﻿CHAPTER 
  XV 
  

   THE 
  

  

  203. 
  Introductory. 
  Since 
  they 
  usually 
  grow 
  in 
  water, 
  the 
  

   alg03 
  are 
  sometimes 
  spoken 
  of 
  as 
  pond 
  scums, 
  water 
  mosses, 
  

   sea 
  mosses, 
  and 
  seaweeds. 
  Both 
  fresh 
  and 
  salt 
  waters 
  serve 
  

   as 
  growing 
  places 
  for 
  the 
  algse, 
  and 
  they 
  often 
  appear 
  in 
  such 
  

   abundance 
  that 
  great 
  mats 
  of 
  plants 
  are 
  formed 
  upon 
  or 
  below 
  

   the 
  surface 
  of 
  the 
  water. 
  There 
  are 
  many 
  different 
  kinds 
  of 
  

   algre, 
  and 
  they 
  are 
  divided 
  into 
  four 
  groups, 
  the 
  chief 
  distin- 
  

   guishing 
  character 
  being 
  their 
  color. 
  The 
  most 
  common 
  algse 
  

   have 
  a 
  distinctly 
  green 
  color 
  and 
  are 
  known 
  as 
  the 
  green 
  algce. 
  

   Others, 
  also 
  quite 
  common, 
  have 
  their 
  chlorophyll 
  mixed 
  with 
  

   a 
  blue 
  coloring 
  matter, 
  so 
  that 
  they 
  are 
  bluish 
  green, 
  and 
  these 
  

   are 
  known 
  as 
  the 
  blue-green 
  algce. 
  These 
  two 
  groups 
  of 
  algse 
  

   are 
  found 
  chiefly 
  in 
  fresh 
  waters. 
  There 
  are 
  two 
  other 
  groups 
  

   found 
  usually 
  in 
  salt 
  waters. 
  In 
  one 
  of 
  these, 
  the 
  brown 
  algce, 
  

   there 
  is 
  a 
  brown 
  coloring 
  matter 
  mixed 
  with 
  the 
  green, 
  and 
  

   in 
  the 
  other, 
  the 
  red 
  algce, 
  the 
  green 
  is 
  obscured 
  by 
  red, 
  which 
  

   often 
  becomes 
  most 
  striking 
  in 
  its 
  hues. 
  . 
  

  

  204. 
  The 
  blue-green 
  algae. 
  1 
  Masses 
  of 
  these 
  blue-green 
  

   plants 
  may 
  often 
  be 
  seen 
  in 
  pools 
  of 
  stagnant 
  water 
  or 
  upon 
  

   wet 
  soil. 
  Sometimes 
  they 
  appear 
  as 
  slimy 
  coatings 
  upon 
  

   sticks, 
  stones, 
  and 
  poorly 
  cleaned 
  watering 
  troughs 
  for 
  farm 
  

   animals, 
  and 
  they 
  may 
  also 
  appear 
  as 
  free-floating, 
  dirty 
  mats 
  

   or 
  balls. 
  The 
  blue-green 
  algae 
  are 
  very 
  widely 
  distributed, 
  

   and 
  there 
  are 
  many 
  kinds, 
  of 
  which 
  Nostoc 
  and 
  Oscillatoria 
  

   are 
  quite 
  common 
  and 
  may 
  serve 
  to 
  illustrate 
  the 
  group. 
  

  

  1 
  In 
  structural 
  details 
  blue-green 
  algse 
  are 
  probably 
  more 
  closely 
  related 
  

   to 
  bacteria 
  than 
  to 
  other 
  algse, 
  but 
  because 
  of 
  their 
  general 
  appearance 
  and 
  

   habits 
  of 
  living 
  they 
  are 
  mentioned 
  briefly 
  with 
  the 
  algae. 
  

  

  219 
  

  

  