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  INTRODUCTION 
  TO 
  BOTANY 
  

  

  307. 
  Air 
  and 
  temperature. 
  In 
  the 
  earlier 
  sections 
  of 
  this 
  

   book 
  the 
  air 
  relations 
  of 
  plants 
  have 
  been 
  presented 
  hi 
  the 
  

   various 
  discussions 
  of 
  photosynthesis 
  and 
  of 
  plants 
  of 
  differ- 
  

   ent 
  regions. 
  Under 
  most 
  circumstances 
  the 
  air 
  is 
  likely 
  to 
  

   contain 
  enough 
  of 
  the 
  proper 
  gases, 
  and 
  not 
  too 
  much 
  of 
  harm- 
  

   ful 
  ones, 
  to 
  enable 
  plants 
  to 
  thrive. 
  In 
  large 
  cities 
  and 
  hi 
  the 
  

   vicinity 
  of 
  manufacturing 
  establishments, 
  especially 
  smelters, 
  

   harmful 
  gases 
  are 
  present 
  in 
  quantities 
  that 
  often 
  kill 
  plant 
  

   life. 
  For 
  fifteen 
  miles 
  or 
  more 
  about 
  certain 
  copper 
  smelters 
  

   all 
  plant 
  life 
  has 
  been 
  killed. 
  In 
  commercial 
  greenhouses 
  and 
  

   in 
  homes 
  the 
  plants 
  may 
  exist 
  in 
  the 
  presence 
  of 
  illuminat- 
  

   ing 
  gas, 
  but 
  1 
  part 
  of 
  illuminating 
  gas 
  in 
  80,000 
  parts 
  of 
  air 
  

   will 
  prevent 
  carnations 
  from 
  flowering, 
  and 
  other 
  plants 
  are 
  

   prevented 
  from 
  flowering 
  and 
  often 
  killed 
  by 
  such 
  gases. 
  

  

  In 
  the 
  vicinity 
  of 
  cement 
  manufacturing 
  plants 
  the 
  cement 
  

   gets 
  into 
  the 
  stomata 
  of 
  the 
  leaves, 
  becomes 
  hard 
  and 
  closes 
  

   the 
  stomata, 
  and 
  eventually 
  results 
  in 
  killing 
  all 
  plant 
  life. 
  

  

  308. 
  Prevention 
  of 
  plant 
  diseases. 
  The 
  development 
  of 
  plant- 
  

   industries 
  has 
  tremendously 
  stimulated 
  the 
  study 
  of 
  plant 
  dis- 
  

   eases. 
  Many 
  volumes 
  have 
  been 
  written 
  upon 
  the 
  topic, 
  and 
  

   every 
  agricultural 
  experiment 
  station 
  issues 
  bulletins 
  from 
  

   time 
  to 
  time, 
  to 
  advise 
  people 
  about 
  the 
  latest 
  discoveries 
  in 
  

   regard 
  to 
  the 
  nature 
  of 
  important 
  diseases 
  and 
  the 
  means 
  of 
  

   preventing 
  them. 
  The 
  ways 
  in 
  which 
  these 
  diseases 
  operate 
  

   are 
  various. 
  They 
  may 
  use 
  the 
  food 
  material 
  made 
  by 
  the 
  

   host 
  plant, 
  as 
  is 
  supposed 
  to 
  be 
  true 
  in 
  case 
  of 
  wheat 
  and 
  oat 
  

   rusts; 
  they 
  may 
  also 
  consume 
  or 
  supplant 
  important 
  tissues, 
  

   as 
  when 
  corn 
  and 
  oat 
  smut 
  occupy 
  the 
  grains 
  of 
  their 
  hosts; 
  

   they 
  may 
  stop 
  the 
  vascular 
  tissues, 
  as 
  is 
  the 
  case 
  with 
  the 
  

   bacteria 
  that 
  produce 
  the 
  brown 
  wilt 
  of 
  cabbage 
  and 
  other 
  

   related 
  plants; 
  or 
  they 
  may 
  excrete 
  substances 
  that 
  are 
  

   poisonous 
  to 
  the 
  host 
  plants. 
  

  

  Diseases 
  due 
  to 
  animals 
  such 
  as 
  plant 
  lice 
  or 
  aphids, 
  scale 
  

   insects, 
  and 
  larger 
  insects 
  are 
  extremely 
  destructive. 
  The 
  

   intricate 
  nature 
  of 
  such 
  diseases 
  may 
  be 
  shown 
  by 
  using 
  as 
  

   an 
  illustration 
  the 
  aphids 
  that 
  often 
  produce 
  serious 
  injury 
  

  

  