  The 
  divisions 
  follow 
  one 
  an- 
  

   other 
  in 
  such 
  a 
  way 
  that 
  whole 
  colonies, 
  the 
  descendants 
  of 
  one 
  

   individual, 
  are 
  often 
  grouped 
  together 
  (fig. 
  174). 
  Obviously 
  

   Pleurococcus 
  is 
  a 
  very 
  simple 
  plant 
  in 
  its 
  structure, 
  nutrition, 
  

   and 
  reproduction. 
  

  

  208. 
  Spirogyra. 
  One 
  of 
  the 
  most 
  abundant 
  of 
  the 
  common 
  

   floating 
  pond 
  scums 
  is 
  the 
  green 
  alga, 
  Spirogyra. 
  The 
  plant 
  

   is 
  sometimes 
  called 
  " 
  brook 
  silk 
  " 
  because 
  of 
  its 
  soft, 
  silken 
  

   texture, 
  by 
  means 
  of 
  which 
  it 
  may 
  generally 
  be 
  distinguished 
  

   from 
  other 
  fresh-water 
  algse. 
  It 
  is 
  usually 
  very 
  bright 
  green, 
  

   except 
  when 
  it 
  gathers 
  in 
  dense 
  masses 
  at 
  the 
  surface 
  of 
  the 
  

   water, 
  when 
  it 
  may 
  be 
  dark 
  yellowish 
  green. 
  Spirogyra 
  is 
  a 
  

   many-celled 
  plant, 
  with 
  cylindrical 
  cells 
  arranged 
  end 
  to 
  end, 
  

   and 
  all 
  held 
  in 
  a 
  common 
  sheathing 
  plant 
  wall. 
  Also, 
  each 
  

   cell 
  contains 
  one 
  or 
  more 
  peculiar 
  spirally 
  arranged 
  chloro- 
  

   plasts, 
  each 
  of 
  which 
  extends 
  almost 
  or 
  quite 
  the 
  entire 
  length 
  

   of 
  the 
  cell 
  (fig. 
  175). 
  There 
  are 
  different 
  numbers 
  of 
  chloro- 
  

   plasts 
  in 
  the 
  cells 
  in 
  different 
  species 
  of 
  Spirogyra. 
  A 
  layer 
  

   of 
  cytoplasm 
  usually 
  lies 
  just 
  within 
  the 
  cell 
  wall, 
  and 
  strands 
  

   of 
  cytoplasm 
  run 
  to 
  the 
  nucleus, 
  which 
  is 
  suspended 
  in 
  the 
  

   central 
  part 
  of 
  the 
  cell 
  (fig. 
  175). 
  

  

  209. 
  The 
  nutrition 
  and 
  growth 
  of 
  Spirogyra. 
  In 
  the 
  water 
  

   in 
  which 
  Spirogyra 
  lives 
  there 
  are 
  dissolved 
  the 
  carbon 
  dioxide 
  

   and 
  other 
  inorganic 
  materials 
  from 
  which 
  foods 
  are 
  made. 
  In- 
  

   deed, 
  there 
  is 
  much 
  water 
  within 
  the 
  plant 
  itself, 
  -as 
  may 
  be 
  

   demonstrated 
  by 
  careful 
  drying, 
  by 
  which 
  it 
  is 
  sometimes 
  

   found 
  that 
  as 
  much 
  as 
  98 
  per 
  cent 
  of 
  this 
  plant 
  is 
  water. 
  

   That 
  photosynthesis 
  is 
  carried 
  on 
  is 
  often 
  made 
  evident 
  by 
  

   the 
  oxygen 
  bubbles 
  which 
  arise 
  from 
  the 
  plants 
  that 
  are 
  

   active. 
  It 
  is 
  obvious 
  that 
  this 
  plant 
  can 
  expose 
  more 
  chloro- 
  

   phyll 
  to 
  the 
  light, 
  and 
  hence 
  can 
  do 
  more 
  photosynthetic 
  

   work, 
  than 
  can 
  Pleurococcus. 
  

  

  