﻿FOODS 
  IN 
  PLANTS 
  43 
  

  

  and 
  the 
  supply 
  of 
  materials 
  for 
  growth 
  is 
  maintained 
  by 
  the 
  

   addition 
  of 
  new 
  portions 
  of 
  sugar 
  coming 
  from 
  the 
  tuber. 
  1 
  

  

  42. 
  Channels 
  through 
  which 
  food 
  is 
  transported. 
  Many 
  

   kinds 
  of 
  living 
  tissue 
  serve 
  as 
  channels 
  for 
  the 
  conveyance 
  

   of 
  food 
  from 
  one 
  part 
  of 
  the 
  plant 
  body 
  to 
  another. 
  The 
  

   main 
  route 
  for 
  the 
  transportation 
  of 
  food 
  in 
  flowering 
  plants 
  

   is 
  through 
  special 
  tubular 
  cells 
  which 
  form 
  the 
  sieve 
  tubes, 
  so 
  

   called 
  from 
  the 
  perforated 
  plates 
  found 
  at 
  the 
  ends 
  or 
  along 
  

   the 
  sides 
  of 
  the 
  nearly 
  cylindrical 
  cells 
  of 
  which 
  the 
  tubes 
  

   are 
  built 
  up. 
  In 
  dicotyledons 
  these 
  sieve 
  tubes 
  occupy 
  a 
  

   region 
  of 
  the 
  stem 
  immediately 
  outside 
  of 
  the 
  cambium, 
  as 
  

   shown 
  at 
  o 
  in 
  figure 
  42, 
  A 
  and 
  at 
  si 
  in 
  figure 
  43. 
  The 
  fact 
  that 
  

   most 
  of 
  the 
  plant 
  food 
  prepared 
  in 
  the 
  leaves 
  is 
  carried 
  down 
  

   through 
  the 
  sieve 
  layer 
  of 
  the 
  bark 
  is 
  well 
  shown 
  by 
  the 
  be- 
  

   havior 
  of 
  a 
  willow 
  cutting 
  from 
  which 
  a 
  ring 
  of 
  bark 
  has 
  been 
  

   removed. 
  If 
  the 
  cutting 
  is 
  stood 
  with 
  its 
  lower 
  end 
  in 
  water 
  

   but 
  with 
  the 
  girdled 
  part 
  out 
  of 
  water, 
  enough 
  constructive 
  

   material 
  will 
  pass 
  down 
  through 
  the 
  sieve 
  layer 
  to 
  send 
  out 
  

   roots 
  from 
  the 
  upper 
  edge 
  of 
  the 
  ring, 
  but 
  few 
  or 
  none 
  will 
  

   appear 
  at 
  its 
  lower 
  edge. 
  In 
  the 
  meantime 
  water 
  is 
  freely 
  

   carried 
  upward 
  through 
  the 
  sapwood. 
  In 
  early 
  times 
  the 
  

   process 
  of 
  clearing 
  woodlands 
  for 
  farming 
  purposes 
  was 
  

   made 
  less 
  laborious 
  by 
  girdling 
  the 
  trees, 
  which 
  soon 
  died 
  

   and 
  at 
  length 
  fell 
  and 
  were 
  burned. 
  Would 
  the 
  girdling 
  

   process 
  be 
  more 
  effective 
  if 
  a 
  good 
  deal 
  of 
  the 
  sapwood, 
  as 
  

   well 
  as 
  the 
  bark, 
  were 
  removed 
  from 
  the 
  ring 
  ? 
  In 
  woody 
  

   dicotyledonous 
  stems 
  there 
  are 
  radiating 
  lines 
  of 
  cells 
  (med- 
  

   ullary 
  rays) 
  running 
  outward 
  from 
  the 
  center 
  toward 
  the 
  cir- 
  

   cumference 
  of 
  the 
  stem. 
  Food 
  is 
  stored 
  in 
  these 
  rays, 
  and 
  

   they 
  are 
  also 
  lines 
  of 
  conduction 
  of 
  foods. 
  

  

  43. 
  Food 
  storage. 
  In 
  the 
  trunks 
  of 
  trees 
  stored 
  food 
  may 
  

   be 
  present 
  in 
  various 
  forms, 
  as 
  starch, 
  sugar, 
  oil, 
  and 
  proteins. 
  

   In 
  the 
  autumn 
  many 
  kinds 
  of 
  sapwood 
  turn 
  deep 
  blue 
  or 
  black 
  

  

  1 
  It 
  is 
  not 
  possible 
  here 
  to 
  go 
  into 
  details 
  concerning 
  the 
  transportation 
  

   of 
  other 
  kinds 
  of 
  plant 
  food 
  than 
  starch 
  and 
  the 
  sugars. 
  That 
  of 
  proteins 
  is 
  

   especially 
  difficult 
  to 
  trace. 
  

  

  