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  INTRODUCTION 
  TO 
  BOTANY 
  

  

  these 
  threads 
  absorb 
  food 
  from 
  the 
  algae. 
  The 
  fungus 
  seems 
  

   to 
  hold 
  the 
  combination 
  in 
  compact 
  form 
  and 
  to 
  enable 
  it 
  to 
  

   secure 
  a 
  foothold 
  in 
  places 
  where 
  neither 
  the 
  alga 
  nor 
  the 
  

   fungus 
  could 
  live 
  alone. 
  In 
  the 
  lichen 
  we 
  have 
  therefore 
  a 
  

   combination 
  of 
  plants 
  --an 
  alga 
  and 
  a 
  fungus 
  --neither 
  of 
  

   which 
  alone 
  could 
  live 
  long 
  under 
  the 
  conditions 
  of 
  extreme 
  

   exposure 
  in 
  which 
  lichens 
  are 
  often 
  found. 
  

  

  237. 
  Economic 
  significance 
  of 
  lichens. 
  As 
  lichens 
  live 
  in 
  

   exposed 
  rocky 
  places, 
  they 
  serve 
  to 
  bring 
  about 
  the 
  first 
  stages 
  

   of 
  soil 
  formation 
  by 
  the 
  decay 
  of 
  old 
  lichen 
  plants 
  and 
  by 
  the 
  

   breaking 
  up 
  of 
  the 
  surface 
  layers 
  of 
  the 
  rock 
  or 
  other 
  material 
  

  

  FIG. 
  195. 
  A 
  small 
  piece 
  of 
  the 
  interior 
  of 
  a 
  lichen, 
  showing 
  the 
  relation 
  of 
  

   the 
  alga 
  (a) 
  and 
  the 
  fungus 
  (/) 
  

  

  Magnified 
  500 
  diameters. 
  After 
  Bonnier 
  

  

  upon 
  which 
  they 
  grow. 
  Weathering 
  processes 
  also 
  assist 
  in 
  

   crumbling 
  the 
  rock, 
  and 
  after 
  a 
  time 
  there 
  is 
  soil 
  enough 
  to 
  

   permit 
  the 
  growth 
  of 
  larger 
  plants. 
  Several 
  kinds 
  of 
  crusta- 
  

   ceous 
  lichens 
  are 
  usually 
  the 
  forerunners 
  of 
  other 
  vegetation 
  

   in 
  rocky 
  regions 
  which 
  will 
  not 
  permit 
  other 
  forms 
  of 
  vegeta- 
  

   tion 
  to 
  live. 
  The 
  time 
  required 
  to 
  produce 
  enough 
  soil 
  for 
  

   the 
  growth 
  of 
  other 
  plants 
  depends 
  largely 
  upon 
  the 
  nature 
  

   of 
  the 
  rock 
  and 
  the 
  climate. 
  It 
  is 
  said 
  that 
  on 
  some 
  lava 
  

   beds, 
  after 
  almost 
  two 
  hundred 
  years 
  from 
  their 
  formation, 
  

   crustaceous 
  lichens 
  are 
  still 
  in 
  some 
  places 
  the 
  only 
  plants 
  

   to 
  be 
  found. 
  Lichens 
  are 
  important 
  as 
  food 
  for 
  herbivorous 
  

   animals 
  in 
  regions 
  where 
  other 
  kinds 
  of 
  food 
  are 
  scarce 
  or 
  

   where 
  for 
  a 
  part 
  of 
  the 
  year 
  other 
  vegetation 
  is 
  not 
  available. 
  

  

  