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  INTRODUCTION 
  TO 
  BOTANY 
  

  

  People 
  now 
  know 
  that 
  it 
  is 
  of 
  the 
  greatest 
  importance 
  to 
  

   till 
  the 
  soil 
  during 
  droughts, 
  in 
  order 
  that 
  it 
  may 
  not 
  lose 
  

   its 
  moisture. 
  An 
  illustration 
  will 
  help 
  in 
  understanding 
  this. 
  

   If 
  two 
  pieces 
  of 
  loaf 
  sugar 
  are 
  jtlaeed 
  one 
  upon 
  the 
  other, 
  the 
  

   lower 
  one 
  held 
  in 
  the 
  thuml) 
  and 
  linger 
  and 
  the 
  other 
  left 
  

   lying 
  loosely 
  upon 
  the 
  first 
  and 
  not 
  touching 
  the 
  lingers 
  at 
  

   all, 
  and 
  if 
  the 
  lower 
  one 
  is 
  then 
  placed 
  in 
  contact 
  with 
  water, 
  

   two 
  important 
  facts 
  are 
  shown: 
  The 
  lower 
  piece 
  takes 
  up 
  

   water 
  freely, 
  but 
  the 
  upper 
  one, 
  though 
  lying 
  upon 
  the 
  lower 
  

   wet 
  piece, 
  becomes 
  wet 
  only 
  after 
  a 
  long 
  time. 
  Close 
  con- 
  

   nection 
  between 
  the 
  solid 
  particles 
  is 
  necessary 
  for 
  the 
  rapid 
  

   upward 
  passage 
  of 
  the 
  water. 
  

  

  When 
  soils 
  are 
  compact, 
  moisture 
  from 
  the 
  deeper 
  portions 
  

   passes 
  upward 
  freely, 
  as 
  in 
  the 
  lower 
  lump 
  of 
  sugar, 
  and 
  

   evaporates 
  into 
  the 
  air. 
  If, 
  however, 
  the 
  surface 
  is 
  kept 
  loose 
  

   and 
  finely 
  pulverized, 
  so 
  that 
  the 
  particles 
  are 
  less 
  closely 
  con- 
  

   nected, 
  moisture 
  does 
  not 
  readily 
  pass 
  through 
  it, 
  and 
  there 
  i> 
  

   not 
  so 
  much 
  loss 
  from 
  evaporat 
  ion. 
  The 
  roots 
  of 
  plants, 
  being 
  

   more 
  deeply 
  placed, 
  are 
  in 
  contact 
  with 
  the 
  moist 
  soil 
  from 
  

   which 
  a 
  supply 
  of 
  water 
  may 
  be 
  secured. 
  The 
  depth 
  to 
  which 
  

   roots 
  are 
  known 
  to 
  go 
  in 
  regions 
  where 
  the 
  water 
  is 
  found 
  

   only 
  at 
  great 
  depth 
  is 
  discussed 
  in 
  section 
  22. 
  

  

  It 
  has 
  been 
  generally 
  supposed 
  that 
  tilling 
  the 
  soil 
  serves 
  

   the 
  twofold 
  purpose 
  of 
  regulating 
  the 
  moisture 
  supply 
  for 
  

   growing 
  plants 
  and 
  of 
  preventing 
  the 
  growth 
  of 
  weeds. 
  K\- 
  

   pcriments 
  upon 
  the 
  cultivation 
  of 
  corn 
  seem 
  to 
  show, 
  however, 
  

   that 
  if 
  all 
  the 
  weeds 
  are 
  removed 
  without 
  disturbing 
  the 
  soil, 
  

   the 
  yield 
  is 
  practically 
  the 
  same 
  as 
  when 
  the 
  soil 
  is 
  tilled. 
  1 
  

  

  303. 
  Dry 
  farming 
  and 
  irrigation. 
  In 
  regions 
  with 
  a 
  very 
  

   scanty 
  rainfall 
  frequent 
  tillage 
  of 
  the 
  surface 
  seems 
  to 
  enable 
  

   the 
  soil 
  to 
  hold 
  most 
  of 
  the 
  water 
  that 
  falls, 
  and 
  in 
  this 
  way 
  a 
  

   crop 
  may 
  be 
  grown, 
  somet 
  imes 
  every 
  season 
  and 
  sometimes 
  every 
  

   second 
  season. 
  Evidently 
  much 
  work 
  is 
  necessary 
  to 
  enable 
  

   the 
  soil 
  to 
  accumulate 
  enough 
  moisture 
  to 
  supply 
  the 
  needs 
  of 
  

  

  1 
  "The 
  Weed 
  Factor 
  in 
  the 
  Cultivation 
  of 
  Corn," 
  Bulletin 
  257, 
  Bureau 
  

   of 
  Plant 
  Industry, 
  U.S. 
  Dept. 
  Agr., 
  1912. 
  

  

  