﻿FUNGI 
  AND 
  FUNGOUS 
  DISEASES 
  OF 
  PLANTS 
  249 
  

  

  236. 
  The 
  structure 
  of 
  a 
  lichen. 
  A 
  lichen 
  is 
  not 
  a 
  single 
  

   plant, 
  but 
  a 
  combination 
  of 
  fungi 
  and 
  algae 
  living 
  together 
  

   in 
  such 
  a 
  close 
  relationship 
  that 
  it 
  looks 
  like 
  a 
  single 
  plant. 
  

   There 
  may 
  be 
  many 
  individual 
  fungi 
  and 
  many 
  individual 
  

   algre 
  in 
  this 
  relation, 
  but 
  the 
  combination 
  is 
  spoken 
  of 
  as 
  the 
  

   lichen 
  plant. 
  The 
  fungal 
  part 
  of 
  the 
  lichen 
  is 
  usually, 
  though 
  

   not 
  always, 
  a 
  member 
  of 
  the 
  sac-bearing 
  class 
  of 
  fungi, 
  and 
  

  

  FIG. 
  194. 
  The 
  "bearded 
  moss" 
  lichen 
  (Usnea 
  barbata) 
  growing 
  upon 
  the 
  

  

  branches 
  of 
  a 
  spruce 
  tree 
  

  

  consequently 
  lichens 
  are 
  often 
  classified 
  with 
  sac 
  fungi. 
  This 
  

   is 
  obviously 
  a 
  somewhat 
  questionable 
  classification, 
  but 
  for 
  

   lack 
  of 
  a 
  better 
  one 
  we 
  shall 
  use 
  it. 
  The 
  algae 
  that 
  enter 
  into 
  

   the 
  formation 
  of 
  lichens 
  are 
  usually 
  unicellular 
  forms 
  resem- 
  

   bling 
  Pleurococcus. 
  When 
  the 
  lichen 
  is 
  dissected, 
  the 
  green 
  

   cells 
  of 
  the 
  algae 
  and 
  the 
  white 
  threads 
  of 
  the 
  fungi 
  may 
  be 
  

   seen 
  (fig. 
  195). 
  The 
  chlorophyll 
  of 
  the 
  algae 
  enables 
  the 
  

   lichen 
  to 
  manufacture 
  carbohydrate 
  foods. 
  Individual 
  alga 
  

   cells 
  are 
  often 
  closely 
  wound 
  about 
  by 
  threads 
  of 
  the 
  fungus 
  ; 
  

  

  