﻿FOODS 
  IN 
  PLANTS 
  41 
  

  

  of 
  a 
  weak 
  solution 
  of 
  sugar 
  in 
  water. 
  On 
  its 
  arrival 
  at 
  the 
  

   storage 
  region 
  (for 
  example, 
  the 
  tuber 
  of 
  the 
  potato 
  plant) 
  

   the 
  dissolved 
  sugar 
  is 
  reconverted 
  into 
  starch 
  by 
  the 
  action 
  

   of 
  minute, 
  colorless 
  corpuscles 
  of 
  protoplasm, 
  known 
  as 
  leuco- 
  

   plasts. 
  The 
  starch 
  grains 
  deposited 
  for 
  storage 
  (fig. 
  28) 
  are 
  

   many 
  times 
  larger 
  and 
  show 
  a 
  far 
  more 
  definite 
  structure 
  

   than 
  those 
  formed 
  in 
  the 
  chloroplasts 
  during 
  photosynthesis. 
  

   40. 
  Diffusion 
  and 
  osmosis. 
  It 
  is 
  clear 
  that 
  plant 
  food 
  must 
  

   pass 
  from 
  within 
  certain 
  cells 
  into 
  others, 
  and 
  the 
  way 
  in 
  which 
  

   this 
  occurs 
  may 
  be 
  made 
  clearer 
  by 
  the 
  use 
  of 
  simple 
  illustra- 
  

   tions. 
  If 
  some 
  dried 
  raisins 
  or 
  prunes 
  are 
  placed 
  in 
  pure 
  water 
  

   it 
  will 
  soon 
  be 
  noted 
  that 
  the 
  outside 
  membranes, 
  which 
  at 
  

   first 
  were 
  contracted 
  and 
  wrinkled, 
  have 
  become 
  distended. 
  

   Water 
  has 
  passed 
  through 
  the 
  fruit 
  coating 
  and 
  is 
  retained 
  

   within 
  it. 
  After 
  a 
  longer 
  time 
  it 
  will 
  be 
  found 
  that 
  the 
  water 
  

   outside 
  the 
  fruit 
  has 
  become 
  sweet, 
  owing 
  to 
  the 
  outward 
  pas- 
  

   sage 
  of 
  dissolved 
  sugar 
  from 
  within 
  the 
  fruit. 
  If 
  a 
  little 
  molasses 
  

   is 
  poured 
  into 
  a 
  straight-sided 
  jar, 
  and 
  a 
  disk 
  of 
  porous 
  paper 
  

   is 
  placed 
  so 
  as 
  to 
  cover 
  the 
  molasses 
  (to 
  prevent 
  instantaneous 
  

   mixing), 
  and 
  water 
  is 
  then 
  carefully 
  poured 
  upon 
  the 
  disk 
  of 
  

   paper, 
  the 
  water 
  will 
  for 
  a 
  considerable 
  time 
  appear 
  clear 
  and 
  

   colorless. 
  Only 
  after 
  some 
  hours 
  will 
  the 
  molasses 
  rise 
  and 
  

   mingle 
  much 
  with 
  the 
  water, 
  or 
  the 
  latter 
  perceptibly 
  thin 
  the 
  

   molasses. 
  This 
  process, 
  by 
  which 
  two 
  liquids 
  in 
  contact 
  become 
  

   mixed 
  by 
  the 
  interchange 
  of 
  inconceivably 
  minute 
  portions 
  

   (molecules) 
  of 
  both 
  liquids, 
  is 
  called 
  diffusion. 
  The 
  tendency 
  

   is 
  for 
  the 
  two 
  liquids 
  to 
  become 
  completely 
  intermingled, 
  so 
  

   that 
  at 
  last 
  all 
  portions 
  of 
  the 
  mixture 
  are 
  of 
  precisely 
  the 
  

   same 
  composition. 
  Similarly, 
  if 
  a 
  dense 
  liquid 
  surrounds 
  a 
  

   plant 
  cell, 
  water 
  passes 
  more 
  rapidly 
  outward 
  than 
  inward, 
  and 
  

   the 
  remaining 
  internal 
  parts 
  of 
  the 
  cell 
  collapse, 
  or 
  plasmolyze 
  

   (fig. 
  29), 
  because 
  of 
  the 
  loss 
  of 
  water. 
  This 
  merely 
  illustrates 
  

   the 
  fact 
  that 
  interchange 
  of 
  liquids 
  may 
  take 
  place 
  in 
  either 
  

   direction 
  through 
  a 
  membrane, 
  but 
  is 
  more 
  rapid 
  from 
  the 
  

   less 
  dense 
  to 
  the 
  more 
  dense 
  liquid. 
  The 
  mingling 
  of 
  liquids 
  

   that 
  are 
  separated 
  by 
  a 
  partition 
  which 
  one 
  or 
  both 
  of 
  them 
  

  

  