﻿WEEDS 
  

  

  335 
  

  

  312. 
  How 
  weeds 
  injure 
  the 
  farm 
  and 
  garden. 
  1 
  Although 
  

   some 
  weeds 
  are 
  of 
  use 
  as 
  food 
  for 
  man 
  or 
  the 
  lower 
  animals, 
  

   aud 
  a 
  few 
  have 
  medicinal 
  properties, 
  their 
  presence 
  in 
  the 
  farm 
  

   or 
  garden 
  is 
  on 
  the 
  whole 
  most 
  harmful 
  in 
  the 
  following 
  ways 
  : 
  

  

  1. 
  Weeds 
  take 
  

   moisture 
  needed 
  

   by 
  useful 
  plants. 
  

  

  2. 
  Weeds 
  rob 
  

   the 
  soil 
  of 
  val- 
  

   uable 
  salts, 
  such 
  

   as 
  nitrates 
  and 
  

   potash 
  compounds, 
  

   and 
  it 
  is 
  probable 
  

   chat 
  they 
  may 
  add 
  

   secretions 
  that 
  are 
  

   injurious. 
  

  

  3. 
  Weeds 
  shade 
  

   other 
  plants, 
  thus 
  

   weakening 
  them 
  

   by 
  hindering 
  pho- 
  

   tosynthesis. 
  

  

  4. 
  A 
  few 
  para- 
  

   sitic 
  weeds, 
  like 
  

   the 
  clover 
  dodder 
  

   (fig. 
  243), 
  rob 
  

   their 
  hosts 
  of 
  

   plant 
  food. 
  

  

  5. 
  Some 
  weeds 
  

   harbor 
  parasitic 
  

   fungi 
  or 
  insects 
  

   (such 
  as 
  the 
  potato 
  

   beetle) 
  which 
  are 
  

  

  injurious 
  to 
  use- 
  

   ful 
  plants. 
  

  

  FIG. 
  242. 
  The 
  common 
  ragweed 
  (Ambrosia 
  

   artemisiaefolid) 
  

  

  The 
  group 
  of 
  flower 
  clusters 
  at 
  the 
  left 
  and 
  the 
  leaf 
  at 
  

   the 
  right 
  are 
  considerably 
  reduced, 
  and 
  the 
  central 
  plant 
  

   is 
  much 
  reduced. 
  The 
  slender 
  stem 
  is 
  characteristic 
  of 
  

   individuals 
  grown 
  in 
  the 
  shade; 
  plants 
  grown 
  in 
  the 
  

   sunlight 
  are 
  much 
  shorter 
  and 
  very 
  rohust 
  

  

  1 
  See 
  also 
  Bergen 
  and 
  Caldwell, 
  Practical 
  Botany. 
  Ginn 
  and 
  Company, 
  

   Boston. 
  

  

  