﻿Miscellaneous. 
  65 
  

  

  normally 
  during 
  the 
  momentary 
  pause, 
  when, 
  owing 
  to 
  the 
  contrac- 
  

   tion 
  of 
  the 
  vacuole, 
  their 
  liquid 
  contents 
  have 
  time 
  to 
  accumulate 
  

   and 
  distend 
  them. 
  

  

  l$ow 
  if 
  a 
  specimen 
  of 
  sufficient 
  size, 
  say 
  a 
  Paramoeeium, 
  be 
  placed 
  

   under 
  certain 
  unfavourable 
  conditions 
  (among 
  which 
  is 
  deficiency 
  of 
  

   oxygen), 
  we 
  find 
  that 
  the 
  contractile 
  vacuoles 
  contract 
  less 
  perfectly 
  

   and 
  at 
  longer 
  intervals, 
  the 
  plasmatic 
  canals 
  become 
  persistently 
  

   visible 
  and 
  enlarged, 
  the 
  whole 
  animal 
  becomes 
  dropsical, 
  its 
  thin 
  

   cuticle 
  bursts, 
  and 
  its 
  protoplasm, 
  no 
  longer 
  protected 
  by 
  the 
  

   * 
  Hautschicht 
  ' 
  and 
  cuticle, 
  and 
  exposed 
  directly 
  to 
  the 
  water, 
  disin- 
  

   tegrates 
  into 
  " 
  diffluence." 
  

  

  Again, 
  if 
  a 
  vegetable 
  cell 
  bo 
  wounded 
  in 
  water 
  and 
  its 
  protoplasm 
  

   passes 
  out 
  it 
  rounds 
  off 
  and 
  surrounds 
  itself 
  with 
  a 
  Hautscbicht 
  ; 
  

   tben 
  vacuoles 
  appear 
  inside 
  ; 
  these 
  enlarge 
  enormously 
  and 
  finally 
  

   burst; 
  the 
  protoplasm 
  so 
  exposed 
  without 
  a 
  Hautschicbt 
  to 
  the 
  

   water 
  disintegrates 
  into 
  diffiuence, 
  just 
  like 
  the 
  Cdiate 
  whose 
  con- 
  

   tractile 
  vacuole 
  works 
  inefficiently. 
  This 
  pathological 
  vacuolation 
  

   and 
  diffluence 
  do 
  not 
  occur 
  if 
  to 
  the 
  water 
  there 
  be 
  added 
  a 
  suffi- 
  

   ciency 
  of 
  sugar, 
  saltpetre, 
  glycerine, 
  or 
  other 
  innocuous 
  osmotic 
  

   substance 
  ; 
  and 
  when 
  the 
  protoplasm 
  has 
  the 
  power 
  of 
  excreting 
  a 
  

   complete 
  cellulose 
  wall, 
  as 
  in 
  Vaucheria, 
  the 
  incipient 
  vacuolation 
  

   is 
  arrested 
  on 
  the 
  formation 
  of 
  the 
  wall. 
  

  

  The 
  explanation 
  of 
  both 
  series 
  of 
  facts 
  is 
  the 
  same, 
  as 
  worked 
  out 
  

   by 
  a 
  number 
  of 
  botanists 
  from 
  Von 
  Mold 
  to 
  De 
  Vries 
  : 
  — 
  

  

  Protoplasm 
  contains 
  in 
  its 
  interstices 
  substances 
  of 
  bigh 
  osmotic 
  

   value 
  ; 
  its 
  outer 
  layer 
  at 
  least, 
  wbile 
  freely 
  pervious 
  to 
  water, 
  is 
  

   slightly 
  if 
  at 
  all 
  pervious 
  to 
  these 
  substances 
  even 
  in 
  solution. 
  

   Hence, 
  when 
  protoplasm 
  is 
  immersed 
  in 
  water, 
  cavities 
  or 
  vacuoles 
  

   form 
  in 
  its 
  substance 
  containing 
  solutions 
  of 
  these 
  substances, 
  which 
  

   continue 
  to 
  enlarge 
  by 
  attraction 
  of 
  water 
  from 
  without 
  ; 
  the 
  enlarge- 
  

   ment 
  produces 
  a 
  tension 
  which 
  De 
  Vries 
  and 
  others 
  have 
  by 
  various 
  

   methods 
  determined 
  to 
  be 
  at 
  least 
  three 
  atmospheres, 
  and 
  wbich 
  may 
  

   reach 
  fifteen 
  atmospheres. 
  Now, 
  naked 
  protoplasm 
  has 
  very 
  little 
  

   toughness 
  ; 
  it 
  yields 
  readily 
  to 
  the 
  increasing 
  tension 
  and 
  to 
  the 
  

   expansion 
  of 
  the 
  vacuole, 
  and 
  finally 
  bursts 
  and 
  disintegrates. 
  On 
  

   the 
  other 
  hand, 
  cellulose 
  and 
  chitinous 
  cyst-walls 
  are 
  sufficiently 
  

   tough 
  to 
  resist; 
  and 
  equilibrium 
  is 
  attained 
  when, 
  after 
  a 
  certain 
  

   amount 
  of 
  stretching, 
  the 
  elasticity 
  of 
  the 
  wall 
  balances 
  tbe 
  tension 
  

   of 
  tbe 
  vacuoles 
  due 
  to 
  osmosis. 
  

  

  If, 
  however, 
  in 
  the 
  absence 
  of 
  a 
  tough 
  wall 
  the 
  vacuole, 
  instead 
  

   of 
  extending 
  indefinitely 
  and 
  bursting 
  irregularly, 
  (a) 
  opens 
  by 
  a 
  

   minute 
  pore, 
  (6) 
  contracts 
  regularly 
  as 
  it 
  expels 
  its 
  contents, 
  (c) 
  

   closes 
  up 
  simultaneously 
  with 
  the 
  completion 
  of 
  their 
  expulsion, 
  — 
  ■ 
  

   then 
  no 
  part 
  of 
  the 
  inner 
  protoplasm 
  is 
  exposed 
  directly 
  to 
  the 
  water, 
  

   and 
  we 
  have 
  a 
  mechanism 
  which 
  expels 
  regularly 
  the 
  plasmatic 
  juice 
  

   or 
  cell- 
  sap 
  when 
  over-diluted 
  and 
  over-abundant, 
  and 
  which 
  prevents 
  

   the 
  destruction 
  of 
  the 
  protoplasm 
  by 
  bursting 
  and 
  diffluence. 
  Tbis 
  

   is 
  the 
  mechanism 
  of 
  tbe 
  contractile 
  vacuole, 
  which 
  is 
  thus 
  a 
  physio- 
  

   logical 
  necessity 
  to 
  the 
  naked 
  cell 
  living 
  in 
  water, 
  just 
  as 
  the 
  kidney 
  

   is 
  to 
  the 
  multicellular 
  animal 
  organism. 
  

  

  Ann. 
  & 
  Mag. 
  N. 
  Hist. 
  Ser. 
  6. 
  Vol 
  iii. 
  5 
  

  

  