﻿206 
  INTRODUCTION 
  TO 
  BOTANY 
  

  

  by 
  tissue, 
  so 
  that 
  they 
  form 
  nodules, 
  or 
  tubercles 
  (figs. 
  167 
  

   and 
  168). 
  Within 
  these 
  tubercles 
  the 
  bacteria 
  are 
  able 
  to 
  

   take 
  the 
  uncombined 
  nitrogen 
  from 
  the 
  air 
  of 
  the 
  soil 
  and 
  

   combine 
  it 
  with 
  other 
  substances 
  in 
  such 
  a 
  way 
  as 
  to 
  form 
  

   nitrates. 
  The 
  tubercle 
  bacteria 
  are 
  known 
  as 
  the 
  nitrogen- 
  

   fixing 
  bacteria, 
  since 
  they 
  take 
  free 
  nitrogen 
  from 
  the 
  air 
  and 
  

   combine 
  it 
  with 
  other 
  things 
  so 
  as 
  to 
  make 
  plant 
  foods. 
  

  

  Since 
  the 
  ordinary 
  grain-producing 
  plants 
  must 
  have 
  nitro- 
  

   gen 
  in 
  order 
  to 
  grow, 
  and 
  since 
  they 
  can 
  use 
  it 
  only 
  in 
  the 
  

   nitrate 
  form, 
  the 
  significance 
  of 
  these 
  tubercle 
  bacteria 
  to 
  

   agricultural 
  plants 
  is 
  evident. 
  Agriculturists 
  have 
  known 
  for 
  

  

  a 
  long 
  time 
  that, 
  after 
  growing 
  

   a 
  crop 
  of 
  clover 
  or 
  peas, 
  the 
  soil 
  

   is 
  in 
  better 
  condition 
  for 
  growing 
  

   other 
  crops, 
  but 
  the 
  way 
  in 
  which 
  

   this 
  is 
  brought 
  about, 
  and 
  the 
  

   relation 
  of 
  bacteria 
  to 
  the 
  process, 
  

   are 
  matters 
  of 
  recent 
  knowledge. 
  

  

  The 
  clover, 
  alfalfa, 
  peas, 
  and 
  beans 
  

   FIG. 
  168. 
  Bacteria, 
  or 
  bacteroids 
  , 
  i 
  ,1 
  -, 
  i 
  

  

  ,,\ 
  vi. 
  STOW 
  better 
  when 
  the 
  tubercle 
  

   (meaning 
  ' 
  f 
  like 
  bacteria") 
  which 
  

  

  grew 
  in 
  the 
  root 
  tubercles 
  1 
  bacteria 
  are 
  present 
  (fig. 
  Ib9), 
  

  

  and 
  the 
  added 
  nitrates 
  left 
  by 
  

  

  decay 
  of 
  the 
  tubercles 
  and 
  the 
  plants 
  upon 
  which 
  they 
  grew 
  

   enrich 
  the 
  soil 
  for 
  subsequent 
  crops. 
  

  

  There 
  are 
  many 
  other 
  kinds 
  of 
  soil 
  bacteria, 
  one 
  kind 
  liv- 
  

   ing 
  in 
  such 
  a 
  way 
  as 
  to 
  denitrify 
  nitrate 
  compounds, 
  thus 
  

   working 
  in 
  a 
  manner 
  opposite 
  to 
  some 
  of 
  the 
  above 
  groups. 
  

  

  192. 
  Bacteria 
  and 
  the 
  destruction 
  of 
  foods. 
  From 
  what 
  has 
  

   been 
  said 
  in 
  preceding 
  sections 
  it 
  is 
  evident 
  that 
  all 
  our 
  fruits, 
  

   vegetables, 
  meats, 
  etc. 
  are 
  at 
  times 
  in 
  danger 
  of 
  destruction 
  

   by 
  bacteria 
  or 
  other 
  organisms. 
  How 
  to 
  prevent 
  this 
  destruc- 
  

   tion 
  has 
  been 
  one 
  of 
  the 
  important 
  problems 
  since 
  civilization 
  

   began. 
  Surplus 
  production 
  of 
  food 
  is 
  useless 
  unless 
  some 
  of 
  

   it 
  may 
  be 
  kept 
  for 
  future 
  needs. 
  

  

  1 
  From 
  Lipman, 
  Bacteria 
  in 
  Relation 
  to 
  Country 
  Life. 
  The 
  Macmillan 
  

   Company. 
  

  

  